Question. A light and a heavy body have equal kinetic energy. Which one is moving fast?
Ans : The lighter body is moving fast.
Question. State the relation between kWh and joule. Define 1 watt.
Ans : 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 kinetic energy of a body be negative?
Ans : No as mass and velocity cannot be negative
Question. What is the energy of a body due to its motion called?
Ans :Kinetic energy.
Question. What is the SI unit of kinetic energy?
Ans : Joule.
Question. Can any object have mechanical energy even if its momentum is zero? Explain.
Ans : 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. Give one example each of potential energy (i) due to position (ii) due to shape.
Ans : (i) Potential energy due to position : Water stored in dam has potential energy.
(ii) Potential energy due to shape : In a toy car, the wound spring possesses potential energy and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.
Question. What kind of energy transformation takes place when a body is dropped from a certain height?
Ans : When a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy.
Question. Can energy be destroyed? Can energy be created?
Ans : No, energy can neither be created nor be destroyed.
Question. A cell converts one form of energy into another. Name the two forms.
Ans : It converts chemical energy into electrical energy.
Question. Name one unit of power bigger than watt.
Ans : A unit bigger than watt is kilowatt.
Question. A spring which has been kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans : The PE of the spring gets converted into KE of acid molecules whose temperature rises.
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.
Ans : 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.
Ans : 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. A body of mass 2 kg is moving with a speed of 20 m/s.
Find the kinetic energy.
Ans : KE = 0.5 mv2
= 0.5 × 2 × 20 × 20 = 400 J
Question. At what rate is electrical energy consumed by a bulb of 60 watt?
Ans : A 60 watt bulb consume electrical energy at the ratio of 60 joule per second.
Question. Which type of energy is present in an atom?
Ans : Nuclear energy.
Question. Define energy.
Ans : 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?
Ans : 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. (a) What is meant by potential energy? Is potential energy vector or scalar quantity?
(b) Give one example of a body having potential energy.
Ans : (a) The energy possessed by a body by virtue of its position or configuration. It is a scalar quantity.
(b) Stretched string of a bow.
Question. When is the work done by a force said to be negative?
Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work.
Ans : We know that work done W = Fs cos q , where q is the angle between F and S. Clearly, W will be – ve, if q is between 90° and 180° because then cos q will be –ve. Consider the case of a body falling under gravity. The body experiences an upward frictional force and downward force due to gravity. Since, the body is moving downwards, the work done by force
to gravity will be +ve but that is against the upward thrust will be –ve.
Question. A light and a heavy body have the same momentum.
Which one will have greater kinetic energy?
Ans :
As K =p2/2m and p is same for both. Hence, K∝ m/1 Thus, the lighter body has more kinetic energy thanbthe heavier body.
Question. (i) What is a closed system?
(ii) State the law of conservation of energy.
Ans : (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.
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.
Ans : 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 no work on the motion.
Question. A body moves along a circular path. How much work is done in doing so? Explain.
Ans : In case of a body moving along a circular path, the force (centripetal) is always along the radius while displacement is tangential. Hence, work done W = FS
cos 90° = 0 as angle between F and S is 90°.
Question. A battery lights a bulb. Describe the energy changes involved in the process.
Ans : 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?
Ans : 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. (a) How much work is done when a force of 1 N moves a body through a distance of 1 m in its direction?
(b) Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.
Ans : (a) 1 J of work is done.
(b) Yes, it is possible when force acts at right angles to the direction of motion of the body. Example :
Gravitational force of Earth acts on a satellite at right angles to its direction of motion.
Question. Is it possible that a body is in accelerated motion tinder a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example.
Ans : 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. Does the potential energy of a spring increase or decrease when it is compressed?
Ans : The potential energy of the spring increases because work is done on it when it is compressed.
Question. A spring is compressed, what change is expected in the potential energy of the spring
Ans : When a spring is compressed, its potential energy is used up to changing its shape.
Question. A man rowing a boat upstream is at rest with respect to the shore. Is he doing work?
Ans : 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?
Ans : 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. What will happen to the kinetic energy of a body if its mass is doubled?
Ans : Its kinetic energy will be doubled.
Question. What will happen to the kinetic energy of a body if its velocity is halved?
Ans : The kinetic energy of the body will become one-fourth.
Question. When a constant force is applied to a body moving with constant acceleration, is the power of the force constant? If not, how would force have to vary with speed for the power to be constant?
