Work & Energy — Set 4
Physics · कार्य और ऊर्जा · Questions 31–40 of 70
If a force acts on a body but the displacement is zero, the work done is?
Correct Answer: A. Zero
Work depends on displacement along the force direction. If displacement is zero, then W = F s cosθ becomes zero. A common example is pushing a wall that does not move.
What is the dimensional formula of power?
Correct Answer: A. [M L^2 T^-3]
Power is work done per unit time. Work has dimension [M L^2 T^-2], and dividing by T gives [M L^2 T^-3]. This matches the unit watt as joule per second.
A person of weight 600 N climbs a staircase of height 5 m. How much work is done against gravity?
Correct Answer: D. 3000 J
Work against gravity equals force times vertical height. Here W = 600 x 5 = 3000 J. This work increases the person’s gravitational potential energy.
Work done in stretching a spring from its natural length to extension x is?
Correct Answer: A. (1/2)kx^2
Spring force increases from 0 to kx, so average force is (0 + kx)/2. Work = average force x displacement gives W = (1/2)kx^2. This work is stored as elastic potential energy.
1 calorie of heat is approximately equal to?
Correct Answer: D. 4.2 J
Calorie is a traditional heat unit, and its SI equivalent is in joules. 1 calorie is approximately 4.2 joules. Food energy is often measured in kilocalories (kcal).
The law of conservation of energy states that energy?
Correct Answer: A. Can neither be created nor destroyed, only transformed
The correct answer is 'Can neither be created nor destroyed, only transformed'. Energy is conserved in an isolated system. It can change form, but the total energy remains constant. This principle applies to mechanical, thermal, electrical, and other forms.
A force of 10 N acts opposite to displacement of 3 m. What is the work done?
Correct Answer: D. -30 J
Use W = F s cosθ with θ = 180° because directions are opposite. cos 180° = -1, so W = 10 x 3 x (-1) = -30 J. Negative work removes energy from the motion.
A force is conservative if the work done by it over a closed path is?
Correct Answer: B. Zero
For a conservative force, work depends only on endpoints. In a closed path, the initial and final positions are the same, so work is zero. This property is used to define potential energy functions.
Find the gravitational potential energy of a 2 kg mass at height 5 m. (Take g = 9.8 m/s^2)?
Correct Answer: B. 98 J
Use PE = mgh with m = 2, g = 9.8, and h = 5. So PE = 2 x 9.8 x 5 = 98 J. Gravitational potential energy increases linearly with height.
If a spring’s extension is doubled, its elastic potential energy becomes?
Correct Answer: C. Four times
Elastic potential energy in a spring is proportional to x^2. Doubling x makes x^2 four times. Therefore the stored energy becomes four times.