NCERT Solutions for Class 9 - Work and Energy

Question:  A force of 7 N acts on an object. The displacement is. say 8 m. in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Answer:  When a force F acts on an object to displace it through a distance S in its direction. then the work done W on the body by the force is given by: 
Work done = Force × Displacement
W=F×S
Where.
F=7N
S=8m
Therefore. work done. W = 7 × 8
= 56 Nm
= 56 J

Question:  When do we say that work is done?
Answer:  Work is done whenever the given conditions are satisfied:  (i) A force acts on the body. (ii) There is a displacement of the body caused by the applied force along the direction of the applied force.

Question: 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 F displaces a body through a distance S in the direction of the applied force. then the work done W on the body is given by the expression: 
Work done = Force × Displacement
W=F×s
   

Question: Define 1 J of work.
Answer: 1 J is the amount of work done by a force of 1 N on an object that displaces it through a distance of 1 m in the direction of the applied force.

Question: A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m 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
W=F×d
Where.
Applied force. F = 140 N
Displacement. d = 15 m