Ans :We know that,
power (p) = force (f) × velocity (v)
Since the body is moving with accleration, V changes
and as a result of that P also changes, F being constant.
For P to be constant, FV = constant or F \ V
Thus, as V increases, F should decrease to keep P constant.
Question. What is the SI unit of energy?
Ans : The SI unit of energy is joule.
Question. Does a body at rest possess any kinetic energy?
Ans : No.
Question. A spring which is kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans : The potential energy of the spring gets converted into heat energy (kinetic energy of acid molecules). Due to this heat, the temperature of the acid rises.
Question. How much energy will an electric motor of 1 horsepower consume in one second?
Ans : An electric motor will consume 746 joule of energy per second.
Question. Give the mathematical relation between power, force and velocity.
Ans : Power (P) = force (F) × velocity (V) = (F × V)
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?
Ans : 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. Give the SI unit of potential energy.
Ans : The SI unit of potential energy is joule.
Question. What do you mean by transformation of energy?
Ans : It is the change of energy from one form of energy into another form of energy.
Question. When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy?
Ans : 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.
Ans : (i) Joule, (ii) Erg, (iii) Kilowatt hour.
Question. Name the practical unit of power in engineering.
Ans : Horsepower.
Question. Name at least six forms of energy.
Ans : (i) Chemical energy
(ii) Heat energy
(iii) Light energy
(iv) Electrical energy
(v) Sound energy
(vi) Solar energy
Question. How much work is done by a force of 10 N in moving an object through a distance of 4 m in the direction of the force?
Ans :
Work done = Force × Displacement
= F × s
= (10 N) × (4 m)
= 40 joule or 40 J.
Question. Define potential energy.
Ans : 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.
Ans : Potential energy.
Question. What is power? How do you differentiate kilowatt from kilowatt hour?
Ans : Power is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy.
Question. Calculate the work done in lifting 200 kg of water through a vertical height of 6 m.
Ans : (i) Work done in lifting a body
= Weight of body × vertical distance
(ii) The work done in lifting
= W = mgh
= 200 kg × 10 m/s2 × 6 m = 1200 J
Question. How many watt are there in 1 horsepower?
Ans : 746 watt.
Question: In a tug-of-war one team gives way to the other. What work is being done and by whom?
Answer : The winning team does work. The work is equal to the product of the resultant force and the displacement undergone by the losing team.
Question: What will cause greater change in kinetic energy of a body? Changing its mass or changing its velocity?
Answer : Changing its velocity.
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: 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: 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: Will work be done by a man who pushes a wall?
Answer : No.
Question: Name the form of energy associated in each case :
(i) A flying bird.
(ii) A man climbing the stairs.
(iii) A compressed watch spring.
(iv) A fast moving object.
Answer : (i) Mechanical energy
(ii) Mechanical energy
(iii) Potential energy
(iv) Kinetic energy
Question: Name the various forms of energy.
Answer : The various forms of energy are :
(i) Potential energy
(ii) Kinetic energy
(iii) Mechanical energy
(iv) Heat energy
(v) Chemical energy
(vi) Electrical energy
(vii) Light energy.
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: 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: What is the work done on a body moving in a circular path?
Answer : Zero, because force and displacement are perpendicular to each other.
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: 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: What is the relationship between megawatt and watt?
Answer : 1 megawatt = 106 watt
Question: What is the relationship between megawatt and joules per second?
Answer : 1 megawatt = 106 joule/second
1 MW = 106 js–1
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 × h
= 0.25 × 10 × 2.5 = 6.25 J
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: What is the amount of work done by a force when a body moves in a circular path?
Answer :
Work done is given by the expression W = Fs cos q
. When a body moves in a circular path, then the
displacement (s) is zero. Therefore, work done is
W = F × 0 = 0.
Question: Name the common forms of the mechanical energy.
Answer : The common forms of the mechanical energy are :
(i) Kinetic energy
(ii) Potential energy
Question: Does every change in energy of the body involve work?
Answer : Yes.
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: The masses of scooter and bike are in the ratio of 2 : 3 but moving with same speed of 108 km h–1. Compute the ratio of their kinetic energy.
Answer : The energy possessed by a body by virtue of its motion.
Given, m1/m2=2/3
2 , the ratio of KE is equal to the ratio of their masses if they have the same velocity, therefore,ratio of their KE is also 2 : 3.
