NCERT 9th Class (CBSE) Science: Energy and Work

Question: A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Answer: Given, W = 1000 J, t = 10 s, R = ?
Using p = W/t = 1000/10 = 100 W

Question: Define average power.

Answer: When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power.

Question: Define energy.

Answer: Energy is the ability of a body to do work. It is also defined as the capacity to do work.

Question: A body performs no work. Does it imply that the body possesses no energy?

Answer: When a body does not perform any work, it never implies that the body has no energy. The body may have energy but still does not perform any work, e.g., a book placed on a table has potential energy but is not performing any work.

Question: What is the SI unit of energy?

Answer: The SI unit of energy is joule.

Question: Does a body at rest possess any kinetic energy?

Answer: No.

Question: What will happen to the kinetic energy of a body if its mass is doubled?

Answer: Its kinetic energy will be doubled.

Question: What will happen to the kinetic energy of a body if its velocity is halved?

Answer: The kinetic energy of the body will become one-fourth.

Question: By how much will the speed of a body, of fixed mass, increase if its kinetic energy becomes four times its initial kinetic energy?

Answer: The speed is doubled.

Question: Can a body possess energy even if it is not in motion?

Answer: Yes, it can possess potential energy.

Question: Define potential energy.

Answer: It is defined as the energy possessed by a body by virtue of its position or change in shape.

Question: Name the energy possessed by a stretched rubber band lying on the table.

Answer: Potential energy.

Question: Give the SI unit of potential energy.

Answer: The SI unit of potential energy is.joule.

Question: What do you mean by trans- formation of energy?

Answer: It is the change of energy from one form of energy into another form of energy.

Question: Can energy be destroyed? Can energy be created?

Answer: No

Question: A cell converts one form of energy into another. Name the two forms.

Answer: It converts chemical energy into electrical energy.

Question: Name one unit of power bigger than watt.

Answer: A unit bigger than watt is kilowatt.

Question: When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy?

Answer: A stretched bow possesses potential energy on account of a change in its shape. To shoot an arrow; the bow is released. The potential energy of the bow is converted into the kinetic energy of the arrow.

Question: Name at least three commonly used units of energy.

Answer: (i) Joule (ii) Erg (iii) Kilowatt hour.

Question: Name the practical unit of power in engineering.

Answer: Horsepower.

Question: Name at least six forms of energy.

Answer:

1. Chemical energy
2. Heat energy
3. Light energy
4. Electrical energy
5. Sound energy
6. Solar energy

Question: How many watt are there in 1 horsepower?

Answer: 746 watt.

Question: What is horsepower?

Answer: It is a unit of power.

Question: Name the commercial unit of measurement of energy.

Answer: Kilowatt-hour

Question: Define Kilowatt-hour.

Answer: One – kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour.

Question: Define the term ‘watt’.

Answer: One watt is the power of an appliance which consumes energy at the rate of 1 Joule per second.

Question: What is the common name of ‘1 kWh’ of electrical energy?

Answer:It is commonly known as ‘ 1 unit’ of electricity.

Question: A light and a heavy body have equal kinetic energy. Which one is moving fast?

Answer: The lighter body is moving fast.

Question: State the relation between kW h and joule. Define 1 watt.

Answer: 1 kW h – 1000 W h = 1000 Js^-1 x 60 x 60 s = 3.6 x 10⁶ J
1 watt is the power of an agent which can do one joule of work in one second.

Question: Is it possible that a body be in accelerated motion under a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example.

Answer: Yes, it is possible, when the force is perpendicular to the direction of motion. The moon revolving round the earth under the centripetal force of attraction of the earth but earth does not do any work on the motion of The moon.

Question: Define work. How is work measured? When is work done by a force negative?

Answer: Work is said to be done if force acting on an object displaces it through a certain distance.

It is measured as the product of force and displacement.

Work done is negative if force and displacement are in the opposite direction.

Question: What is the work done by the force of gravity in the following cases?
(a) Satellite moving around the earth in a circular orbit of radius 35000 km.
(b) A stone of mass 250 g is thrown up through a height of 2.5 m.

Answer:

(a) Zero, as the displacement in one complete revolution is zero.
(b) Given m = 250 g = 0.25 kg, h = 2.5 m, g = 10 ms^-2, W = ?
Now, W = FS = mg x h = 0.25 x 10 x 2.5 = 6.25 J

Question: A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your Answer.

Answer: The work done is zero. This is because the gravitational force and displacement are perpendicular to each other.

Question: The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Answer: It does not violate the law of conservation of energy. Whatever, is the decrease in PE due to loss of height, same is the increase in the KE due to increase in velocity of the body.

Question: What are the various energy transformations that occur when you are riding a bicycle?

Answer: The chemical energy of the food changes into heat and then to muscular energy. On paddling, the muscular energy changes into mechanical energy.

Question: Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Answer: Energy transfer does not take place as no displacement takes place in the direction of applied force; the energy spent is used to overcome inertia of rest of the rock.

Question: An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Answer: Since the body returns to a point which is on the same horizontal line through the point of projection, no displacement has taken place against the force of gravity; therefore, no work is done by the force due to gravity.