NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 1
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Question 1:
Q. A force of acts on an object. The displacement is, say, in the direction of the force (Figure). Let us take it that the force acts on the object through the displacement. what is the work done in this case?
Answer:
When force acts on an object to displace it through a distance in its direction, then the work done on the body by the force is given by:
Work done Force Displacement
Where,
N
m
Therefore, work done,
Question 1:
Q. When do we say that work is done?
Answer:
Work is done whenever the given conditions are satisfied:

A force acts on the body.

There is a displacement of the body caused by the applied force along the direction of the applied force.
Question 2:
Q. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Answer:
When a force diaplaces a body through a distance in the direction of the applied force, then the work doen on the body is given by the expression:
Work doneForceDisplacement
Question 3:
Q. Define 1 j of work
Answer:
1 J is the amount of work done by a force of on an object that displaces it through a distance of 1 in the direction of the applied force.
Question 4:
Q. A pair of bullocks exerts a force of on a plough. The field being ploughed is long. How much work is done in ploughing the length of the field?
Answer:
Work done by the bullocks is given by the expression:
work done force displacement
Applied force,
Displacement,
Hence, 2100 of work is done in ploughing the length of the field.