ICSE Class 8 Physics Chapter 04 Energy

Energy

Theme: Building on previous learning on energy, the emphasis in this class is on the introduction of gravitational potential energy to children. Look at a swinging bob of a pendulum. When it is at its extreme position (the highest point of its motion), it has gravitational potential energy. When it reaches its mean position (lowest point), it has maximum speed and it has high kinetic energy. In this case, one form of energy changes into other, according to the law of conservation of energy. Energy is the ability to do work. Work is said to be done when a force acting on an object changes the position of the object. For the special case when the object changes its position along the direction of the force, work is given by the product of the force and distance moved by the object. But different persons may take different time to do the same work. Rate of doing work is called power. So energy and power are different physical quantities, having different units. In many situations, the focus is on the power and not energy. For e.g. the power of a motor which works is paid for the electricity consumed, is actually paid for the energy consumed.

In This Chapter You Will Learn To

Define work

Express work in proper unit

Calculate work done in simple cases

Define kinetic energy

Express kinetic energy in proper units

Solve simple problems based on kinetic energy

Define potential energy

Define gravitational potential energy

Solve simple problems based on gravitational potential energy

Describe energy transformation in daily life situation

Distinguish between energy and power

Plan an experimental investigation or demonstration using scientific processes

Identify/select on the basis of attributes

Learning Objectives

Revising previous concepts learnt by children.

Building on child's previous learning.

Explaining concept of work done with examples from daily life.

Calculating work done in simple cases and expressing result in proper unit.

Explaining of kinetic energy and potential energy.

Explaining of gravitational potential energy.

Solving of problems on kinetic and potential energy.

Demonstrating kinetic and potential energy using a simple pendulum.

Engaging children in problem solving tasks on KE and PE.

Explaining and discussing with children energy transformation in daily life situations/activities.

Explaining the difference between energy and power.

Citing examples of different applications of conservation of energy (Roller coaster, Production of hydroelectricity etc.) with children making energy conversion diagrams and deduce that energy is conserved.

Knowing Concepts

Concept of work.

Unit of work (joule).

Calculation of Work done in simple cases.

Kinetic energy.

Basic concept.

Potential energy.

Basic concept.

Gravitational potential energy.

Calculation of kinetic and potential energies from a set of given data (simple problems and assuming g = 10 m/s).

Energy transformation in common daily life situations.

Difference between energy and power.

Work

In everyday life, all of us do some work. We do work when we play, when we go to school, when we go upstairs, when we pedal a bicycle, when we lift a load and so on.

In common language, we use the word work much casually. For example, if we push a wall, we say that work is done by us. While reading a book, we say that we are working. But actually no work is done in these activities (Fig. 4.1).

Teacher's Note

In physics, work has a specific meaning that differs from everyday usage - work requires both force and displacement in the direction of that force, which is why pushing a stationary wall does no work.

In Physics, the word work has a special meaning. Work is said to be done only when a body changes its position or moves on applying a force on it. No work is said to be done, if there is no motion produced in the body even when a force acts on it. Thus,

Work is said to be done if the force applied on a body moves it. If no motion takes place, no work is said to be done.

For example, a cyclist pedalling a cycle does work, a horse pulling a cart does work, an engine pulling a train does work, a coolie lifting a box does work, a boy going upstairs does work, a boy lifting a book does work (Fig. 4.2).

When a crane picks up a car involved in an accident and takes it to a workshop, we say that work is done.

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

A boy climbing up stairs, does work (Fig. 4.3).

Work is also done by a force if the force applied on a body changes its size or shape. For example, if a boy squeezes a toothpaste tube or gum tube or a rubber ball, he does work in changing the shape of the tube or ball (Fig. 4.4).

Similarly, a boy does work in stretching a rubber string in which size of the string increases.

A person does no work if there is no change in position or no motion even after

application of force. For example, a boy pushing a car or a heavy stone does no work if the car or stone does not move (Fig. 4.5), although he may get tired.

A person pushing against a wall also does no work since he is not able to move the wall. Similarly, a coolie does no work while standing with a heavy box on his head, as there is no motion, although he may get tired holding it (Fig. 4.6). But the coolie does work when he raises the heavy box to his head.

Thus, following two conditions must be fulfilled for work to be done:

(1) A force must act on the body.

(2) The force must produce change in position i.e., motion or produce change in size or shape of the body.

Teacher's Note

Students often confuse getting tired with doing work - but fatigue is not the same as work in physics; work requires actual displacement or shape/size change.

