Energy II, or Conservation of Energy
by Jacob Spindel, January 1998
Table of Contents
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New Quantities
Here's the quantities you can know:
- PE Potential Energy
- KE Kinetic Energy
- PE_{s} Spring Potential Energy
- V_{o} Initial velocity
- V Final Velocity
- F Restoring Force
- k Spring Constant
- d Distance
All of these are defined on other pages except for restoring force, spring constant, and spring potential energy, which are explained below.
Defining Spring Constant and Restoring Force
Springs spring. That is, they push back when they're compressed. When the spring
"pushes back", it is exerting a force, which can be measured in Newtons. We can
calculate how much a spring will push back. To do this, use the following formula:
F=-k*x
where F is the force, k is something called the spring constant, and x is the distance
the spring has been compressed (not the length of the spring!). What's the deal
with that "spring constant" thing, anyway? It's just a characteristic of the spring.
Not every spring pushes back the same amount, so the spring constant of a spring,
measured in N/m, is how we calculate information based on the specific spring.
Defining Spring Potential Energy
How exactly does a spring push back when it has been compressed? No, not
magic dwarves. A compressed spring has energy stored in it. You know, energy. That
thing we
measure in Joules (J). Our other new formula tells us exactly how much energy.
PE_{S}=1/2*k*x^{2}
Formulas
So now we have all the formulas we need for solving spring force and energy problems
with:
1. PE=m*g*h
2. PE_{S}=1/2*k*x^{2}
3. KE=1/2*m*v^{2}
4. W=F*s
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General Problem Solving Strategy:
The most important thing to remember in this lesson is that energy is CONSERVED. You can't create energy or get rid of it. Energy can change forms, but it cannot appear or disappear. All of these problems will involve a "before" and an "after." In other words, something will happen in the problem, like an object speeding up or slowing down, that will cause energy to change forms. However, at the beginning of the problem and at the end of the problem, the total amount of energy will remain the same because energy is CONSERVED. Your job will be to know when energy changes from one form to another. Possible forms of energy include potential energy, spring potential energy, kinetic energy, and work. An object can have all four kinds of energy simultaneously. If you add all the different kinds of energy the object has together, you will find its total energy. If the object changes in some way (for example, it falls, it speeds up, or it slows down), it may have less of one type of energy and more of a different type of energy than it had before. However, the total energy will be the same throughout the problem.
- Read the problem.
- Figure out what two different points in time are where you know information about what's going on in the problem.
- Go through the problem and figure out what is given or implied.
Place the information in a table divided into columns and rows. Make each column represent
a
different place for energy to be stored, such as PE, PE_{S}, KE, and W (Work).
Also divide the table into two rows. Each row represents one point in time.
Usually, you will know all the information at one of the points in time, and you will
be
looking for a quantity at the other point in time. For example, a moving car may change
the amount of potential and kinetic energies it has as it is moving along.
- Find any formula that will allow you to calculate anything that you don't know, and
apply it.
- Add what you just found in the last step to your table of knowns.
- Check to see if you have found the answer. If not, repeat the previous two steps until you are done.
And remember, only you can prevent forest fires.
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Example Problem 1
All of Bob's friends jumped off a bridge, so he did too. The bridge is 12 m above the water and Bob has a mass of 68 Kg. With what velocity will Bob hit the water?
This is a common problem of something falling, which means that its potential energy goes to kinetic energy. Our two points in time are:
1. Bob is on top of the bridge, thinking he's a hot shot.
2. Bob is at his fastest velocity, which is when he's just about to hit the water, crying for his mommy.
Bob initially has potential energy because he is elevated from the ground. We can calculate his potential energy using the PE formula:
PE=m*g*h
With numbers:
PE=68*9.8*12=7996.8 J
This potential energy will, as promised, become kinetic energy. So, we can set the number we found for potential energy equal to the kinetic
energy formula:
KE=1/2*m*v^{2}=7996.8 J
We know Bob's mass. If we plug that into this formula, then we can solve
for v, which is the velocity we're looking for.
1/2*68*v^{2}=7996.8
v=15.3362 m/s, or 15 m/s with sig-figs.
