Work and Energy

Science

NCERT

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?

Solution :

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$$\\$ $= 56 J$

2   When do we say that work is done?

Solution :

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.

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

Solution :

When a force FN displaces a body through a distance S mts in the direction of the applied force, then the work done W on the body is given by the expression: Work done = Force × Displacement$\\$ $W = F × s$

4   Define $1 J $ of work.

Solution :

$1 J$ 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.

5   A pair of bullocks exerts a force of $140 N$ on a plough. The field being ploughed is $15m$ long. How much work is done in ploughing the length of the field?

Solution :

Work done by the bullocks is given by the expression:$\\$ Work done = Force × Displacement$\\$ W = F × s$\\$ Where,$\\$ Applied force, F = $140 N$$\\$ Displacement, s = $15 m$$\\$ W = $140 × 15 = 2100 J$$\\$ Hence,$ 2100 J$ of work is done in ploughing the length of the field.