Q1Multiple choice
When a body falls freely towards the earth, then its total energy
- (a)increases
- (b)decreases
- (c)remains constant
- (d)first increases and then decreases
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(c) remains constant
Chapter 11
Class 9 Science · 28 questions · 27 with answers
Questions
Q2Multiple choice
A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process the potential energy of the car
- (a)does not change
- (b)becomes twice to that of initial
- (c)becomes 4 times that of initial
- (d)becomes 16 times that of initial
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(a) does not change
Q3Multiple choice
In case of negative work the angle between the force and displacement is
- (a)00
- (b)450
- (c)900 (d ) 1800
Q4Multiple choice
An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass is 3.5 kg. Both spheres are dropped simultaneously from a tower. When they are 10 m above the ground, they have the same
- (a)acceleration
- (b)momenta
- (c)potential energy
- (d)kinetic energy
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(a) acceleration
Q5Multiple choice
A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. The work done against the gravitational force will be (g =10 m s–2)
- (a)6 ×103 J
- (b)6 J
- (c)0.6 J
- (d)zero
Show answer
(d) zero
Q6Multiple choice
Which one of the following is not the unit of energy?
- (a)joule
- (b)newton metre
- (c)kilowatt
- (d)kilowatt hour 16-04-2018
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(c) kilowatt
Q7Multiple choice
The work done on an object does not depend upon the
- (a)displacement
- (b)force applied
- (c)angle between force and displacement
- (d)initial velocity of the object
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(d) initial velocity of the object
Q8Multiple choice
Water stored in a dam possesses
- (a)no energy
- (b)electrical energy
- (c)kinetic energy
- (d)potential energy
Show answer
(d) potential energy
Q9Multiple choice
A body is falling from a height h. After it has fallen a height , it will possess
- (a)only potential energy
- (b)only kinetic energy
- (c)half potential and half kinetic energy
- (d)more kinetic and less potential energy
Show answer
(c) half potential and half kinetic energy
Q10Short answer
A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?
Show answer
Initial velocity = v, then v ′ = 3 v Initial kinetic energy = m v2 1 1 1 Final kinetic energy (K.E.) = m v ′ 2 = m (3 v )2 = 9 ( m v 2 ) 2 2 2 (K.E) initial : (K.E) final = 1:9
Q11Short answer
Avinash can run with a speed of 8 m s–1 against the frictional force of 10 N, and Kapil can move with a speed of 3 m s–1 against the frictional force of 25 N. Who is more powerful and why?
Show answer
Power of Avinash P A = F A . v A = 10 × 8 = 80 W The power of Kapil Pk = F k . v k = 25 × 3 = 75 W So, Avinash is more powerful than Kapil.
Q12Short answer
A boy is moving on a straight road against a frictional force of 5 N. After travelling a distance of 1.5 km he forgot the correct path at a round about (Fig. 11.1) of radius 100 m. However, he moves on the circular path for one and half cycle and then he moves forward upto 2.0 km. Calculate the work done by him. Fig. 11.1
This question refers to a figure in the original PDF.
Show answer
F = 5 N W = F.S W = 5 × [1500 + 200 + 2000] = 18500 J.
Q13Short answer
Can any object have mechanical energy even if its momentum is zero? Explain.
Show answer
Yes, mechanical energy comprises both potential energy and kinetic energy. Momentum is zero which means velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.
Q14Short answer
Can any object have momentum even if its mechanical energy is zero? Explain.
Show answer
No. Since mechanical energy is zero, there is no potential energy and no kinetic energy. Kinetic energy being zero, velocity is zero. Hence, there will be no momentum. W mgh m ×10 × 10
Q15Short answer
The power of a motor pump is 2 kW. How much water per minute the pump can raise to a height of 10 m? (Given g = 10 m s–2)
Show answer
P= = ⇒ = 2000 W ∆t ∆t 60 OPNNN or m = =1200 kg
Q16Short answer
The weight of a person on a planet A is about half that on the earth. He can jump upto 0.4 m height on the surface of the earth. How high he can jump on the planet A? WORK AND ENERGY 69 16-04-2018
Show answer
Since, weight of the person on planet A is half that on the earth, acceleration due to gravity there, will be 1/2 that on the earth. Hence he can jump double the height with the same muscular force. The potential energy of the person will remain the same on the earth and on planet A. Thus, m g1h 1 = m g2h 2 if g1 = g then g2 = g , h 1 = 0.4 gh g ×0.4 1 1 = Then h2 = g g 2 2 or h 2 = 0.4 × 2 = 0.8m
Q17Short answer
The velocity of a body moving in a straight line is increased by applying a constant force F, for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.
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v 2 – u2 = 2 a.s v 2 -u 2 This gives s= 2a F= m a we can write work done (W) by this force F as v 2 -u 2 W= ma = 1 m v2 − 1 m u2 = (K.E)f – (K.E)i 2a 2 2
Q18Short answer
Is it possible that an object is in the state of accelerated motion due to external force acting on it, but no work is being done by the force. Explain it with an example.