That's about 34 miles per hour, by the way. I guess it's just not Bob's day.
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Example Problem 2
Tamika thought that example 1 was so stupid that she started using a spring to shoot rocks at Jacob. Her spring has a spring constant of 50 N/m, and she compresses it 0.25 m each time. Then she releases it, shooting each rock horizontally. The rocks are all identical and have a mass of 0.5 kg. What is the velocity of the rocks right after she shoots them (before they have a chance to be influenced by gravity or air resistance)?
Our two points in time for this problem are:
1. Tamika has compressed the spring so that it is storing spring potential energy.
2. Tamika has released the spring so that it transfers its potential energy to the rock's kinetic energy (in other words, the spring springs and pushes the rock).
First, we need to find the total energy stored in the first point in time, which is simply the spring potential energy because there are no other sources of energy in this problem.
We can calculate how much potential energy the spring has by using the spring potential energy formula:
PE_{spring}=-1/2*k*x^{2}
Next, we plug the numbers into this formula:
PE_{spring}=-1/2*50*-(0.25)^{2}=6.25 J
(The 0.25 is negative because when you compress a spring, the distance it's compressed is represented by a negative number.)
In the second point in time, all this energy is going to kinetic energy, so we set 6.25 equal to the kinetic energy formula.
6.25=1/2*m*v^{2}
If we plug the value we know for m into this formula, we can solve for v.
6.25=1/2*0.5*v^{2}
v=5 m/s
(That's about 11.18 mph, by the way. I think I'll sue Tamika.)
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Sample Problems
The answers to each problem follow it in parentheses. They also link to a
solution to the problem. Try the problem, check your answer, and go to
the solution if you do not understand.
1.
A 1250 Kg car going 23 m/s can coast how far up a very tall hill if it loses no energy to friction? (27 m)
2.
How far will the car in the previous problem coast up the hill if it
loses 150,000 J of energy to friction on the way up? (15 m)
3.
A 873 Kg car going 12 m/s at the top of a 6.2 m tall hill is going how fast
at the bottom? (No loss to friction)(16 m)
4.
A 312 Kg rocket ship in deep space fires an engine that produces 516 N of thrust,
for a distance of 32 m. If the rocket ship was initially at rest, what is its final
velocity?(10. m)
5.
What is the final velocity of a .452 Kg object initially at rest if you exert a force
of 6.50 N on it vertically for a distance of 12.5 m?
(10.7 m)
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6.
A 100. Kg rollercoaster has a speed of 8.0 m/s on the top of a hill that is 6.4 m
tall. What is its speed on the top of a 2.4 m tall hill?(12 m)
7.
A coasting 980 Kg car going 21 m/s at the top of a 15 m tall hill is brought to rest
by a crash barrier at the bottom of the hill in a distance of 12.4 m. What force
did the barrier exert on the car, and what acceleration did the car undergo in stopping?(29,000 N, 30. m/s/s or 3.02 "g"s)
8.
A 120. Kg bicyclist going 5.60 m/s at the bottom of a 2.00 m tall hill exerts a forward
force of 200. N for 10.0 m as he climbs the hill. What is his speed at the top
of the hill?
(5.05 m/s)
9.
A 150 Kg rollercoaster car is going 12 m/s at the top of a 12 m tall hill, and then
rolls into the station at a height of 3.0 m where it is brought down to a speed of
6.0 m/s with a braking force of 8900 N. Over what distance must the force be exerted?
(2.4 m)
10.
A coasting 1150 Kg car going 21 m/s hits a puddle that is 13 m long. It leaves the
puddle going 18 m/s. What force did the puddle exert on the car?
(5200 N)
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11.
The engines on a 45 Kg rocket fired vertically burn for a distance of 25 m generating
a force of 620 N. (Assume for this problem that the rocket's mass remains constant)
What is the rocket's speed just after the engines quit burning? To what height does
the rocket rise in the air before falling back to earth?
(14 m/s, 35 m)
12.
A spring with a constant of 124 N/m is compressed 16.2 cm. How much potential energy does it store? How much is released if the spring is allowed to expand only from 16.2 cm to 12.5 cm?
