**Category : **6th Class

Work force and energy are interrelated with each other. When a body moves from one place to another on applying a force it is said that the work is done. Work is said to be not done if body does not move or does not change its position on application of force on it. The unit for the measurement of work done is Joule and represented by first letter of its name, "J". The unit for the measurement of force is Newton and represented by the first letter of its name "N". If displacement and work done is known then the applied force can be calculated.

Energy is the capacity to do the work. Heavy work require more energy and all living things obtain energy from the food they eat. In this chapter, we will study about these three important physical quantities work, force and energy.

**Work**

Work is said to be done if a force is applied on a body and body changes its states or physical shape. If the applied force and displacement are in parallel and in the same direction then the work done is said to be positive. The work done is said to be negative if the displacement is in opposite direction.

Let the body A is at rest at a point M, and a force of IN is applied on the body.

The following three conclusions can be obtained by applying the force on the body A:

(i) If body A changes its position from M to N then the work is done.

(ii) If body A changes its shape is also work is done.

(iii) If body A, neither changes is position nor its shape then no work is done.

No displacement and no change in shape

When an amount of force is applied on a body, the rate of displacement of the body also depends on the mass of the body. Therefore, heavier body requires more force to displace. The work is said to be done even if the body moves from its position or it changes its shape. The mass of the body is not considered for calculating the work done.

When a force of 1 N is applied on a body and body changes its position by 1 metre, the work is said to be done and work done is calculated by the following formula;

Work done = Force x Displacement = 1 N x 1 m = 1 Nm = 1 Joule.

Therefore, the applied force \[=\frac{Work\,done}{Displacement}\]

And displacement \[=\frac{Work\,done}{Applied\,force}\]

Let us assume that the body L is in rest at point P. A force of 250 N is applied on the body to push it. Body L does not move and remains in its original state.

Hence the displacement\[(d)=0\].

Therefore, the work done = Force x displacement. Now, substituting the value of force and displacement, thus, work done = 250 N x 0 = 0. Here, work done = 0, it means no work is done.

Let the force of 300 N is acting on a body. The displacement due to applied force is 3 m. Therefore, the work done = Force x displacement = 300 N x 3 = 900 J.

**Force due to Earth's Gravity**

According to the Newton's law of universal gravitation, every object in the universe is attracted by the other and the exerted force between two bodies is directly proportional to the product of their masses and inversely proportional to the square of distance between them. Therefore, the exerted force between two bodies, and \[{{m}_{2}}=\frac{{{m}_{1}}\times {{m}_{2}}}{{{r}^{2}}}\]

Where, G = Gravitational constant, ml and m are the masses of two objects, and r is the distance between them.

When a body falls free from a certain height, the speed of the body falling on the Earth surface depends on the mass of the body and acceleration due to the gravity of the Earth. This is called standard acceleration fall. Gravity is denoted by the small letter g. The precise value of the earth's gravity is and is considered only, 9.8.

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