The energy converted to heat during braking primarily comes from which energy form?

• A. Kinetic energy
• B. Potential energy
• C. Nuclear energy
• D. Thermal energy

Answer: (A)

When you brake, the car’s motion is stored as kinetic energy. The braking system uses friction to oppose that motion, doing negative work on the car and converting that kinetic energy into heat. So the heat you feel in the brakes comes from the kinetic energy the car had while moving (1/2 m v^2) being transformed into thermal energy in the brake components and surrounding air. Potential energy isn’t the primary source of the heat during braking, unless the car started on a slope and then fully converted that potential energy to kinetic first; the braking heat itself is from the kinetic energy at the moment of braking. Nuclear energy isn’t involved.