(1.63 J, 0.66 J)
13.
A 2.0 g penny is pushed down on a vertical spring, compressing the spring by 1.0 cm. The force constant of the spring is 40. N/m. How far above this original position will the penny fly if it is released?
(10. cm)
14.
A 745 Kg rollercoaster car travelling at a speed of 3.50 m/s at an elevation of 11.25 m is stopped in the station at an elevation of 10.21 m by a spring over a distance of 67 cm. What is the spring constant of the spring?
(5.4 * 10^{4} N)
15.
A spring-loaded marble gun shoots 12.5 gram marbles off of a (Frictionless!) horizontal table that is 91 cm high. The spring in the gun has a spring constant of 52 N/m. By how much do you need to compress the spring to land a marble on the floor 45 cm horizontally from the edge of the table?
(1.6 cm)
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Solutions to Sample Problems
Jacob's Guarantee: If the following solutions are not right, then they're wrong.
1.
A 1250 Kg car going 23 m/s can coast how far up a very tall hill if it loses no energy
to friction?
Our two points in time are going to be:
1. The car is going at 23 m/s and hasn't started up the hill yet.
2. The car has gone as far up the hill as it can go, and it has stopped moving.
So, if you look at the formulas, the first one won't work just yet because you don't
know height or PE, and the second one one work because this problem doesn't even have
a spring in it. However, the third one will work since you know the initial
velocity.
KE=1/2*m*v^{2}
Putting in numbers:
KE=1/2*1250*23^{2}=330625
So now you know these quantities:
- m = 1250 Kg
- V_{0} = 23 m/s
- V_{final} = 0 (assumed)
- KE=330625 J
- Height = ?
So again you look at the formulas. Remember, if the car stops moving because
it moves upward, the kinetic energy will become potential energy. So now you can
use the first problem, because kinetic energy becomes equal to potential energy:
PE=m*g*h
Plugging in numbers:
330625=1250*9.8*h
so h (the height) = 26.9897 m, or 27 m with sig figs.
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2.
How far will the car in the previous problem coast up the hill if it loses 150,000
J of energy to friction on the way up?
This problem is basically the same as the last one. We're going to transfer that kinetic
energy to potential energy. However, this time, not all of the energy goes to potential
energy. This time, some of it is lost to friction. We have to subtract this amount
first.
Here's what we know:
- m = 1250 Kg
- KE=330625 J
- Energy loss to friction = 150000 J
- Height = ?
Amount of energy left to go to potential energy=KE - energy lost to friction
330625-150000=180625 J
180625 J are left to be transferred to potential energy. Set it equal to the potential
energy formula again.
PE=m*g*h
Plugging numbers into the formula:
180625=1250*9.8*h
So
h (Height) = 14.744 m, or 15 m with sig figs.
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3.
A 873 Kg car going 12 m/s at the top of a 6.2 m tall hill is going how fast
at the bottom? (No loss to friction)
Our two points in time are:
1. The car is at the top of the hill, moving at 12 m/s.
2. The car is at the bottom of the hill, moving at a speed that we don't know yet.
At the first point in time, the car is off the ground, so it has potential energy. We can calculate that with the information we have:
PE=m*g*h
With numbers:
PE=873*9.8*6.2=53043.5 J
The car is also moving at the top of the hill, so it also has kinetic energy.
KE=1/2*m*v^{2}
With numbers:
KE=1/2*873*12^{2}=62856 J
The total energy this car has is simply PE+KE=53043.5+62856=115899 J.
So now, we know:
- V=12 m/s
- h = 6.2 m
- m = 873 kg
- PE= 53043.5 J
- KE = 62856 J
- Total energy = 115899 J.
Now, let's figure out what's going to happen to the car in the second point in time. It's not going to be on the hill anymore. This means that it won't have any potential energy because it's on the ground. The potential energy will be transferred to kinetic energy, which will add to the kinetic energy the car already had. All it's energy will be kinetic:
KE=115899 J.
We also know that KE=1/2*m*v^{2}
So 115899=1/2*m*v^{2}
Plugging in the number we know for the mass of the car gives us:
115899=1/2*873*v^{2}
Solve this equation for v, and you'll find that v=16.3 m/s, or 16 m/s with sig-figs.
