A 1.5 kg object is dropped from rest from a height of 8.0 m above the ground. What is its kinetic energy just before it hits the ground? (Ignore air resistance and take g = 10 m/s².)
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Correct answer
C. 120 J
Principle or equation
Conservation of mechanical energy: the loss in gravitational potential energy equals the gain in kinetic energy when only gravity does work. KE = mgh.
Why this answer is correct
The object starts from rest, so initial kinetic energy is zero. The gravitational potential energy lost is mgh = 1.5 × 10 × 8.0 = 120 J. This becomes kinetic energy just before impact.
Example
A 2 kg object dropped from 10 m has KE = 2 × 10 × 10 = 200 J just before hitting the ground.
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