CBSE Class 9 Physics Work And Energy Worksheet Set C

Question. Name at least six forms of energy.

Answer: (i) Chemical energy
(ii) Heat energy
(iii) Light energy
(iv) Electrical energy
(v) Sound energy
(vi) Solar energy

Question. How many watt are there in 1 horsepower?

Answer: 746 watt.

Question. List two essential conditions for work to be done.

Answer: (i) A force must act and
(ii) There should be displacement in the body.

Question. What is the SI unit of work done and power?

Answer: Joule and Watt.

Question. What is power? What is its SI unit?

Answer: It is defined as the rate of doing work. Its unit is watt.

Question. Find the energy in kWh consumed in 10 hours by a machine of power 500 W.

Answer: W = P × t
= 500 × 10 = 5000 Wh = 5 kWh.

Question. When is work said to be done against the force of gravity?

Answer: When a body is lifted, the work is done against the force of gravity.

Question. Justify giving proper reasoning whether the work done in the following case is positive or negative - Work done by resistive force of air on a vibrating pendulum in bringing it to rest.

Answer: Work done is negative because the resistive force of air always acts opposite of the direction of motion of the vibrating pendulum.

Question. The work done in lifting a box on to a platform does not depend upon how fast it is lifted up. Explain your answer giving proper reasoning.

Answer: The work done (W) in lifting a box through a distance
(s) against the gravitational force (F) is given by W = Fs. Hence, it is obvious that it is independent of the rate at which the box is lifted.

Question. Write an expression for the work done in lifting a body of mass ‘m’ through a vertical height ‘h’.

Answer: Work done W = mgh, where g is acceleration due to gravity.

Question. When a book is lifted from a table, against which force work is done?

Answer: Work is done against the force of gravity.

Question. What is the work done when the force acting on the body and the displacement produced in the body are at right angles to each other?

Answer: Zero.

Question. Is it possible that some force is acting on a body but still the work done is zero?

Answer: Yes, when force acts at an angle of 90° with the displacement.

Question. What is the work done in the situation shown below?

Answer: Zero.

Question. What is the relationship between watt and joules per second.

Answer: 1 Watt = 1 joule/second
1 W = 1 Js^–1

Question. What is the relationship between kilowatt and joule per second?

Answer: I know = 1000 joule/second
or, 1 kW = 1000 Js^–1

Question. A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement.What is the work done in this case?

Answer: Given,
displacement = 8 m,
Force = 7 N
Now, Work done = Force × Displacement
= 7 × 8 = 56 J

Question. When do we say that work is done?

Answer: Work is said to be done when a force causes displacement of an object in the direction of applied force.

Question. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Answer: Work done = Force × Displacement

Question. What is the kinetic energy of an object?

Answer: The energy possessed by a body by virtue of its motion is called kinetic energy.

Question. Write an expression for the kinetic energy of an object.

Answer: The expression is KE = 1/2 mv² where ‘m’ is the mass and v is the velocity of the body.

Question. Define 1 watt of power.

Answer: When a work of 1 joule is done in 1 s, the power is said to be one watt.

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

Answer: Given, W = 1000 J, t = 10 s, P = ?
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. What is the relationship between horse power and watt?

Answer: 1 horsepower = 746 watt
1 hp = 746 W

Question. How much energy will an electric motor of 1 horsepower consume in one second?

Answer: An electric motor will consume 746 joule of energy per second.

Question. Give the mathematical relation between power, force and velocity.

Answer: Power (P) = force (F) × velocity (V)
= (F × V)

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

Answer: No.

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. A man rowing a boat upstream is at rest with respect to the shore. Is he doing work?

Answer: The man is doing work relative to the stream because he is applying force to produce relative motion between the boat and the stream. But he does zero work relative to the shore as the displacement relative to the shore is zero.

Question. What type of energy is stored in the spring of a watch?

Answer: When we wind a watch, the configuration of its spring is changed. The energy stored in the spring is obviously potential in nature (elastic potential energy to be more accurate).

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 transformation of energy?

Answer: It is the change of energy from one form of energy into another form of 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. 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 kWh and joule. Define 1 watt.

Answer: 1 kWh – 1000 Wh = 1000 Js^–1 × 60 × 60 s
= 3.6 × 106 J
1 watt is the power of an agent which can do one joule of work in one second.

Question. Can any object have mechanical energy even if its momentum is zero? Explain.

Answer: Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.

Question. Which type of energy is present in an atom?

Answer: Nuclear energy.

Question. Which type of energy is present in a battery?

Answer: Chemical energy.

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 × h
= 0.25 × 10 × 2.5 = 6.25 J

Question. A battery lights a bulb. Describe the energy changes involved in the process.
Answer.Within the electric cell of the battery the chemical energy changes into electrical energy. The electric energy on flowing through the filament of the bulb, first changes into heat energy and then into the light energy.

Question. What kind of energy transformations take place at a thermal power station?
Answer.At a thermal power station, the chemical energy of coal is changed into heat energy which is further changed into electrical energy with the help of an electric generator.

Question. Name the transformation of energy involved in the following cases :
(a) When a body is thrown upwards.
(b) When a body falls from the top of a hill.
(c) When coal burns.
(d) When a gas burns.
(e) When water falls from a height.
Answer.
(a) Kinetic energy into potential energy.
(b) Potential energy into kinetic energy.
(c) Chemical energy into heat energy.
(d) Chemical energy into heat energy.
(e) Potential energy into kinetic energy.

