Work & Energy — Set 3
Physics · कार्य और ऊर्जा · Questions 21–30 of 70
A 20 N force acts on a body and produces a displacement of 2 m at an angle of 60° to the force. What is the work done?
Correct Answer: C. 20 J
Use W = F s cosθ with F = 20, s = 2, and θ = 60°. So W = 20 x 2 x 0.5 = 20 J. Only the parallel component of force does work.
Kinetic energy is a?
Correct Answer: C. Scalar quantity
Kinetic energy has magnitude but no direction, so it is scalar. It depends on mass and the square of speed. The SI unit of kinetic energy is joule.
In uniform circular motion with constant speed, the kinetic energy of the body is?
Correct Answer: A. Constant
Kinetic energy depends on speed, not direction of motion. In uniform circular motion, speed remains constant though direction changes. Therefore kinetic energy remains constant.
If a body has momentum p and mass m, its kinetic energy is?
Correct Answer: B. p^2/(2m)
Using p = mv, substitute v = p/m into KE = (1/2)mv^2. This gives KE = (1/2)m(p^2/m^2) = p^2/(2m). It shows kinetic energy increases with the square of momentum.
A 0.5 kg ball moves with speed 4 m/s. What is its kinetic energy?
Correct Answer: B. 4 J
Use KE = (1/2)mv^2 with m = 0.5 and v = 4. So KE = 0.5 x 0.5 x 16 = 4 J. Squaring the speed makes kinetic energy change quickly with v.
Work done by gravity when a body moves between two heights depends on?
Correct Answer: A. Only the initial and final heights
Gravity is a conservative force, so its work depends only on position change. Therefore it depends only on initial and final heights. The work by gravity equals the negative change in gravitational potential energy.
Energy stored in a stretched or compressed spring is called?
Correct Answer: C. Elastic potential energy
A deformed spring stores energy due to its change in shape. This stored energy is called elastic potential energy. It can convert into kinetic energy when the spring is released.
If only conservative forces act on a system, its mechanical energy?
Correct Answer: A. Remains constant
Conservative forces do not dissipate mechanical energy. So the sum of kinetic and potential energy stays constant. This is called conservation of mechanical energy.
A machine takes 500 J input energy and gives 400 J useful output. What is its efficiency?
Correct Answer: A. 80%
Efficiency = (useful output/input) x 100%. So efficiency = (400/500) x 100 = 80%. Efficiency is always less than or equal to 100% in real machines.
Power is defined as?
Correct Answer: D. Rate of doing work
Power tells how quickly work is done. It is defined as work done per unit time. The SI unit of power is watt.