Question: Define power. Write commercial unit and SI unit of electrical energy. An electrical geyser of 1.5 kW works for 2 hours. Find the electrical energy units consumed in a day.
Answer : Power is defined as the rate of doing work. SI unit s joule and kWh is the commercial unit of electrical energy.
Given, P = 1.5 kW,
t = 2 hours,
E = P × t = 1.5 × 2 = 3 kWh
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: 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 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: What is meant by the term horsepower (hp)?
Answer : Horsepower is another commercial unit power
1 hp = 746 W
1W = 746
1 hp = 0.0013 hp
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 = 2
1 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: 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: What is the relationship between horse power and watt?
Answer : 1 horsepower = 746 watt
1 hp = 746 W
Question:An electric kettle of 500W is used to heat water everyday for 2 hours. Calculate the number of unit of electrical energy consumed by it in 10 days.
Answer : E = Pt = 500 W × 10 × 2h
= 10000 Wh
= 10 kWh = 10 unit
Question: How does the kinetic energy of a body change if the mass of the body is halved?
Answer: If the mass of a body is heaved the kinetic energy is also halved.
Question: A car and a truck are moving with the same velocity of 60 km/hr–1, which one has more kinetic energy?
Answer : Truck has more kinetic energy as kinetic energy is directly proportional to the mass.
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: What is the relationship between watt and joules per second.
Answer : 1 Watt = 1 joule/second
1 W = 1 Js–1
<1M>
(Q.1) Kinetic energy of a moving object is the energy
(A) Possessed by an object due to its motion
(B) That increases with speed
(C) Equal to the work done on it to make it acquire that velocity.
(D) All of these
(Q.2) Negative value of work done indicates that
(A) Force and displacement are in the same direction
(B) More than one force is acting on the object
(C) Displacement and force are in opposite directions
(D) Both (2) and (3
(Q.3) Unit of work done is
(A) Joule
(B) Newton meter
(C) Calorie
(D) Both (1) and (2)
(Q.4) Work done is defined as
(A) Product of force and displacement
(B) Distance through which the object is moved
(C) Mass of the object getting displaced
(D) Product of force and mass
(Q.5) Scientific concept of work suggest that work is said to be done if
(A) A force acts on an object
(B) The object must be displaced
(C) acceleration should be there.
(D) Only (1) and (2)
(Q.6) 1 kilowatt = -----
(A) 1000 W
(B) 1000 J s –1
(C) 1000 N m s –1
(D) All of these
(Q.7) Commercial Unit of power is kilowatt-hour (kW h)
(A) 1 kW h = 3.6 x 106 J
(B) 1 kWh is the energy consumed in one hour at the rate of 1000 J s –1
(C) 1 kWh = 1 unit of electrical energy
(D) All these statements are correct
(Q.8) An electrical appliance of 500W is used for 5 hours per day. Energy consumed in 30 days will be
(A) 2.5 kW h
(B) 25 kW h
(C) 75 kW h
(D) None of these
(Q.9) What is energy of a body?
(Q.10) Define kinetic energy.
(Q.11) What is potential energy?
(Q.12) What will be the work done by a force if displacement of the body is zero?
(Q.13) When is work done on a body positive?
(Q.14) Can energy be converted from one form to another?
(Q.15) Define power.
(Q.16) What will be the new kinetic energy of a body if its speed is doubled?
(Q.17) What is the energy change involved when a battery lights a bulb?
(Q.18) Sun is said to be the ultimate source of energy. Solar energy gets transformed into
(A) Chemical energy during photosynthesis
(B) Heat energy in drying food grains
(C) Electrical energy in solar cells
(D) All of these
(Q.19) Various forms of energy include
(A) Mechanical Energy
(B) Chemical Energy
(C) Light Energy
(D) All of these
(Q.20) Law of conservation of energy states that
(A) Energy can neither be created nor be destroyed
(B) Energy can only be converted from one form to the other
(C) Total energy before and after the transformation remains the same
(D) All of these
(Q.21) Work done by a force can be
(A) Only positive
(B) Only negative
(C) Both positive and negative
(D) None of these
(Q.22) Capacity of doing work is termed as
(A) Pressure
(B) Energy
(C) Force
(D) Displacement
(Q.23) Unit of energy is
(A) Same as the unit of work
(B) Joule
(C) Both (1) and (2)
(D) Neither (1) nor (2)
(Q.24) Kinetic energy of a moving object can be represented mathematically as
(A) Ek = ½ m v2
(B) Ek = mgh
(C) Both of these
(D) None of these
(Q.25) An object of 2 kg is moving with a velocity of 5 m/s. If its velocity is doubled, the kinetic energy will become
(A) 100J
(B) 25J
(C) 200J
(D) 2.5J
(Q.26) Potential energy of an object is the energy possessed by it by virtue of
(A) Its configuration
(B) Its position
(C) Its position or configuration
(D) It motion.
