Question

2
which of the following is an example of a situation where mechanical energy is not conserved?
a a cart on a frictionless roller coaster track
b a ball falling in a vacuum
c a skater traveling across smooth ice
d a box being pushed across a rough surface

Explanation:

Mechanical energy is conserved when only conservative forces (like gravity, elastic force) act, and non - conservative forces (like friction, air resistance) are absent.

• Option A: A cart on a frictionless roller coaster track has only gravity and normal force (conservative or no work - doing force) acting. So mechanical energy is conserved.
• Option B: A ball falling in a vacuum has only gravity (conservative force) acting. Mechanical energy is conserved.
• Option C: A skater traveling across smooth ice has negligible friction (non - conservative force). Mechanical energy is approximately conserved.
• Option D: A box being pushed across a rough surface has friction (a non - conservative force) doing work. Friction converts mechanical energy into thermal energy, so mechanical energy is not conserved.

Answer:

D. a box being pushed across a rough surface