Question

Identify the energy changes in the following cases.
A: A car is moving up a hill.
B: Photographic film is exposed to sun-light.
A) In ‘A’ mechanical energy in a moving car is converted to kinetic energy and in ‘B’ potential energy is converted to chemical energy.
B) In ‘A’ potential energy in a moving car is converted to kinetic energy and in ‘B’ chemical energy is converted to light energy.
C) In ‘A’ kinetic energy in a moving car is converted to potential energy and in ‘B’ potential energy is converted to light energy.
D) In ‘A’ kinetic energy in a moving car is converted to potential energy and in ‘B’ light energy is converted to chemical energy.

Answer 1

Hint: We are given two extremely different situations to analyse and understand the energy conversions happening in them. We need to understand the difference in the energies at different instants of an object under the given circumstances.

Completion answer:
Let us consider the two different situations given to us individually.
A: A car moving up the hill – We know that a body to move upward initially requires an energy to move which is converted and stored as the potential energy of the car. This initial energy by virtue of its upward motion is the kinetic energy. The kinetic energy of the car at the valley is completely converted to the potential energy at the top of the hill as –
\[\begin{align}
  & K{{E}_{down}}=P{{E}_{hill}} \\
 & \text{i}\text{.e}\text{.,} \\
 & \dfrac{1}{2}m{{v}^{2}}=mgh \\
\end{align}\]
The car moving uphill converts from potential energy to the kinetic energy.
B: Photographic film is exposed to sun-light – We know that a photographic plate is sensitive to light. The chemical composition of the plate changes when the plate is exposed to light. The photographic film when exposed to sunlight undergoes a chemical reaction, i.e., the light energy from the sun is converted to chemical energy for the reaction to occur.
The Silver bromide on the photographic film decomposes to silver grains on exposing the film to the sunlight.
We understand that in ‘A’ the kinetic energy is converted to potential energy and in ‘B’ the light energy is converted to chemical energy.

The correct answer is option D.

Note:
The car moving uphill on reaching the top of the hill has the maximum potential energy given that the car comes to a halt after the climb. The silver halides on the photographic plates decompose to metallic silver making it opaque as in a photograph.