# Work and Energy

(1) In our daily life any mental and physical activity is considered as work. But in science work is the displacement of any object by applied force.

Eg:- If we lift an object high. The book rises up (Displacement) with force applied on it by us.

Table below shows the example of work done and not done in physics

 SN Example Displacement of the object Force applied on the object Result 1. Reading book sitting one place No No No work done 2. Lift book from place A to place B Yes Yes (On book) Yes work has done 3. Pushing a rock No Yes (On rock) No work done 4. Pushing a Table from place A to place B Yes Yes (on table) Yes, work has done 5. Standing at one position with lots of weight in your hand for some time No Yes (On weight) No work done 6. Walked with lots of weight from place A to place B Yes Yes (On human body to move) Yes work has done

(2) Work done by a constant force

(i) When displacement in the direction of the force applied

A brick is resting on the table. Apply constant force and brick displaced in the direction of force applied. The work done on an object is called positive if displacement of the object is in the direction of the applied force.   (ii) When displacement in the opposite direction of the applied force

When we lift an object, the displacement of the object is in upward but force applied on it downward by the Earth. Therefore, the work done on the object is negative. The work done on an object is called negative if displacement of the object is in the opposite direction of the applied force.   (iii) When displacement or force applied on the object is zero the work done on the object is zero.

(3) The capacity of an object to do work is known as energy.

The unit of energy is same as work because it measures in work. Therefore the SI unit of the energy is joule.

1 j is the energy required to do 1 j of work. Sometime energy can be measured in kilojoule (kj).

1 kj = 1000 j

(i) Forms of energy

(a) Kinetic energy is the energy possess by an object due to motion.

Or

The kinetic energy of an object moving with a certain velocity is equal to the work done on it to make it acquire that velocity.    (b)Potential energy is the energy possessed by a body due to its change in position or shape and not used to cause change in the velocity or speed of the object.

Eg:- When we stretched rubber , energy transferred to rubber band is potential energy.

The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.  The work done by gravity depends on the difference in vertical heights of the initial and final positions of the object and not on the path along which the object is moved.

In figures below potential energy of the object is equal, ie, mgh. (4) Law of conservation of energy states that energy can only be converted from one form to another; it can neither be created nor destroyed.

Therefore,

Total energy before transformation = Total energy after transformation

Eg:- If an object falls from a tower its total energy before and after transformation remains same.

Let mass of the object = m

Let height of the tower = h

Let acceleration = g

Let initial velocity of the object = u

Final velocity of the object = v

Total energy at the starting of the object  Total energy after the object starts falling  Total energy when the object about to touch the ground    Helping Topics

NCERT solutions, class 9

Practice sheet, class 9 