ICSE Class 6 Physics Chapter 5 Work and Energy

Chapter 5: Work and Energy

Syllabus

1. (a) Work is done when an applied force moves an object in the direction of the force. Examples from daily life.

(b) Work = Force × Distance moved (in the direction of the force).

2. Work-energy relationship; units of work.

3. Energy is the ability to do work.

(a) Different forms of energy - kinetic, potential, heat, light, magnetic, electrical, muscular, sound, chemical energy.

(b) Interchangeability of energy from one form to another - identification of such transformations.

(c) Energy chains - all start with the Sun.

Work

We do different types of work in our everyday life. We play, we carry a load, say, our school bag, we pedal a cycle, we go for jogging and so on. These are few examples of physical work. We may read, think, solve problems, etc. These are few examples of mental work.

In physics, we recognise only physical or mechanical work and that too if there is some displacement, i.e., the body has moved from one place to another place. In science, we do not recognise mental work as work done, although it also consumes energy.

Work Done

A force tends to move an object upon which it acts. If the object moves in the direction of the force, work is said to be done. Work is also said to be done if the force applied on a body changes its position, decreases or increases its speed, changes its direction of motion, etc.

Example 1: When a crane picks up an accidental car and takes it to a workshop, we say that work is done.

Example 2: In a school playground, while playing football, when a boy hits the ball and runs towards the goal, we say that work is done.

If on applying force on an object, the motion of the object changes or the direction of motion changes, the work is said to be done by the force.

Sometimes, work is said to be done even when there is no displacement, rather there is a change in the shape or size of the body.

Example 3: Squeezing a rubber ball, squeezing toothpaste from the tube.

Example 4: A small child sitting at a place is making different shapes of objects with the help of plasticine. He is doing work, because he is changing the shape of the plasticine.

No Work Done

If you apply a force on a heavy object and the object does not move, no work is done though the force is applied on it.

Is work done when leaves fly in the wind?

Is work done when you close your eyelids?

Example 5: If two teams of a tug-of-war apply the pulling force with equal amount, no work is done because there is no change in the position of both the teams.

Reading is a kind of mental work; no work is said to be done.

No work is done in case of pushing a wall.

No work is said to be done if there is no displacement in the position of the car to which the force is applied.

No work is done if the force applied on both sides is equal.

Example 6: If you push a wall, you waste a lot of energy. But no work is said to be done since there is no displacement.

Example 7: There may be a case when a man applies a force to push his car forward but the car does not move inspite of a great deal of strong pushing. Although the man has applied much force, no work has been done because the force neither succeeded in moving the car nor did it change its shape.

Example 8: When a porter is standing still on a railway platform with a heavy luggage on his head, he is not doing any work. Since there is no displacement, hence no work is said to be done.

From the above examples we come to the conclusion that work is said to be done only if the following two conditions are satisfied:

(a) A force must be applied on the object.

(b) The object should move from its resting position or there should be a change in the shape or size of the object.

Definition of Work

When a force displaces a body in the direction of applied force then the product of the force and displacement is called the work done by the force on the body.

Example 9: Suppose a person A pushes an object through a distance of 10 metres, while another person B pushes the same object through a distance of 20 metres by the same force. We say that work done by person B is more than the work done by person A. This means that for a given force the work depends on the distance moved by the object in the direction of the applied force.

Suppose you push a trolley (load) weighing 50 kg to a distance of 10 metres, while your brother pushes another trolley (load) weighing 100 kg to the same distance. Then the work done by your brother is more than that of yours, as your brother had to apply more force. This means that work depends on the magnitude of the force applied.

Hence, we conclude that the work done depends on the following two factors:

(a) force applied and

(b) the displacement (shortest possible distance in the direction of force applied).

We can now define work as the product of force and displacement.

Work = Force × Displacement

= F × S

Unit of Work

The unit of work is, therefore, the product of a unit of force and a unit of distance. Since the unit of force is newton (N) and the unit of distance is metre (m), therefore the unit of work is newton-metre (Nm). A newton-metre is also called a joule (J) named after J.P. Joule, a scientist who did a number of experiments in the field of energy. Sometimes, a larger unit called kilojoule (kJ) is also used.

Unit of work = 1 Newton × 1 metre

= 1 Nm

= 1 Joule

and

1kJ = 1000 J

Do You Know?

James Prescott Joule (1818-1889) was a British Physicist and famous for extensive work on heat and conversion of heat into work. The S.I. unit of work and energy is named after him. He verified the law of conservation of energy. He was also instrumental to work with Lord Kelvin to develop the Kelvin scale of temperature.

Teacher's Note

When you carry your school bag up a flight of stairs or push a shopping cart down the aisle, you are doing work by applying force over a distance. Understanding this concept helps explain why certain tasks make you tired.

Examples and Solutions

Example 11: If a boy lifts a textbook weighing 10 N from the floor to a table of height 2 m. How much work does he do?

Solution:

Work done = Force × Distance moved in the direction of the force

= 10 N × 2 m = 20 J

Example 12: A man needs a force of 2000 N to pull his car. He pulls it to a distance of 30 m. How much work is done by him?

Solution:

Work done = Force × Distance moved in the direction of the force.

= 2000 N × 30 m

= 60000 J or 60 kJ

Example 13: A man drags a trunk through a distance of 5 m and for that he does 30000 J of work. What is the force required?

Solution:

Work = Force × Distance moved in the direction of force

30000 J = F × 5 m

F = 30000 J / 5 m = 6000 N

Example 14: A force of 1500 N is required to do a work of 20000 J to drag an object. Through what distance will it move?

Solution:

Work = Force × Distance moved in the direction of force

Work / Force = Distance

Distance moved in the direction of force

= 20000 J / 1500 N = 13.33 m

Energy

If a boy while playing football runs about all over the field or if he cycles to some distance, he gets tired. He has used something to do these things. If he had stayed at home and sat down at a place, he would not have felt so tired. What has he used to do these things? He has used his energy.

Everything in this universe is either matter or energy.

Matter is the physical state of a substance which exists in nature in different forms and occupies some space.

Energy is the ability or capacity to do work.

To do more amount of work, we need more amount of energy.

After doing sufficient amount of work, one feels tired because lot of energy has been spent during this process. Hence we say, there is a direct relationship between work and energy. The energy is the cause and work is its effect. The unit of energy is joule (J) which is the same as that of work.

Different Forms of Energy

Energy exists in several forms. Some forms are explained below.

Mechanical Energy: Whenever a body is in motion or it is taken to a certain height with respect to the earth's surface, it possesses with mechanical energy.

Mechanical Energy: The energy possessed by a body either due to its state of rest or state of motion is called mechanical energy.

Mechanical energy is of two forms, namely, kinetic energy and potential energy.

Kinetic Energy: The energy that the moving bodies possess is called motional energy or kinetic energy. (The word kinetic comes from the Greek word kinesis which means motion). Swinging pendulum, running water, and moving air are examples of bodies that possess kinetic energy.

The amount of kinetic energy depends on the mass and velocity of the moving object.

Example: An apple falling from a tree, blowing wind, a bird flying, etc are few examples of kinetic energy.

The energy possessed by a body due to its motion is called kinetic energy.

KE = \(\frac{1}{2} mv^2\)

where m is the mass of the body, v is its velocity and KE is the kinetic energy possessed by the body.

You might have observed that a bullet is capable of penetrating into human body, a storm can uproot big trees, etc. These are all because of the high speed which increases the kinetic energy of the bullet or air by a very large amount.

Teacher's Note

A moving bicycle has kinetic energy, which is why it can knock you down if you're not careful. The faster it moves, the more energy it has to do damage.

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