Work and Energy – Science – (NCERT Solutions for Class 9)

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  1.  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?

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    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 = 7 N
    S = 8 m
    Therefore, work done, W = 7 × 8
    = 56 Nm
    = 56 J

  1.  When do we say that work is done?

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    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.
  2. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

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    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
  3. Define 1 J of work.

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    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.
  4. 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?

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    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

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