Work & Energy — Set 5
Physics · कार्य और ऊर्जा · Questions 41–50 of 70
A motor does 1200 J of work in 2 s. What is its power?
Correct Answer: D. 600 W
Power is P = W/t. Here P = 1200/2 = 600 W. A higher power means the same work is done in less time.
If friction force f acts over displacement s opposite to motion, the work done by friction is?
Correct Answer: A. -fs
Friction acts opposite to displacement, so the angle is 180°. Work is W = f s cos 180° = -fs. This negative work is usually converted into thermal energy.
Mechanical energy of a body is the sum of?
Correct Answer: A. Kinetic and potential energy
Mechanical energy includes energy of motion and energy of position. These are kinetic energy and potential energy respectively. Their sum helps analyze many motion problems.
Kinetic energy increases with speed according to?
Correct Answer: D. K
Kinetic energy is given by KE = (1/2)mv^2. Because of the square, small speed increases can raise KE a lot. This is why stopping distance grows quickly with speed.
A force of 50 N acts on a body causing a displacement of 4 m at 30° to the displacement. What is the work done? (cos 30° ≈ 0.866)?
Correct Answer: D. 173 J
Use W = F s cosθ with F = 50, s = 4, and θ = 30°. So W = 50 x 4 x 0.866 ≈ 173 J. Only the component along displacement contributes to work.
When a body falls freely downward (ignoring air resistance), its gravitational potential energy mainly converts into?
Correct Answer: D. Kinetic energy
During free fall, height decreases so gravitational potential energy decreases. That lost potential energy appears mainly as kinetic energy. If air resistance is present, some energy becomes heat and sound.
Work done in lifting a body slowly through height h against gravity is equal to?
Correct Answer: A. Increase in potential energy
Slow lifting means kinetic energy change is negligible. The work done against gravity becomes an increase in gravitational potential energy. This equals mgh when g is constant.
If a force acts parallel to the velocity of a body, instantaneous power can be written as?
Correct Answer: B. P = Fv
Power is rate of doing work, and for parallel force, work rate equals force times speed. So P = Fv when force and velocity are in the same direction. If they are at angle θ, then P = Fv cosθ.
A pump lifts water with force 1000 N at a speed of 2 m/s vertically. What power is required?
Correct Answer: D. 2000 W
Power for a constant vertical lift is P = Fv. Here P = 1000 x 2 = 2000 W. This represents energy transferred per second to the water.
1 watt is equal to?
Correct Answer: A. 1 J/s
Watt is the SI unit of power, which is work per unit time. So 1 watt equals 1 joule per second. Power measures the rate of energy transfer.