Difference between revisions of "Net Force"

Main Page > Giancoli Physics (5th ed) Solutions > Blank Problem Template

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Problems

1. What is the weight of a 3.4 kg mass?

F = ma

Weight = force

F = 3.4 x 9.8 = 33.32 N

2. What mass has a weight of 720. N?

F = ma

F/a = m

720 N/9.81 m/s = 73.4 m/s

3. Bob must exert 240. N of force on a 980. Kg car to move it at a constant speed up an incline. (The frictional and gravity force is 240. N) What is the acceleration of the car if he exerts a force of 350. N?

F = ma

F/m = a

(350 N - 240 N)/980 kg = .112 m/s2

4. What is the acceleration of a 6.0 Kg object hanging on a string that is under a tension of 80. N? 30. N? (Make up positive)

F = ma

6 kg * 9.81 m/s = 58.9 N

A. (80 N - 58.9 N) = (6 kg)a

21.1 N = (6 kg)a

a = 3.5 m/s2

B. (30 N - 58.9 N) = (6 kg)a

(-28.9 N) = (6 kg)a

a = -4.8 m/s2

5. . What force is needed to accelerate a 60.0 Kg cart and rider from rest to 4.20 m/s in 2.50 seconds when the friction force is 24.0 N?

v/t = a

(4.2 m/s)/(2.5 s) = 1.68 m/s2

F = ma

(F - 24 N) = (60 kg)(1.68 m/s2)

F = 125 N

6. What is the mass of a box that moves at a constant velocity along a surface with a force of 15 N, and accelerates at +4.2 m/s/s when you exert +26 N? (The box is moving in the same direction as the forces)

F = ma

(26 N - 15 N) = m(4.2 m/s2)

m = 2.6 kg

7. If you exert a force of 60. N on a car, it moves at a constant velocity. (i.e. there is a frictional force of 60. N) What is its mass if when you exert 80. N on it, it accelerates from rest to 2.0 m/s in 100. seconds?

F = ma

(2.0 m/s) / (100 s) = .02 m/s2

(80 kg - 60 kg) = m(.02 m/s2)

m = 1000 kg

8. A 60.0 Kg rocket accelerates upward from rest reaching a height of 23.4 m in 3.00 seconds. What must be the thrust of the engine?

SUVAT!!!

s = ut + .5at2

23.4 = .5a(3)2

a = 5.2 m/s2

F = ma

(F - (9.81m/s2 * 60 kg)) = 60 kg * 5.2 m/s2

F = 901 N

9. It takes 45 N to make a 10. kg cart move at a constant speed. What force does it take to make the cart accelerate at 3.2 m/s/s in the direction it is moving?

F = ma

(F - 45 N) = 10 kg * 3.2 m/s2

F = 77 N

10. What tension would accelerate a 5.0 Kg object suspended on a string upwards at 6.2 m/s/s? Downwards?

F = ma

A. (F - (5 kg * 9.81 m/s2) = 5 kg * 6.2 m/s2

F = 80 N

B. (F - (5 kg * 9.81 m/s2) = 5 kg * -6.2 m/s2

F = 18 N

11. A 45.00 gram rocket accelerates upward from 0 to 12.00 m/s in .05000 seconds. What must be the thrust of the engines?

SUVAT!!! AGAIN!!!

v = u + at

12 m/s = 0 m/s + a(.05 s)

a = 240 m/s2

F = ma

(F - (.045 kg * 9.81 m/s2)) = .045 kg * 240 m/s2

F = 11.24 N

12. A rocket has engines that produce 60. N of thrust. What is its mass if it accelerates upward at 40. m/s/s?

Here comes a whole bunch of Algebra...

F = ma

(60 N - (mg)) = m(40 m/s2)

1.5 N/m/s2 - mg / 40 m/s2 = m

1.5 N/m/s2 = mg / 40 m/s2 + m

1.5 N/m/s2 = mg + 40m / 40 m/s2

60 N = 40m + 9.81m

60 N = 49.81m

m = 1.2 kg

-2.33 m/s/s

+281 N

-7.83 m/s/s

-32.0 kN

-21.6 m/s

+72.6 m/s/s

+9750 N