You Must Know
1. Work is said to be done only when an object moves in the direction of the force applied on it.
2. Work done, by a force, depends upon the magnitude of applied force and the net displacement in the direction of this force.
3. Energy is the capacity to do work.
4. Energy exists in very many common forms. Electrical energy, mechanical energy, chemical energy, sound energy, light energy and heat energy are some of the well-known forms of energy.
5. When a body possesses either potential energy, or kinetic energy, or both, we say that it has mechanical energy.
6. We often observe, or use, devices that ‘work’ by using, one, or the other, of the different well-known forms of energy.
Something To Know
A. Fill in the blanks.
1. The work done depends upon the force applied and the displacement produced.
2. Energy is the capacity to do work.
3. Chemical energy, stored in a battery, can be used to produce light in a torch.
4. Sun is the main source of light energy.
5. An electric bell produces sound from electrical energy.
B. Write True or False for the following statements.
1. Work is done whenever a force is applied on a given object. False
2. A stationery ball does not possess kinetic energy. False
3. A ball, raised to a height, is said to possess chemical energy. False
4. The motion, of the blades of a fan, is due to mechanical energy. False
5. Sound is a form of energy. True
C. Tick (✔) the correct option.
1. In which of the following situations, no ‘work’ is being done?
shifting a box of biscuits from a lower shelf to a higher shelf in the kitchen.
throwing a ball.
pushing a baby stroller on a road.
just holding a book in your hand for two minutes. (✔)
Answer – just holding a book in your hand for two minutes. (✔)
2. The ability, or capacity, of a body to do work is known as-
force
energy (✔)
displacement
motion
Answer – energy(✔)
3. The form of energy, possessed by flowing water, is known as-
potential energy
kinetic energy (✔)
light energy
chemical energy
Answer – kinetic energy(✔)
4. The energy, associated with wind, is an example of-
chemical energy
kinetic energy (✔)
electrical energy
potential energy
Answer – kinetic energy(✔)
5. When a rubber band is stretched, it can be said to have gained-
chemical energy
potential energy (✔)
electrical energy
kinetic energy
Answer – potential energy(✔)
D. Answers the following questions in brief.
1. Write the meaning of the following terms.
(a) work
Answer – When a force is applied on an object and a displacement is produce, then work is said to be done.
(b) energy
Answer – The capacity to do work is called energy.
2. State the conditions under which work is said to have been done.
Answer – Two conditions are required for work to be done. The first condition is the applied force and the second condition is the displacement.
3. Name any three different forms of energy.
Answer – Three forms of energy are as follows;
(a) Mechanical energy
(b) Chemical energy
(c) Light energy
4. State the type of mechanical energy possessed by the following:
(a) a bullet fired from a gun.
Answer – Kinetic energy
(b) the wound up spring of a toy car.
Answer – Potential energy
(c) the water, stored at a height, in a tank. (d) a moving bus.
Answer – Potential energy
(d) a moving bus.
Answer – Kinetic energy
5. State two different uses of light energy.
Answer – Two different uses of light energy are:
(a) Photosynthesis by green plants.
(b) Light is necessary for seeing an object.
E. Answer the following questions.
1. Is there a difference between ‘applying a force’ and ‘doing work’? Give reason for your answer.
Answer – Applying a force’ is like a cause and ‘doing work’ is like a result of that cause. Force can be applied without work being done but work cannot be done without applying a force.
2. Explain, by giving an example, that a moving object can do work.
Answer – When a stone hits a windowpane, it can break the glass of the pane. This happens because of the work being done by the moving stone. This example shows that a moving object can do work.
3. A porter keeps standing at a place while holding a suitcase of mass 20 kg over his head for five minutes. Has he done some work? Give reason for your answer.
Answer – When someone is standing with some object on his bead, he is not doing a work. In this case, displacement does not happen in the object and hence no work is being done. Define the term ‘energy’. Describe an activity to show the interrelationship between work and energy.
Take a cricket ball which is hard and heavy. Take a deep pan and fill it with sand. Now, drop the ball in the pan from a height of 1 metre. Observe and measure the size of the depression which is formed in the sand because of the fallen ball. After that, drop the ball in the pan from a height of 2 metres. Once again observe the size of the depression in sand.
You will notice that the size of the depression increases when the ball is dropped from more height. This happens because of higher potential energy possessed by the ball when it reaches a greater height. The size of depression in sand shows the relative amount of work being done.
4. Define the term ‘energy’. Describe an activity to show the inter-relationship between work and energy.
Answer – The capacity to do work is called energy. The greater the amount of energy, more work can be done. The following activity can illustrate the interrelationship between work and energy.
5. Name a device which converts-
(a) electrical energy into light energy.
Answer – Electric bulb
(b) sound energy into electrical energy.
Answer – Microphone
(c) chemical energy into electrical energy.
Answer – Electric cell
Value Based Question
Kshitij visited his native village during the summer vacations. While taking a stroll in the field, he saw a boy aiming his catapult at a bird. He asked the boy not to do so and to use his energy for helping, rather than harming, others.
1. Which quality of Kshitij’s behaviour do you appreciate?
Answer – Caring attitude for animals.
2. What kind of energy is possessed by the-
(a) stretched string of the catapult?
Answer – Potential energy
(b) the stone when it gets released from the catapult?
Answer – Kinetic energy
3. Do you think that Kshitij did the right thing by asking the boy not to hit the bird?
Answer – Yes, Kshitij was right in asking the boy not to hit the bird. We should not harm or kill innocent creatures.
4. Give your suggestions about how one can use his/her energy for helping, rather than harming, others.
Answer – We can use our energy in feeding a bird or a stray dog, rather than hitting them. We can use our energy in helping others.
Something To Do
1. List some examples in which no work is done on a body even though a force acts on it.
Answer – When a person is trying to push the wall.
When a person is standing at a place with a heavy load over his head.
2.We can easily perform several tasks, like opening a bottle, or lifting heavy objects, easily by using ‘simple machines. Suggest some activities/tasks, which illustrate the usefulness of some common machines.
Answer – There can be many examples, like trying to unscrew a nut with bare hands and then with a wrench.
3. The law of conservation of energy states that ‘energy can neither be created nor be destroyed’; it can only be transformed from one form to another. Keeping this law in mind, complete the table given below for the type of energy transformation that takes place in the different devices listed there.
Answer –
4. Take a 15-minute ‘minifield trip’ of your school library, computer lab, science laboratory, canteen, etc. Spend these 15 minutes making observations about the energy transformations taking place in the different devices being used there. List your observations in your science notebook.
Answer – Students will do this at their own.