According to the law of conservation of energy, in an isolated system: MCQ with Answer and Explanation

According to the law of conservation of energy, in an isolated system:
A. Thermal energy is always zero
B. Potential energy remains constant
C. Kinetic energy remains constant
D. Total mechanical energy remains constant if only conservative forces act
Answer: Option D
Solution (By JKSSB Mock Tests)
Law of conservation of energy states total energy (all forms) is conserved in isolated systems. For mechanical energy (KE + PE) to be conserved, only conservative forces (like gravity, spring) must act; non-conservative forces (friction) convert mechanical energy to heat. Option C precisely states this condition. Options A and B are incorrect as KE and PE individually vary. Option D is false as thermal energy can exist. Memory tip: 'Mechanical energy conserved ⇔ no non-conservative work'. This nuanced understanding is critical for energy problems in competitive exams, where distractors often omit the conservative force condition.

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Practice More Physics Questions

Question #1
The escape velocity from Earth does not depend on
A. Mass of the object
B. Mass of Earth
C. Universal gravitational constant
D. Radius of Earth

Correct Answer: Option A


Explanation:
v_esc = √(2GM/R), independent of m. Heavier object needs more energy, but velocity same. All objects fall at same rate. Important concept.

This question belongs to: Science Physics
Question #2
A body travels first half distance at speed v₁ and second half at v₂. Average speed is:
A. 2v₁v₂/(v₁+v₂)
B. v₁v₂/(v₁+v₂)
C. √(v₁v₂)
D. (v₁+v₂)/2

Correct Answer: Option A


Explanation:
For equal distances: average speed = total distance/total time = 2d/(d/v₁ + d/v₂) = 2v₁v₂/(v₁+v₂). This is harmonic mean, not arithmetic. Memory tip: Equal distances ⇒ harmonic mean of speeds; equal times ⇒ arithmetic mean. Frequently tested kinematics concept distinguishing average speed calculation methods.

This question belongs to: Science Physics
Question #3
In which of the following scenarios is the work done physically zero?
A. A person pushing a heavy wall that does not move.
B. All of the above.
C. A person carrying a heavy load on his head and walking on a horizontal road.
D. A satellite moving in a circular orbit around the Earth.

Correct Answer: Option B


Explanation:
Work (W) = F × d × cos(θ). In scenario A, displacement (d) is zero. In scenario B, the applied force (upward) is perpendicular to the displacement (horizontal), so cos(90°) = 0. In scenario C, the centripetal force (gravity) is perpendicular to the satellite's instantaneous direction of motion, so work done is zero. All cases yield zero work.

This question belongs to: Science Physics