Work, Energy and Power

This chapter has

Work, Energy and Power — solved practice questions

8 JEE Physics questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   A constant force of 10 N acts on a body and displaces it by 5 m in the direction of the force. The work done by the force is:

   • A25 J
   • B50 J
   • C10 J
   • D2 J
   Show answer & solution

   Correct answer: (B) 50 J

   Work = force x displacement (in the direction of force) = 10 N x 5 m = 50 J.

2. Q2easy

   A body of mass 2 kg is moving with a speed of 3 m/s. Its kinetic energy is:

   • A3 J
   • B6 J
   • C9 J
   • D18 J
   Show answer & solution

   Correct answer: (C) 9 J

   KE = (1/2)mv^2 = (1/2)(2)(3^2) = (1/2)(2)(9) = 9 J.

3. Q3easy

   Which of the following forces does zero work on a particle moving in a horizontal circle at constant speed?

   • AAn applied tangential force
   • BKinetic friction opposing motion
   • CAir drag
   • DThe centripetal (net inward) force
   Show answer & solution

   Correct answer: (D) The centripetal (net inward) force

   The centripetal force is always perpendicular to the velocity (and displacement), so W = F·s cos(90 degrees) = 0.

4. Q4medium

   A variable force F = (3x^2 − 2x + 7) N acts on a particle moving along the x-axis. The work done in moving the particle from x = 0 to x = 5 m is:

   • A135 J
   • B100 J
   • C70 J
   • D185 J
   Show answer & solution

   Correct answer: (A) 135 J

   W = integral of F dx = [x^3 − x^2 + 7x] from 0 to 5 = (125 − 25 + 35) = 135 J.

5. Q5medium

   A spring of force constant 100 N/m is stretched from an extension of 0.1 m to 0.2 m. The work done in this process is:

   • A0.5 J
   • B1.0 J
   • C1.5 J
   • D2.0 J
   Show answer & solution

   Correct answer: (C) 1.5 J

   W = (1/2)k(x2^2 − x1^2) = (1/2)(100)(0.04 − 0.01) = 50 x 0.03 = 1.5 J.

6. Q6medium

   A pump lifts 2000 kg of water per minute to a height of 10 m. Taking g = 10 m/s^2, the minimum power of the pump is:

   • A2000 W
   • B3333 W
   • C20000 W
   • D5000 W
   Show answer & solution

   Correct answer: (B) 3333 W

   P = mgh/t = (2000 x 10 x 10)/60 = 200000/60 ≈ 3333 W (about 3.33 kW).

7. Q7medium

   A block of mass 2 kg moving at 4 m/s on a frictionless surface compresses a spring of force constant 800 N/m. The maximum compression of the spring is:

   • A0.05 m
   • B0.10 m
   • C0.15 m
   • D0.20 m
   Show answer & solution

   Correct answer: (D) 0.20 m

   By energy conservation (1/2)mv^2 = (1/2)kx^2, so x = v√(m/k) = 4√(2/800) = 4 x 0.05 = 0.2 m.

8. Q8medium

   If the momentum of a body is increased by a factor of 2, its kinetic energy (for constant mass) becomes:

   • AUnchanged
   • B2 times
   • C4 times
   • D8 times
   Show answer & solution

   Correct answer: (C) 4 times

   KE = p^2/2m, so KE is proportional to p^2. Doubling p multiplies KE by 4.

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