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4.
A 312 Kg rocket ship in deep space fires an engine that produces 516 N of thrust, for a distance of 32 m. If the rocket ship was initially at rest, what is its final velocity?
The two points in time we know are:
1. The rocket ship is firing its engine to produce work.
2. The rocket ship has stopped firing its engine and is at the highest speed it will reach.
At the first point in time, there is only one kind of energy: work. At the second point in time, there is only one kind of energy: kinetic energy (the rocket is moving). This should be pretty easy.
(There is no potential energy anywhere because the rocket is in deep space and there is no gravity.)
We can figure out how much work the rocket is doing with the work formula:
W=F*d
With numbers:
W=516*32=16512 J
Let's check what we know:
- F = 516 N
- d = 32 m
- m = 312 Kg
- W = 16512 J
- V = ?
Remember, the work is going to transfer to kinetic energy, and the amount of kinetic energy we will have is equal to the amount of work we had because energy is conserved.
W = KE = 16512 J
Next, we use the formula we have for KE:
KE=1/2*m*v^{2}=16512 J
We also know that m = 312 Kg
1/2*312*v^{2}=16512
v = 10.2882 m/s, or 10. m/s with sig-figs.
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5.
What is the final velocity of a .452 Kg object initially at rest if you exert a force of 6.50 N on it vertically for a distance of 12.5 m?
The two points in time we know in this problem are:
1. Work is being done on an object that is at rest and on the ground.
2. The work is done, and the object is now moving and in the air.
At the first point in time, the only energy is the work, just like the first point in time in the last problem. At the second point in time, the object is moving AND it's in the air. That means that the work has gone to kinetic energy AND potential energy. Sneaky crocodile!
Remember, of course, that if you add the kinetic and potential energies, it will still be equal to the work, because the total amount of energy from each point in time is the same.
In math terms,
W=PE+KE.
Here's what we already know:
- m = 0.452 Kg
- F = 6.5 N
- d = 12.5 m
- V = ?
We know that W=PE+KE. We also know that W=F*d, PE = m*g*h, and KE = 1/2*m*v^{2}This means that:
F*d=m*g*h+1/2*m*v^{2}
Plug in numbers:
6.5*12.5=0.452*9.8*12.5+1/2*0.452*v^{2}
Solve for v and you'll find that it's 10.7011 m/s, or 10.7 m/s with sig-figs.
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6.
A 100. Kg rollercoaster has a speed of 8.0 m/s on the top of a hill that is 6.4 m tall. What is its speed on the top of a 2.4 m tall hill?
Our two points in time for this problem are:
1. The rollercoaster is on a 6.4 m hill going 8.0 m/s.
2. The rollercoaster is on a 2.4 m hill going at a speed we don't know yet.
Here is what you start with:
- h_{1} = 6.4 m
- h_{2} = 2.4 m
- m = 100. Kg
- V_{1} = 8.0 m/s
- V_{2} = ?
At the first point in time, the roller coaster has potential energy (it's on a hill) and kinetic energy (it's moving). At the second point in time, the roller coaster still has potential energy and kinetic energy. However, it has more kinetic energy and less potential energy than at the first point in time. The total energy from each point in time will be equal:
PE_{1}+KE_{1} = PE_{2} + KE_{2}
We can calculate the initial potential energy with the potential energy formula:
PE_{1} = m*g*h_{1}
PE_{1} = 100*9.8*6.4 = 6272 J.
We also have enough information to calculate the initial kinetic energy:
KE_{1} = 1/2*m*v^{2}
KE_{1} = 1/2*100*8.0^{2} = 3200 J.
And we can calculate the final potential energy:
PE_{2} = m*g*h_{2}
PE_{2} = 100*9.8*2.4 = 2352 J
Substitute the numbers we have into the formula PE_{1}+KE_{2}=PE_{2}+KE_{2}:
6272+3200 = 2352+KE_{2}
Solving this reveals that KE_{2} = 7120 J.