(Q.27) Potential energy of an object at a height h is given as
(A) Ep = ½ m v2
(B) Ep = m g h
(C) Ep = w
(D) All of these
(Q.28) Gravitational Potential Energy of an object will
(A) Increase by increasing the path along which the object is moved
(B) Decrease by increasing the path along which the object is moved
(C) Not affected by changing the path, provided the overall height is same
(D) None of these
(Q.29) An object of 12 kg mass is placed at a height of _____ m, if its Ep = 240 J and g = 10 ms-2
(A) 4 m
(B) 2m
(C) 8m
(D) 1m
(Q.30) Total mechanical energy of an object is
(A) Potential Energy + Kinetic Energy = Constant
(B) m g h + ½ m v2 = constant
(C) Both (1) and (2)
(D) None of these
(Q.31) Rate of doing work is termed as
(A) Force
(B) Mechanical energy
(C) Power
(D) Momentum
(Q.32) Unit of power is
(A) Joule
(B) Watt
(C) Calorie
(D) Newton
(Q.33) The potential energy of a freely falling object decreases progressively.
(A) The law of conservation of energy is violated
(B) Potential energy gets converted into kinetic energy progressively
(C) Sum of Potential Energy and Kinetic Energy at any point during the free fall remains constant
(D) Both (2) and (3)
(Q.34) When a freely falling object hits the ground, its kinetic energy is
(A) Converted into heat energy
(B) Used to form a crater in the ground
(C) Collides and then rebounds
(D) Any of the three are possible
(Q.35) A man X takes 60 m tall stairs to reach the top of a building and another man Y takes 55 m tall stairs to reach the top of the same building. Which of the following statement is true?
(A) Work done by the person X is more than the work done by the person Y
(B) Work done by the person Y is more than the work done by the person X
(C) Work done by both of them is same
(D) None of these
(Q.36) Two bodies of same mass have velocities in ratio 2:3.What will be the ratio of their kinetic energies?
(A) 2 : 3
(B) 4 : 9
(C) √2 : √3
(D) 8 : 27
(Q.37) What type of energy transformation takes place in a battery cell?
(A) Chemical energy into electrical energy
(B) Electrical energy into chemical energy
(C) Potential energy into electrical energy
(D) Mechanical energy into electrical energy
(Q.38) What is the unit for electrical energy?
(A) KW
(B) KWh
(C) Watt
(D) Js
(Q.39) Which of the following has magnitude as well as direction?
(A) Work
(B) Force
(C) Kinetic energy
(D) Potential energy
(Q.40) What type of energy transformation takes place in a cracker?
(A) Kinetic energy into sound energy
(B) Chemical energy into sound and light energy
(C) Potential energy into sound and light energy
(D) Kinetic energy into sound and light energy
<2M>
(Q.41) A constant force of 10N displaces a body through 5 m. Find the work done by the force?
(Q.42) What will be the work done if a stone of mass 2 kg is raised through a height of 10cm?
(Q.43) What are the conditions needed for work to be done?
(Q.44) An electric bulb of 60w is used for 5 hours a day. Calculate the energy consumed in one day by the tube?
(Q.45) An object of mass 10kg is moving with speed 4 m/sec.What is the kinetic energy of the object?
(Q.46) An object of mass 10kg is at a certain height above the ground. If the potential energy of the object is 200j find the height of the object from the ground?
(Q.47) A person is holding a heavy bag on his head for 20 minutes and gets tired. Has he done some work or not? Explain.
<3M>
(Q.48) A ball of mass 2kg is kept on a tower of height 30m. Find its potential energy at this point. If it is allowed to fall freely, find its kinetic energy when it just touches the ground?