Factors affecting the amount of work done

Experimentally it is found that the amount of work done by a force depends on the following two factors:

(1) The magnitude of the force applied, and

(2) The distance moved by the body in the direction of force.

(1) Dependence of the amount of work done on the magnitude of the force applied on the body: Work done is more if the force applied to move the body is more.

Example: More work is done by us if we lift a bucket full of water from the ground floor to the first floor than if we lift an empty bucket to the same height. The reason is that we have to apply a greater force to lift the bucket full of water than to lift the empty bucket.

(2) Dependence of the amount of work done on the distance moved by the body in the direction of force: Work done is more if the distance moved by the body in the direction of force is more.

Example: More work is done by us if we lift a bucket of water from the ground floor to the second floor than if we lift the same bucket from the ground floor to the first floor.

Definition of Work

We define the work done as follows:

The work done by a force on a body is equal to the product of the force applied and the distance moved by the body in the direction of force i.e.,

Work done = Force - distance moved in the direction of force.

In Fig. 4.7, let a force F be applied on a body which moves it from the position A to the position B by a distance d in the direction of force, then the work done by the force on the body is

\[W = F \times d\]

Obviously, if a force acts on a body and the body does not move, no work is done (i.e., W = 0 if d = 0).

The work done by a force is zero if the body moves in a direction perpendicular to the direction of force. For example, when a stone tied at the end of a string is whirled in a horizontal circular path, the motion of stone is always normal to the force of tension in the string as shown in Fig. 4.8. Therefore, the work done by the force of tension on the stone is zero.

Similarly, in motion of earth around the sun, the force of attraction on earth by the sun is always normal to the direction of motion of earth, so no work is done by the gravitational force of sun on the earth.

Similarly, if a coolie moves on a plane road with luggage on his head, the work done by him against the force of gravity (i.e., weight) is zero. Since, distance moved by him is normal to his weight.

Teacher's Note

The concept that work equals zero when force and displacement are perpendicular helps explain why carrying something horizontally requires no work against gravity - only vertical motion does work against gravity.

Units of Work

(1) We have read that the S.I. unit of force is newton (N) and that of distance is metre (m). Hence, S.I. unit of work is newton - metre (N m) or joule (symbol J). The unit joule has been named after the name of the scientist James Prescott Joule.

Definition of joule: Since,

\[1 \text{ joule} = 1 \text{ newton} \times 1 \text{ metre}\]

or

\[1 \text{ J} = 1 \text{ N} \times 1 \text{ m}\]

i.e., one newton - metre is called one joule.

Thus, one joule of work is said to be done if one newton force when acting on a body moves it by 1 metre in the direction of force.

A bigger unit of work is kilo joule (symbol kJ) and Mega joule (symbol MJ)

\[1 \text{ kJ} = 1000 \text{ J and } 1 \text{ MJ} = 10^6 \text{ J}\]

(2) If we measure the force in kgf and the distance in metre, the unit of work is kgf - m.

Since, 1 kgf = 9.8 newton (precisely),

\[1 \text{ kgf} \times \text{m} = 9.8 \text{ newton} \times \text{metre} = 9.8 \text{ joule}\]

(Assuming that the force of gravity on a mass of 1 kg is 9.8 N).

In chapter 3, you have learnt that the unit of moment of force is newton - metre (symbol N m). But it is not written as joule (J). Only for work and energy, the product, newton - metre is written as joule. This is to distinguish moment of force from the work or energy.

Energy

When work is done on a body, its energy increases. In other words, the work done on the body is stored in it in the form of energy. But if work is done by the body, its energy decreases. In other words, energy is spent when a body does work.

For example, a boy while playing football runs all over the field and he spends energy in doing work [Fig. 4.9]. He will continue to play football till he possesses energy.

Teacher's Note

Energy is what enables us to do work - when we run and play, we use up energy stored in our bodies, which is why we get tired.

Thus, we can define energy as follows:

Energy is the capacity of doing work.

Relationship between work and energy

It is experienced that more you run on a bicycle or more you run all around the playground, more you feel tired. The reason is that a lot of energy is spent in doing work by you. Thus, to do more amount of work, we need to spend more energy. Hence, we can say that there is a direct relationship between work and energy.

Similarly, the work done on a body in changing its state is said to be the energy possessed by the body.

For example, if a body is moved from the ground to a height, work is done on the body against the force of gravity and the body at the height is said to possess energy. Similarly, if a body initially at rest is made to move, work is done on the body and the body

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