We also know that KE=1/2*m*v^{2}, so:
7120=1/2*m*v^{2}
7120=1/2*100*v^{2}
v = 11.9331 m/s, or 12 m/s with sig-figs.
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7.
A coasting 980 Kg car going 21 m/s at the top of a 15 m tall hill is brought to rest by a crash barrier at the bottom of the hill in a distance of 12.4 m. What force did the barrier exert on the car, and what acceleration did the car undergo in stopping?
Here
is what you start with:
- m = 980 Kg
- v = 21 m/s
- h = 15 m
- d = 12.4 m
- F = ?
- a = ?
Our two points in time will be:
1. The car is on top of the hill going 21 m/s.
2. The car is at the bottom of the hill and it has been stopped by the crash barrier.
The car has two different types of energy at the first point in time: it is on a hill, so it has potential energy, and it is moving, so it has kinetic energy. How much?
PE=m*g*h
With numbers:
PE=980*9.8*15=144060 J.
KE=1/2*m*v^{2}
With numbers:
KE=1/2*980*21^{2}=216090 J.
Total energy=PE+KE=144060+216090=360150 J.
In the second frame, the car has come off the hill, and it has been stopped. That means that it no longer has potential or kinetic energy. That energy had to go somewhere. You can't just get rid of energy. Not even Congress is that wasteful. The wording of the problem makes it pretty obvious where this energy is going: the car is doing work on the crash barrier. The energy goes to work. The energy isn't going to any other source, so:
Total energy=W.
154350=F*d.
And we know the distance, d, so:
360150=F*12.4.F=29044.35 N, or 29000 N with sig-figs.
But wait! There's more!
Now that we know the force, it won't be hard to find the acceleration of the car. It's time to use a formula from the good old days:
F=m*a.
Force equals mass times acceleration. We know the force and the mass, and we want the acceleration.
29044.35=980*a.
a=29.637 m/s, or 30. m/s with sig-figs.
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8.
A 120. Kg bicyclist going 5.60 m/s at the bottom of a 2.00 m tall hill exerts a forward force of 200. N for 10.0 m as he climbs the hill. What is their speed at the top of the hill?
Here is what you start with:
- m = 120. Kg
- v_{0} = 5.60 m/s
- h_{0}=0 (the bottom of the hill)
- h_{final}=2.00 m
- F = 200. N
- d = 10.0 m
- v = ?
Our two points in time will be:
1. The biker is at the bottom of the hill, traveling at 5.60 m/s, exerting a force of 200. N, happily ringing the bell on his bike.
2. The biker is at the top of the hill and has finished exerting the force.
At the first point in time, the biker is moving, so he has kinetic energy:
KE=1/2*m*v^{2}
With numbers:
KE=1/2*120.*5.60^{2}=1881.6 J.
The only other source of energy at the first point in time is the work done by the biker:
W=F*d
With numbers:
W=200.*10.0 m = 2000 J.
So the total energy equals W+KE=2000+1881.6 = 3881.6 J.
At the second point in time, the biker is on top of a hill, so he has potential energy. The problem says he has speed at the top of the hill (but we don't know how much yet), so the biker has some amount of kinetic energy. We can figure out how much potential energy he has:
PE=m*g*h
With numbers:
PE=120.*9.8*2=2352 J.
We know what the total energy is because we already found it at the first point in time in the problem. We also know, since the only types of energy at the second point in time are PE and KE, that PE+KE=Total Energy.
Plugging in numbers reveals:
2352+KE=3881.6
KE=1529.6 J.
And we know that KE=1/2*m*v^{2}
So 1529.6=1/2*m*v^{2}
m = 120. Kg, so:
1529.6=1/2*120.*v^{2}
v=5.049 m/s, or 5.05 m/s with sig-figs.
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9.
A 150 Kg rollercoaster car is going 12 m/s at the top of a 12 m tall hill, and then rolls into the station at a height of 3.0 m where it is brought down to a speed of 6.0 m/s with a braking force of 8900 N. Over what distance must the force be exerted?
Here is what you start with:
- m = 150 Kg
- v_{0} = 12 m/s
- h_{0} = 12 m
- v = 6 m/s
- h = 3.0 m
- F = 8900 N
- d = ?
