Problem Set 6.8: | 1 | 3 | 4 | 5 | 6 | 17 | 19 | 21 | 23 | 25 | 28 | 29 | 31Go up
- by Sonja Scherer, Cooper Spear, and Chris Murray 2001

1. A 75.0-kg firefighter climbs a flight of stairs 10.0 m high. How much work is required?

Well, work is given by
W = Fs (if the force and the displacement are parallel, which I am going to assume)
The force needed to move the firefighter up the ladder is her weight, or
F = ma
F = (75 kg)(9.8 N/kg) = 735 N
And finally, W = Fs = (735 N)(10.0 m) = 7350 Nm or 7350 J

3. How much work did the movers do (horizontally) pushing a 150-kg crate 12.3 m across a rough floor without acceleration, if the effective coefficient of friction was 0.70?

Work is given by
W = Fs (if the force and the displacement are parallel, which I am going to assume)
The force would be the force of friction,
The normal force is the weight of the 150 kg crate or (150 kg)(9.80 N/kg) = 1470 N
So applying the friction formula: (Since it is not accelerating, the force needed is exactly the kinetic friction force)
Ffr = µkFN
Ffr = (.70)(1470 N) = 1029 N
And finally, W = Fs = (1029 N)(12.3 m) = 12656.7 Nm or 1.3 x 104 J

4. A car does 7.0x104 J of work in traveling 2.8 km at a constant speed.  What was the average retarding force acting on the car?

s = 2.8 km = 2800 m
Work is given by
W = Fs (if the force and the displacement are parallel, which I am going to assume)
so F = W/s = (7.0 x 104J)/(2800 m) =
25 N

5. How high will a 0.325-kg rock go if thrown straight up by someone who does 115 J of work on it? Neglect air resistance.

Work is given by
W = Fs (if the force and the displacement are parallel, which I am going to assume)
The force needed to move the rock upward is its weight, or
F = ma
F = (.325 kg)(9.8 N/kg) = 3.185 N
And finally,
W = Fs, so
F = W/s = (115 J)/(3.185 N) = 36.1 m

6. A hammerhead with a mass of 2.0 kg is allowed to fall onto a nail from a height of .40 m . What is the maximum amount of work it could do on the nail? Why do people not just "let it fall" but add their own force to the hammer as it falls?

The work it can do on the nail is the same as what was done to lift it, so
Work is given by
W = Fs (if the force and the displacement are parallel, which I am going to assume)
The force needed to lift the hammer is its weight, or
F = ma
F = (2.0 kg)(9.8 N/kg) = 19.6 N
And finally, W = Fs = (19.6 N)(.40 m) = 7.84 Nm or 7.8 J
People push on the hammer to give it more energy.  Since energy is the ability to do work, the more kinetic energy the hammer has when it strikes the nail, the more work it can do.

17. At room temperature, an oxygen molecule, with mass of 5.31 x 10-26 kg, typically has a KE of about 6.21 x 10 -21 J. How fast is it moving?

The formula for kinetic energy is:
Ek = 1/2mv2
We want the velocity, so let's solve for it first, as I don't really want to do math with numbers in scientific notation:
2
Ek = mv2
2
Ek/m = v2
v =
Ö{2Ek/mÖ{2(6.21 x 10 -21 J)/(5.31 x 10-26 kg) =  483.6 m/s = 484 m/s

19. How much work is required to stop an electron (m = 9.11 x 10-31 kg) which is moving with a speed of 1.90 x 106 m/s

You would need to do an amount of work equal to its kinetic energy.  (Or rather, the electron would do work on you - giving you energy)
The formula for kinetic energy is:
Ek = 1/2mv2
Ek = 1/2(9.11 x 10-31 kg)(1.90 x 106 m/s)2 =  1.64436x10-18 J = 1.64x10-18 J

21. An automobile is traveling along a highway at 90 km/h. If it travels instead at 100 km/h, what is the percent increase in the automobile's  kinetic energy?

We don't know the mass of the car, but don't let that get you down.  We will just have to be clever.
The formula for kinetic energy is:
Ek = 1/2mv2
now, we'll let v1 = 90 km/hr, and v2 = 100 km/hr
The percent increase is just one thing, divided by another times 100, so we have
%increase =
Ek1/Ek1  = {1/2mv12}/{1/2mv22- now the constants 1/2 and m cancel out:
%increase = Ek1/Ek1  = {v12}/{v22}  = {(100 km/hr)2}/{(90 km/hr)2} = 1.23 or 23% increase
The units don't matter, as they cancel out too.  Life is good.

23. A baseball (m = 140 g) traveling 35 m/s moves a fielder's glove backward 25 cm when the ball is caught. What was the average force exerted by the ball on the glove?

m = 140g = .140 kg, s = 25 cm = .25 m
This is a conservation of energy problem.  The idea is that the kinetic energy of the moving baseball goes into the work it takes to move the glove backward.  So I will set the kinetic energy equal to work:
1/2mv2 = Fs
1/2(.140 kg)(35 m/s)2 = F(.25 m)
F =
343 N

25. At an accident scene on a level road, investigators measure a car's skid mark to be 88 m long. It was a rainy day and the coefficient of friction was estimated to be 0.42. Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes. (Why does the car/s mass not matter?)

Here the kinetic energy of the car goes entirely to work done against friction.  (i.e. the tires sliding along the ground)  I will set the kinetic energy equal to work:
1/2mv2 = Fs
Where F is the force of friction given by
Ffr = µkFN - so I will substitute this for the force:
1/2mv2 = Fs = (µkFN)s
And finally, the normal force,
FN is simply the weight in this case.  F = ma, so weight equals mg where g is the acceleration of gravity.  Substituting this in for the normal force:
1/2mv2 =  ( µkmg)s
Notice that at this point I can cancel the mass of the car from both sides, leaving:
1/2v2 =  ( µkg)s
Now we can solve for v:
v2 =  2(µkg)s
v =
Ö{2( µkg)s} =  26.9 m/s = 27 m/s

28. A 220-kg load is lifted 21.0 m   vertically with an acceleration a = 0.150 g by a single cable. Determine (a) the tension in the cable, (b) the net work done on the load, (c) the work done by the cable on the load, (d) the work done by gravity on the load, and (e) the final speed of the load assuming that it started from rest.

a = .150 g = (.150)(9.8 m/s/s) = 1.47 m/s/s
a)
To calculate the tension in the cable, let's do a little Newton's second law here.  We have the force of gravity downward of (220 kg)(9.8 N/kg) = 2156 N downward (-), and the cable tension T upward (+) and an upward acceleration of 1.47 m/s/s upward (+), so our F = ma look like:
<T - 2156 N> = (220 kg)(+1.47 m/s/s)
T = 2479.4 N
c)
The work done by the cable on the load is W = F.s = (2479.4 N)(21.0 m) = 52067.4 J.  This is a positive quantity as the displacement is in the direction of the force.  (The cable pulled up, and it moved up)
d)
The work done by gravity is W = F.s = (2156 N)(21.0 m) = 45276 J.  This is a negative quantity because the displacement is up, but the force of gravity is down.
b)
The Net work done on the load is the sum of the work done by the cable on the load, and the work done by gravity on the load.