What happens to some kinetic energy when friction acts on a moving object?

• A. Some kinetic energy is converted into thermal energy
• B. Kinetic energy increases
• C. Energy is created
• D. The object stops instantly

Answer: (A)

When friction acts on a moving object, some of its kinetic energy is transformed into thermal energy. The frictional force does negative work on the object as surfaces rub against each other, taking energy from the motion and turning it into heat in the surfaces (and sometimes sound or slight deformation). That’s why the object slows down—the kinetic energy it had is being converted to heat rather than increasing. The total energy in the system stays the same, just changing form. It doesn’t create energy or cause the speed to rise, and stopping instantly isn’t typical unless the frictional force is extremely large for a very brief moment; in general, energy is dissipated gradually as heat.