Our two points in time in this problem will be:
1. The roller coaster is on the hill going 12 m/s.
2. The roller coaster is in the station at a lower altitude and speed.
At the first point in time, the coaster is moving and is elevated. That means it has potential energy and kinetic energy. Let's find out how much.
PE=m*g*h
With numbers:
PE=150*9.8*12=17640 J.
KE=1/2*m*v^{2}
With numbers:
KE=1/2*150*12^{2}=10800 J.
Total energy=PE+KE=17640+10800=28440.
At the second point in time, the roller coaster is still moving and elevated, so it still has potential energy and kinetic energy (but not the same amounts as before). However, at the second point in time, some of the roller coaster's energy also goes into the work used to stop the coaster. We can figure out how much potential energy and kinetic energy the roller coaster has at the second point in time:
PE=m*g*h
With numbers:
PE=150*9.8*3=4410 J.
KE=1/2*m*v^{2}With numbers:
KE=1/2*150*6^{2}=2700 J.
We also know that PE+KE+W=Total Energy, because there are no other sources of energy at this point.
4410+2700+W=28440
W=21330 J.
And W=F*d.
21330=8900*d
d=2.397 m, or 2.4 m with sig-figs.
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10.
A coasting 1150 Kg car going 21 m/s hits a puddle that is 13 m long. It leaves the
puddle going 18 m/s. What force did the puddle exert on the car?
Here is what you start with:
- m 1150 Kg
- V_{o} = 21 m/s
- d = 13 m
- V = 18 m/s
- F = ?
Our two points in time for this problem are:
1. The unsuspecting car is coasting toward the puddle but hasn't hit it yet.
2. The car has gone through the puddle and slowed down, and the driver is figuring out how he can use the mud on his car as an Area 4 Project.
At the first point in time,the car is moving, so it has kinetic energy. In fact, that's all it has. We can calculate how much kinetic energy using the KE formula:
KE=1/2*m*v^{2}
With numbers:
KE=1/2*1150*21^{2}=253575 J.
Total energy=KE=253575 J because the car has no other types of energy.
At the second point in time, the car is still moving, so it still has kinetic energy. However, the car has slowed down, so it has less kinetic energy than it had before. We can find how much KE the car has at the second point in time:
KE=1/2*m*v^{2}
With numbers:
KE=1/2*1150*18^{2}=186300 J.
The car has lost some of its kinetic energy. However, that energy had to go somewhere. In fact, it was transferred to the work done by the puddle. The two types of energy at the second point in time are KE and W.
Total Energy=KE+W
We know that the total energy is 253575 and the kinetic energy at the second point in time is 186300 J. So:
253575=186300+W
W=67275
We also know that W=F*d.
W=F*d
67275=F*13
F=5175 N, or 5200 N with sig-figs.
Go back to: Problem Formulas Table of Contents
11.
The engines on a 45 Kg rocket fired vertically burn for a distance of 25 m generating a force of 620 N. (Assume for this problem that the rocket's mass remains constant.) What is the rocket's speed just after the engines quit burning? To what height does the rocket rise in the air before falling back to the earth?
Here's what you start with:
- m = 45 Kg
- d = 25 m
- F = 620 N
- v = ?
- h = ?
Our two points in time for this problem will be:
1. The rocket is on the ground and not moving, but it is doing work with its engines.
2. The rocket has stopped firing its engines and is in the air and moving.
3. The rocket is at the highest point it will reach before it starts falling back to the ground.
(Number of points in time in this problem calculated by Intel.)
Okay, okay. So we actually have three points in time in this problem. But that's because we actually have two problems. We're trying to answer two different questions in this problem, and each question will only deal with two of the points in time.
First, let's look at part A. We're trying to find the velocity of the rocket immediately after the engines stop firing. For this part of the question, we'll only use the point 1 in time and point 2 in time, because we are only comparing the rocket on the ground to the rocket immediately after the engines have stopped firing, and we don't care yet about what the rocket does after that.
At the first point in time, the only energy the rocket has is work. At the second point in time, this energy has transferred so that it has kinetic energy and potential energy but not work.
