Question

When a car is moving on a level road and breaks are applied till it comes to rest, what happens to its mechanical energy?
(a) It increases
(b) It decreases
(c) It becomes zero
(d) It becomes negative

April 24, 2024

Answer

(b) It decreases

Reason — When the brakes are applied, the car experiences friction between the brake pads and the wheels. This friction converts the kinetic energy of the car into other forms of energy, such as heat and sound. As the car eventually comes to rest, its velocity becomes zero, and thus its kinetic energy becomes zero. Therefore, the kinetic energy component of the car’s mechanical energy decreases to zero. As the height above the ground does not change significantly so, the potential energy component remains relatively constant. Hence, mechanical energy (KE + PE) decreases.

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