Question

When brakes are applied to a moving vehicle, then the kinetic energy of the vehicle is converted to

• A. Potential energy
• B. Mechanical energy
• C. Gravitational energy
• D. Heat energy

Hint:

Braking is an energy transformation process: \[ \text{Kinetic Energy } \left(\frac{1}{2}mv^2\right) \xrightarrow{\text{Frictional Work}} \text{Thermal/Heat Energy } (m_{\text{rotor}} \cdot c_p \cdot \Delta T) \] In modern electric vehicles, regenerative braking systems capture a portion of this kinetic energy, converting it into electrical potential energy stored in batteries rather than losing it entirely as heat.

Answer 1

The Correct Option is D

Concept: The operation of a vehicle braking system is governed by the First Law of Thermodynamics, which states that energy can neither be created nor destroyed, but can only change from one form to another. A moving vehicle of mass \(m\) traveling at a velocity \(v\) possesses directional translation kinetic energy (\(E_k\)), expressed as: \[ E_k = \frac{1}{2}m v^2 \] When the operator applies the brakes, mechanical force presses the high-friction brake pads against the rotating surfaces of the brake discs or drums connected to the wheels. Let us evaluate the energy transformation steps:

Step 1: Evaluating the mechanical interaction.
The contact between the non-rotating brake pads and the rotating brake rotors creates a strong frictional resistance force (\(F_{\text{friction}} = \mu \cdot N\)). This force opposes the rotation of the wheels, generating a braking torque that decelerates the vehicle.

Step 2: Computing mechanical work done by friction.
The work done by the friction force over a braking distance \(s\) is given by: \[ W_f = \int_0^s F_{\text{friction}} \, ds \] According to the work-energy theorem, this frictional work must equal the total change in the vehicle's kinetic energy to bring it to a stop: \[ W_f = \Delta E_k = \frac{1}{2}m v^2 - 0 = \frac{1}{2}m v^2 \]

Step 3: Determining the final form of energy.
At the microscopic level, the mechanical work done against friction excites the atoms on the contact surfaces of the brake pads and rotors, increasing their thermal kinetic energy. This microscopic structural excitation manifests macroscopically as a rapid increase in temperature across the brake assembly. Thus, the vehicle's kinetic energy is converted into thermal energy, or heat energy, which is then dissipated into the surrounding atmosphere via convection and radiation. This matches Option (4).