We can find the amount of work energy the rocket has at the first point in time:
W=F*d
With numbers:
W=620*25=15500
Because there is only one source of energy at the first point in time, Total Energy=W=15500 J.
At the second point in time, the rocket is moving and in the air, so it has potential and kinetic energy.
We do know how much potential energy the rocket has. We don't know the height the rocket will have at point 3, but we do know that at point 2 the height is 25 m.
So we can calculate the PE at point 2:
PE=m*g*h
With numbers:
PE=45*9.8*25=11025 J.
With Roman numerals:
PE=VL*IX.IIX*XXV=MMMMMMMMMMMXXV J.
With unidentifiable symbols:
PE=#%*@.!*&/=!@#$% J.
I forget what we're doing.
Oh yeah. The rocket has potential and kinetic energy only at the second point in time, so we know that:
Total energy=PE+KE.
Now we know the total energy and the potential energy, so:
15500=11025+KE
KE=4475 J.
We also know that KE=1/2*m*v^{2}, so:
4475=1/2*45*v^{2}
v=14.102 m/s, or 14 m/s with sig-figs. This is the answer to part A.
Now, in part B, we need to figure out how high the rocket will be at point 3 in time - that is, we need to know how much potential energy it has.
The rocket has the same total amount of energy as before - 15500 J.
Does the rocket have kinetic energy at point 3? When an object flying vertically reaches its highest point, it stops for an instant. The rocket is not moving. It does not have kinetic energy.
With kinetic energy ruled out, the only kind of energy the rocket has at point 3 is potential energy. So:
Total energy=PE
15500=PE
We also know that PE=m*g*h, so:
15500=45*9.8*h
h=35.147 m, or 35 m with sig-figs.
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12.
A spring with a constant of 124 N/m is compressed 16.2 cm. How much potential energy does it store? How much is released if the spring is allowed to expand only from 16.2 cm to 12.5 cm?
Here is what we start with:
- k = 124 N/m
- x_{1} = 0.162 m (Remember to convert from cm!)
- x_{2} = 0.125 m
- PE_{1} = ?
- PE_{1}-PE_{2} = ? (In other words, what is the difference in energy when the spring is partially compressed?)
To find the initial spring potential energy (the answer to part A), we just use the spring potential energy formula:
PE_{S}=1/2*k*x^{2
}
With numbers:
PES=1/2*124*0.1622=1.6271 J, or 1.63 J with sig-figs.
For part B, we want to find how much the energy has decreased after the spring has been partially released. To do this, we need to know how much energy the spring has after it has been partially released:
PES=1/2*k*x2
With numbers:
PES=1/2*124*0.1252=0.96875 J.
To figure out how much energy was lost, just subtract:
1.63-0.96875=0.66 J.
(Of course, the energy isn't really "lost." Energy is conserved! The energy probably went into doing work on the hand of the person holding the spring, or else it pushed on the air and gave the air some extra kinetic energy.)
Go back to: Problem Formulas Table of Contents
13.
A 2.0 g penny is pushed down on a vertical spring, compressing the spring by 1.0 cm. The force constant of the spring is 40. N/m. How far above this original position will the penny fly if it is released?
Here's what we start with:
- m = 0.002 Kg (Remember to convert from grams!)
- x = 0.01 m (Remember to convert from centimeters!)
- k = 40 N/m
- h = ?
And here is the word "banana:"
Our two points in time will be:
1. The penny is on the spring and the spring is compressed.
2. The spring has been released and the penny has reached the highest point in the air it will reach.
At the first point in time, there is only one type of energy: spring potential energy. Let's see how much.
PES=1/2*k*x2
With numbers:
PES=1/2*40*0.012=0.002 J.
Since there are no other types of energy at the first point in time, the total
energy=PES=0.002 J.
At the second point in time, the penny is in the air, but it has stopped moving and the spring is no longer compressed. This means that it has potential energy, but it does not have any other type of energy. So, all of the energy goes to potential energy:
Total energy=PE
0.002 J=PE
We also know that PE=m*g*h, so:
0.002=m*g*h
0.002=0.002*9.8*h
h=0.10204 m, or 10. cm with sig-figs. (You could also answer 0.10 m.)
