Work & Power Problems Answers

WORK & POWER PROBLEMS – ANSWERS

Ralph uses a 2.00x103N force to push his car along a road for 1.00x103m. It takes him 5.00x102s to do this. A) Calculate the amount of work that Ralph did on the car.

W = Fd = 2.00x103N * 1.00x103m = 2.00x106J

B) Calculate the amount of power that he generated.

P = W / t = 2.00x106J / 5.00x102s = 4.00x103 watts

A circus strongman exerts a force of 1500N as he pushes on the side of the building. After pushing for 2 minutes, the building has not moved. How much work did the strongman do? How do you know?

None – nothing moved (zero distance)

Julie must lift three 1.00x102kg boxes up to a ledge that is 2.00m above the ground.

A) Calculate the weight of one of the boxes.

2 2 Fw = mg = 1.00x10 kg * 9.8m/s = 980N

B) How much force must be exerted to lift one box? 980N

C) Calculate the amount of work Julie does to lift one box up to the ledge.

W = Fd = 980N * 2.00m = 1.96 x 103J

D) How much work would have to be done to lift all three boxes up to the ledge, one box at a time?

5.88 x 103 J ( 1.96 x 103 J per box times 3)

E) Calculate the weight of all three boxes.

2 2 3 Fw = mg = 3.00x10 kg * 9.8m/s = 2.94 x 10 N

F) How much force must be exerted to lift all three boxes at once? 2.94 x 103 N

G) Calculate how much work would be done to lift all three boxes up to the ledge at the same time.

W = Fd = 2.94 x 103 N * 2.00m = 5.88 x 103 J H) How does the amount of work done lifting the three boxes, one at a time, compare to the amount of work done lifting the three boxes all at once? Explain why.

Same – same boxes moved to the same height

I) Would Julie generate more power lifting one box at a time to the ledge, or by lifting all three boxes at the same time? Explain your answer.

Three at once – less time to do the same work

A 975N car is traveling east at 25.0m/s. A 50.0N force is applied by the engine to the east for a distance of 225m. What is the final velocity of the car? (Solve using the work – energy theorem! 5 steps.)

2 m = Fw / g = 975N / 9.8m/s = 99.5kg

W = Fd = 50.0N * 225m = 1.13 x 104 J

KE = mv2 / 2 = 99.5kg * 25.02m2/s2 / 2 = 3.11 x 104 J

Total Energy = KE + W = 3.11 x 104 J + 1.13 x 104 J = 4.24 x 104 J

V = √2KE / m = √(2 * 4.24 x 104 J) / 99.5kg = 29.2m/s east