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Question 15 Marks
Describe the energy transformation taking place in an oscillating pendulum.
Answer
When a pendulum swings to$-$and$-$fro, its energy is constantly from potential to kinetic and vice$-$versa. Let us consider the energy transformation in the pendulum at its different positions. At position $B,$ when the bob is raised from position $A$ to position $B$, the work is done on it against the force of gravity. This work done gets stored in it in the form of gravitational potential energy. At this position, the pendulum has only potential energy but no kinetic energy.
At position $A,$ when the bob starts moving down from position $B$ to position $A$, its potential energy goes on decreasing but its kinetic energy goes on increasing. When the bob reaches the mean position $A$, it has only kinetic energy but no potential energy.
At position $C$, as the bob goes from position $A$ towards $C$, its kinetic energy goes on decreasing but its potential energy goes on increasing. On reaching the extreme position $C$, the bob has only potential energy but no kinetic energy.
 
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Question 25 Marks
Describe the work$-$energy relationship by giving a suitable example.
Answer
Energy is the ability of a body to do work. So, there is a direct relationship between energy and work. For example, when an object is at a height from the ground, it has a certain amount of potential energy. When you drop the object, it falls down because of the force of gravity. So, work is being done on the body
While falling down, its potential energy is gradually used up until it becomes zero $($just when the object touches the ground$)$. At any point during its fall, the amount of potential energy which is used up by the body is equal to the distance, the body has travelled $($work done$)$. So, work done by a body $=$ energy change in the body.
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[5 Mark Question Answer] - PHYSICS STD 8 Questions - Vidyadip