Show answer
Yes, it is possible, if an object is moving in a circular path. Because force is always acting perpendicular to the direction of displacement.
Q19Short answer
A ball is dropped from a height of 10 m. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back? (g = 10 m s–2)
Show answer
m g h = m ×10 ×10 = 100m J. Energy is reduced by 40% then the remaining energy is 60m J. Therefore, 60 m = m ×10× h′ or h′ = 6 m OPNN
Q20Short answer
If an electric iron of 1200 W is used for 30 minutes everyday, find electric energy consumed in the month of April.
Show answer
P = = 1.2 kW ONNN t = 60 = 0.5 h E = Power × time × days = 1.2×.5×30 = 18 kW h Long Answer Questions
Q21Long answer
A light and a heavy object have the same momentum. Find out the ratio of their kinetic energies. Which one has a larger kinetic energy?
Show answer
p1 = m 1v 1 p 2 = m 2 v 2 But p1 = p2 or m 1v 1 = m2v2 If m1< m2 then v1> v 2 1 1 (K.E)1 = m1v12 (K.E)2 = m 2v 22 2 2 1 1 1 1 (K.E)1= (m 1v 1 ) v1 = p1 v1 (K.E)2 = (m 2v 2 ) v 2 = = p2 v 2 2 2 2 2 (K.E)1 p 2 v1 v = 2 = 1 (K.E)2 1 p2 v 2 v 2 But v 1> v 2 Therefore, (K.E.)1 > (K.E)2
Q22Multiple choice
An automobile engine propels a 1000 kg car
- (A)along a levelled road at a speed of 36 km h–1. Find the power if the opposing frictional force is 100 N. Now, suppose after travelling a distance of 200 m, this car collides with another stationary car
- (B)of same mass and comes to rest. Let its engine also stop at the same time. Now car (B) starts moving on the same level road without getting its engine started. Find the speed of the car (B) just after the collision.
Show answer
(A) along a levelled road at a speed of 36 km h–1. Find the power if the opposing frictional force is 100 N. Now, suppose after travelling a distance of 200 m, this car collides with another stationary car
(B) of same mass and comes to rest. Let its engine also stop at the same time. Now car (B) starts moving on the same level road without getting its engine started. Find the speed of the car (B) just after the collision.
Q23Multiple choice
A girl having mass of 35 kg sits on a trolley of mass 5 kg. The trolley is given an initial velocity of 4 m s–1 by applying a force. The trolley comes to rest after traversing a distance of 16 m.
- (a)How much work is done on the trolley?
- (b)How much work is done by the girl?
Show answer
(a) How much work is done on the trolley?
Q24Multiple choice
Four men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it.
- (a)How much work is done by the men in lifting the box?
- (b)How much work do they do in just holding it?
- (c)Why do they get tired while holding it? (g = 10 m s–2)
Show answer
(a) How much work is done by the men in lifting the box?
(b) How much work do they do in just holding it?
(c) Why do they get tired while holding it? (g = 10 m s–2)
Q25Long answer
What is power? How do you differentiate kilowatt from kilowatt hour? The Jog Falls in Karnataka state are nearly 20 m high. 2000 tonnes of water falls from it in a minute. Calculate the equivalent power if all this energy can be utilized? (g = 10 m s–2)
Show answer
Power is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy. h = 20 m, and mass = 2000 × 103 kg = 2 × 106 kg m gh 2 ×106 ×1 0 × 20 Power = = W t 60 4 2 = × 10 7 W = ×10 7 W 6 3 work done or energy m gh h
Q26Long answer
How is the power related to the speed at which a body can be lifted? How many kilograms will a man working at the power of 100 W, be able to lift at constant speed of 1 m s–1 vertically? (g = 10 m s–2)
Show answer
Power = = = m. g. ( ) time t t Here = speed power 100 Therefore, m = = =10 kg g × speed 10 ×1
Q27Long answer
Define watt. Express kilowatt in terms of joule per second. A 150 kg car engine develops 500 W for each kg. What force does it exert in moving the car at a speed of 20 m s–1?
Show answer
One watt is the power of an agent which does work at the rate of 1J s–1 1 kilowatt = 1000 J s–1 Total Power = 150 × 500 = 7.5 × 104 W Power 7.5 ×104 Force = = = 3.75 ×10 3 N velocity 20 Force = 3750 N.
Q28Multiple choice
Compare the power at which each of the following is moving upwards against the force of gravity? (given g = 10 m s–2)
- (i)a butterfly of mass 1.0 g that flies upward at a rate of 0.5 m s–1.
- (ii)a 250 g squirrel climbing up on a tree at a rate of 0.5 m s–1. 16-04-2018
Show answer
(i) a butterfly of mass 1.0 g that flies upward at a rate of 0.5 m s–1.
(ii) a 250 g squirrel climbing up on a tree at a rate of 0.5 m s–1. 16-04-2018