(Q.49) A body of mass5 kg is kept on a table. If it is displaced by a force of 10N by 2 m on the table on the same horizontal line, find the work done by the gravitational force.
(Q.50) What is law of conservation of energy?
(Q.51) A man of mass 50kg climbs a tower of height 45m in 5 seconds with the help of a rope. Find the power of the man?
(Q.52) If a particle falls through a height; its potential energy decreases. Does this violate the law of conservation of energy? Explain why?
(Q.53) What is the work done by the force of gravity on a satellite moving round the earth?
(Q.54) What will be the work done to stop a moving cycle of mass 30kg which is moving with speed 54km/hr ?
(Q.55) A freely falling body stops when it hits the ground. What happens to its kinetic energy?
<5M>
(Q.56) If in an office, 10 tubes of 40W, 5 fans of 75W and 2 ACs of 1500Ware used for 8 hours a day.
Calculate the energy consumed per day in commercial units of energy.
(Q.57) Prove that the kinetic energy of a body moving with speed v is equal to (1/2)mv2
(Q.58) What will be the work done to increase the speed of a bike from 18km/h to 54 km/hr if the mass of the car is 100kg?
Exam Questions NCERT Class 9 Science Chapter 11 work And Energy
Question. Name the common forms of the mechanical energy.
Ans : The common forms of the mechanical energy are :
(i) Kinetic energy
(ii) Potential energy
Question. Does every change in energy of the body involve work?
Ans : Yes.
Question. Is it possible that some force is acting on a body but still the work done is zero?
Ans : Yes, when force acts at an angle of 90° with the displacement.
Question. The masses of scooter and bike are in the ratio of 2 : 3 but moving with same speed of 108 km h–1. Compute the ratio of their kinetic energy.
Ans : The energy possessed by a body by virtue of its motion.
Given, m1/m2=2/3
2 , the ratio of KE is equal to the ratio of their masses if they have the same velocity, therefore,ratio of their KE is also 2 : 3.
Question. Define power. Write commercial unit and SI unit of electrical energy. An electrical geyser of 1.5 kW works for 2 hours. Find the electrical energy units consumed in a day.
Ans : Power is defined as the rate of doing work. SI unit s joule and kWh is the commercial unit of electrical energy.
Given, P = 1.5 kW,
t = 2 hours,
E = P × t = 1.5 × 2 = 3 kWh
Question. A force of 10N causes a displacement of 2 m in a body in its own direction. Calculate the work done by force.
Ans : The work done by force = F × s = 10 N × 2 m = 20 J
Question. A body of 5 kg raised to 2 m find the work done.
Ans : The work done to raise a body
= PE
= mgh
= 5 kg × 9.8 × 2 = 98 joule
Question. A moving body of 30 kg has 60 J of KE. Calculate the speed.
Ans :
KE = 0.5 mv2
60 = 0.5 × 30 × v2
60 = 15 v2
v = 2 m/s
Question. What is meant by the term horsepower (hp)?
Ans : Horsepower is another commercial unit power
1 hp = 746 W
1W = 746
1 hp = 0.0013 hp
Question. What is the kinetic energy of an object?
Ans : 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.
Ans : The expression is KE = 2
1 mv2 where ‘m’ is the mass
and v is the velocity of the body.
Question. Define 1 watt of power.
Ans : When a work of 1 joule is done in 1 s, the power is said to be one watt.
Question. Define average power.
Ans : 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. What is the relationship between horse power and watt?
Ans : 1 horsepower = 746 watt
1 hp = 746 W
Question. An electric kettle of 500W is used to heat water everyday for 2 hours. Calculate the number of unit of electrical energy consumed by it in 10 days.
Ans : E = Pt = 500 W × 10 × 2h
= 10000 Wh
= 10 kWh = 10 unit
Question. How does the kinetic energy of a body change if the mass of the body is halved?
Ans : If the mass of a body is heaved the kinetic energy is also halved.
Question. A car and a truck are moving with the same velocity of 60 km/hr–1, which one has more kinetic energy?
Ans : Truck has more kinetic energy as kinetic energy is directly proportional to the mass.
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?
Ans : The speed is doubled.
Question. Can a body possess energy even if it is not in motion?
Ans : Yes, it can possess potential energy.
Question. What is the relationship between watt and joules per second.
Ans : 1 Watt = 1 joule/second
1 W = 1 Js–1