Question. What is the work done by a coolie walking on a horizontal platform with a load on his head?
Answer. In order to balance the load on his head, the coolie applies a force on it in the upward direction, equal to its weight. His displacement is along the horizontal direction. Thus, the angle between force F and displacement is 90°. Therefore, work done
W = Fs cos q = Fs cos 90° = 0.

Question. We wind our watch once a day, what happens to the energy?
Answer.When we wind our watch, we wind the spring inside the watch. As a result, energy is stored in the spring in the form of elastic potential energy. This elastic potential energy is used to make the watch work the whole day.

Question. Define 1 watt of power. A lamp consumes 1000 J of electrical energy in 10 s. Calculate its power.
Answer.
The power of an agent is said to be one watt if it does one joule of work in one second.
(a) Given W = 1000 J, t = 10 s, P = ?
(b) Using the expression,

P =W/t ,
we have P = 1000/10 = 100 W

Question. Distinguish between work, energy and power. State the SI units for each of these quantities.
Answer.Work :It is defined as the product of force applied and the distance moved by the body on the application of the force. In SI unit it is measured in joule.
Energy :It is defined as the capacity of a body to do work. In SI unit it is measured in joule.
Power : It is defined as the rate of doing work. It measures how fast or slow the work is done. In SI unit it is measured in watt unit.

Question. Define 1 J of work.
Answer.Work done = Force x Displacement
Force, F = 1 N and displacement, s = 1 m, then the
work done by the force will be 1 Nm or 1 J. Thus, 1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.

Question. A light and a heavy body have the same momentum. Which one will have greater kinetic energy?
Answer.
As K = p²/2m
and p is same for both. Hence, K∝1\m
Thus, the lighter body has more kinetic energy than the heavier body.

Question. (i) What is a closed system?
(ii) State the law of conservation of energy.
Answer.
(i) A system that does not interact with another is a closed system. Energy in a closed system is neither taken out of it nor taken in from outside.
However, energy transformation may occur inside a closed system.
(ii) The law of conservation of energy states that “the energy can neither be created nor destroyed, but can be transformed from one form to another. In other words, the total energy of a closed system remains constant. Therefore, whenever one from of energy disappear, an equivalent amount of another form appears.

Numerical Questions

Question. A force of 10N causes a displacement of 2 m in a body in its own direction. Calculate the work done by force.
Answer. The work done by force = F × s = 10 N × 2 m = 20 J

Question. How much force is applied on the body when 150 joule of work is done in displacing the body through a distance of 10 m in the direction of force?
Answer.
W = F × s
F =W/s 
= 150/10 =15N

Question. A body of 5 kg raised to 2 m find the work done.
Answer.
The work done to raise a body
= PE
= mgh
= 5 kg × 9.8 × 2 = 98 joule

Question. A work of 4900 J is done on load of mass 50 kg to lift it to a certain height. Calculate the height through which the load is lifted.
Answer. Work done on road to lift = mgh
4900 = 50 × 9.8 h
h = 10 m

Question. An engine work 54,000 J work by exerting a force of 6000 N on it. What is the displacement of the force?
Answer.
S = W/F
= 54000J/6000N = 9 m

Question.A body of mass 2 kg is moving with a speed of 20 m/s. Find the kinetic energy.
Answer.
KE = 0.5 mv²
= 0.5 × 2 × 20 × 20 = 400 J

Question. A moving body of 30 kg has 60 J of KE. Calculate the speed.
Answer.
KE = 0.5 mv²
60 = 0.5 × 30 × v²
60 = 15 v²
v = 2 m/s

Question. A hammer of mass 1 kg falls freely from a height of 2 m. Calculate (I) The velocity and (II) The kinetic energy of the hammer just before it touches the ground. Does the velocity of hammer depend on the mass of hammer?
Answer.
PE = mgh = 1 × 9.8 × 2 = 19.6 J
PE = KE = 0.5 mv²
19.6 = 0.5 × 1 × v²
39.2 = v²
v = 6.29 m/s
No, velocity of hammer does not depend on the mass of the hammer as v = u + at

Question. Calculate the change that should be affected in the velocity of a body to maintain the same KE, if mass of the body is increased to 4 times.
Answer.
Original KE = 1/2 mv₁²
Now, if mass if increase 4 times then,
New KE = 4 × 1/2 mv₂²
1/2 mv₁² = 4 × 1/2 mv₂²
v₁² = 4 v₂²
v₂=v₁/2
Hence, new velocity will be bay the original velocity.

Question. A machine does 192 J of work in 24 sec. What is the power of the machine?
Answer.
P=W/ t= 192J/24sec = 8 W

Question. A weight of 50 kg runs up a hill rising himself vertically 10 m in 20 sec. Calculate power. (Given g = 9.8 ms^–1)
Answer.
P=W/t= mgh/t
(50×9.8×10)/20 = = 245 W

Question. A rickshaw puller pulls the rickshaw by applying a force of 100 N. If the rickshaw moves with constant velocity of 36 kmh^–1. Find the power of rickshaw puller.
Answer.
Force = 100 N
Velocity = 36 km/h
= (36×5)/18 = 10 m/s
Power = Force × Velocity
Power = 100 × 10 = 1000 Watt
Therefore, power of rickshaw puller = 1000 Watt

Question. An athlete weighing 60 kg runs up a staircase having 10 steps each of 1 m in 30 sec. Calculate power (g = 9.8ms^–1).
Answer.
h = 10 × 1m = 10 m
Work done = potential energy
= mgh
= 60 × 9.8 × 10
= 5880 J
Power = W/t = 5880/30 = 196 W