Work and Power - Questions & Answers
1. Work done on the block by other forces as it falls?
If no other forces act, work = 0. If air resistance acts, work is negative.
2. Can total work be zero if net force is not zero?
Yes, in circular motion the net force is perpendicular to motion, so work = 0.
3. Rope pulls body, body pulls rope. Is total work zero?
No, action–reaction act on different objects. The rope’s force does positive work on the body, increasing its energy.
4. Elevator at constant speed?
Total work = 0. Tension does positive work, gravity does equal negative work.
5. What is total work?
It is the sum of works by all forces. It equals the change in kinetic energy.
6. Work done lifting 2 kg by 2 m?
W = mgh = 2 × 9.8 × 2 = 39.2 J.
7. Graph of F vs x giving zero work?
If positive and negative areas cancel (like a sine wave), net work = 0.
8. Work needed for 3 times final speed?
Work ∝ v². So 3 times speed → 9 times more work.
9. With net force, can these be constant?
(a) Speed: Yes, in circular motion.
(b) Velocity: No.
(c) Kinetic energy: Yes, if speed is constant.
(b) Velocity: No.
(c) Kinetic energy: Yes, if speed is constant.
10. Throw a 3 N rock upward (h=15 m, v=25 m/s). Find:
(a) Initial speed ≈ 30.3 m/s.
(b) Max height ≈ 46.9 m.
(b) Max height ≈ 46.9 m.
11. If speed doubles, what happens to kinetic energy?
Kinetic energy becomes 4 times bigger.
12. 6 kg box (3 m/s) compressing spring (k=7500 N/m)?
Maximum compression ≈ 0.085 m (8.5 cm).
13. Work done by centripetal force?
Always zero, since force is perpendicular to motion.
14. 20 kg rock at 8 m/s stops by friction (μ=0.2). Power?
Average power ≈ 157 W lost as heat.
15. Convert 500 MW to kW and W.
500 MW = 500,000 kW = 500,000,000 W.
16. Lifting 30 kg crate (0.9 m). How many in 1 min?
(a) For 0.50 hp (373 W): about 85 crates.
(b) For 100 W: about 23 crates.
(b) For 100 W: about 23 crates.
17. SI unit of power?
Watt (W). 1 W = 1 joule per second.
18. Area under power vs time graph?
It is the energy used. Average power = area ÷ time interval.
19. Relation of power to force and velocity?
P = F · v (dot product). If parallel, P = Fv.
20. Ski tow (15° slope, 300 m, 12 km/h, 50 riders of 70 kg). Power?
Required power ≈ 29.6 kW.