Go back to: Problem Formulas Table of Contents
14.
A 745 Kg rollercoaster car travelling at a speed of 3.50 m/s at an elevation of 11.25 m is stopped in the station at an elevation of 10.21 m by a spring over a distance of 67 cm. What is the spring constant of the spring?
Here is what we know:
- m = 745 Kg
- V0 = 3.50 m/s
- h0 = 11.25 m
- h = 10.21 m
- V = 0 m/s (The car is stopped.)
- x = 0.67 m (Remember to convert from centimeters!>
Our two points in time in this problem will be:
1. The roller coaster is at 11.25 m and moving at a speed of 3.50 m/s.
2. The roller coaster is in the station, the spring has stopped it, and the passengers are hoping that the spring that stopped them doesn't expand again and fling them to their deaths.
(Don't worry, it won't.)
At the first point in time, the roller coaster is on a hill and it's moving. That means that it has potential energy and kinetic energy. How much?
PE=m*g*h
With numbers:
PE=745*9.8*11.25=82136.25 J
KE=1/2*m*v2
With numbers:
KE=1/2*745*3.502=4563.125 J.
The car doesn't have any other types of energy at the first point in time, so the total energy = PE+KE = 86699.375 J.
At the second point in time, the car has stopped, so it no longer has kinetic energy. It still has potential energy, but not the same amount as before. At the second point in time, there is also a compressed spring, so we do have spring potential energy. In fact, the only types of energy that the roller coaster has at the second point in time are regular potential energy and spring potential energy. How much regular potential energy does it have?
PE=m*g*h
With numbers:
PE=745*9.8*10.21=74543.21 J.
We also know that Total energy=PE+PES
So 86699.375=74543.21+PES
PES=12156.165 J.
We also know that PES=1/2*k*x2, so:
12156.165=1/2*k*0.672
k=54159.7906 J, or 5.4 * 104 with sig-figs.
Go back to: Problem Formulas Table of Contents
15.
A spring-loaded marble gun shoots 12.5 gram marbles off of a (Frictionless!) horizontal table that is 91 cm high. The spring in the gun has a spring constant of 52 N/m. By how much do you need to compress the spring to land a marble on the floor 45 cm horizontally from the edge of the table?
Here is what we start with:
- m = 0.0125 Kg (Remember to convert from grams!)
- h = 0.91 m (Remember to convert from centimeters!)
- k = 52 N/m
- dHorizontal = 0.45 m (Remember to convert from centimeters!)
- x = ?
Before we do anything with energy, let's figure out what the initial velocity of the marble has to be just after the spring has released it.
All we need to do this is linear kinematics - it's a cliff problem.
First, let's find out how long the marble will be falling before it hits the ground by using the formula X=X0+1/2*a*t2
with the information we know about how the marble will move vertically.
-0.91=0+1/2*-9.8*t2
t=0.431 s.
The horizontal velocity of the marble will be constant after the spring has released it, so we can figure out what velocity the marble needs in order to travel 0.45 m in 0.431 seconds:
x=v*t
With numbers:
0.45=v*0.431
v=1.04 m/s.
So now we know that the initial velocity of the marble has to be 1.04 m/s, so we just need to figure out how much the spring needs to be compressed to give it that initial velocity.
Now it's time to use energy.
Our two points in time for this problem will be:
1. The marble is at rest but is compressed on the spring.
2. The spring has just been released and the marble is moving.
At the first point in time, there is only one kind of energy: spring potential energy. At the second point
in time, there is also only one kind of energy: kinetic energy. We can figure out how much KE
the marble has at the second point in time:
KE=1/2*m*v2
With numbers:
KE=1/2*0.0125*1.042=0.00676 J.
Because there is only one source of energy at each point in time, PES=Total energy and KE=Total energy, so PES=KE.
0.00676=PES
We also know that PES=1/2*k*x2,so:
0.00676=1/2*52*x2
x=0.01612 m, or 1.6 cm with sig-figs.
Go back to: Problem Formulas Table of Contents
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