Work Energy and Simple Machines Class 9 MCQ Questions Science Chapter 7

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Class 9 Science Chapter 7 Work Energy and Simple Machines MCQ

Work Energy and Simple Machines MCQ

Class 9 Science Chapter 7 MCQ online test

Question 1.
When a horse-pulls a cart, who does the work?
(A) Cart
(B) Wheels
(C) Road
(D) Horse
Answer:
(D) Horse
When a horse-pulls a cart, work is being done by horse.

Question 2.
If 10 N of force is applied to an object, but the object does not move, then how much work being done by the force ?
(A) Zero
(B) 10 J
(C) 10 N
(D) 20 J
Answer:
(A) Zero
If there is no displacement due to application of force, then net work done will be zero.

Question 3.
The work done on an object does not depend upon the [NCERT Exemplar]
(A) displacement
(B) force applied
(C) angle between force and displacement
(D) initial velocity of the object
Answer:
(D) initial velocity of the object
The work done on an object depends upon the force applied, displacement and the angle between force and displacement. Work done does not depend upon initial velocity of the object.

Question 4.
Which one of these is an example of scientific work done?
(A) Playing mobile games when sitting on a sofa.
(B) Studying for board exams.
(C) Pushing a wall and got tired.
(D) Climbing stairs of home.
Answer:
(D) Climbing stairs of home.
Climbing stairs of home involves displacement with the help of some force and scientific work done needs displacement and force acting on the body.

Question 5.
In case of negative work, the force acting on the body and displacement are
(A) perpendicular to each other
(B) parallel to each other
(C) inclined to an angle 30°
(D) anti-parallel to each other
Answer:
(D) anti-parallel to each other
In case of negative work, force F and displacement s are in opposite direction, i.e., anti-parallel to each other.

Question 6.
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 ms^-2) [NCERT Exemplar]
(A) 6 × 10³ J
(B) 6 J
(C) 0.6 J
(D) zero
Answer:
(D) Zero,
gravitational force acting perpendicular to the displacement i.e., angle between direction of force and displacement is 90°.

Question 7.
With the help of figure, write an expression for work in terms of force and displacement.

(A) W = – F × s
(B) W = F /s
(C) W = F × s
(D) W = – \(\frac{F}{s}\)
Answer:
(A) W = – F × s
When a force F displaces a body through a distance s in the opposite direction of applied force, then the work done W on the body is given by
Work done = – Force × Displacement or
W = – F × s

Question 8.
Which of the following statements is/areincorrect? Competency Based Que.
I. Work and energy have different units.
II. Two bodies of unequal masses have equal acceleration at any instant of time when they are dropped from a cliff.
(A) II only
(B) I only
(C) Both I and II
(D) None of these
Answer:
(B) I only
I. Work and energy have same units i.e. joule.
II. Yes, they have equal acceleration i.e. acceleration due to gravity.

Question 9.
A ball is allowed to fall freely from a tower. Which energy is possessed at the mid-way during the fall?
(A) Kinetic only
(B) Potential only
(C) Both potential and kinetic
(D) Heat only
Answer:
(C) Both potential and kinetic
At mid-way of fall, the ball has both kinetic and potential energies.

Question 10.
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 10m above the ground, they have the same [NCERT Exemplar]
(A) acceleration
(B) momentum
(C) potential energy
(D) kinetic energy
Answer:
(A) acceleration
Momentum, kinetic energy and potential energy depend on the mass but acceleration which is acceleration due to gravity is independent of mass.

Question 11.
When speed of the moving bus is tripled, then
(A) kinetic energy is increased by nine-times.
(B) potential energy is increased by nine-times.
(C) momentum is increased by nine-times.
(D) acceleration is doubled.
Answer:
(A) kinetic energy is increased by nine-times.
Kinetic energy, KE = ½ mv²
KE’ = ½ m(3v)²
= 9 (½ mv²)
= 9 KE
Potential energy does not depend on speed.
Momentum, p = mv
p’ = m (3v) = 3 (mv) = 3 p
Momentum is increased by 3 times.

Question 12.
If two stones A and B are dropped from a tower, then which one has maximum kinetic energy ?
(A) Lighter stone
(B) Heavier stone
(C) Both have equal
(D) None of the above
Answer:
(B) Heavier stone
If two stones are dropped from some height, then the heavier stone has greater kinetic energy w.r.t. lighter stone, because kinetic energy is directly proportional to mass.

Question 13.
Which of the following case causes larger change in kinetic energy of a body?
(A) Increasing mass two times.
(B) Increasing velocity two times.
(C) Increasing its potential energy two times.
(D) Increasing mass three times.
Answer:
(B) Increasing velocity two times.
Formula of kinetic energy,
KE = ½ mv² (m = mass, v = velocity)
Changing mass two / three times increases KE only two/three times and there is no dependence of potential energy on kinetic energy.
But increases velocity two times,
KE’ = ½ m (2v)²
= 4 ( ½ mv²)
= 4 KE

Question 14.
Two particles of masses 20 g and 50 g have equal kinetic energies. What will be the ratio of squares of the momentum?
(A) 5 : 2
(B) 5 : 3
(C) 2 : 5
(D) 1 : 5
Answer:
(C) 2 : 5
Kinetic energy,

Question 15.
When the kinetic energy of a body is increased by 800 % the momentum of the body is increased by what percentage?
(A) 2000%
(B) 20%
(C) 200%
(D) 100%
Answer:
(C) 200%
K_f = K_i + 800% of K_i
⇒ K_f = K_i + \(\frac{800}{100}\) K_i = \(\frac{900}{100}\) K_i
⇒ K_f = 9 K_i
⇒ \(\frac{K_f}{K_i}\) = 9
From KE = \(\frac{p^2}{2 m}\)
⇒ p ∝ \(\sqrt{K}\)
\(\frac{p_f}{p_i}=\sqrt{\frac{K_f}{K_i}}\)
= √9 = 3
⇒ p_f = 3 p_i
percentage increase in momentum = \(\frac{p_f-p_i}{p_i}\) × 100
= \(\frac{3 p_i-p_i}{p_i}\) × 100 = 200 %

Question 16.
For given situation, find the potential energy of the ball.

(A) PE = mgh
(B) KE = ½ mv²
(C) PE = zero
(D) PE = Mgh
Answer:
(A) PE = mgh
According to the figure,
PE of ball = Weight of ball × Height
= mg × h = mgh

Question 17.
Water stored in a dam possesses [NCERT Exemplar]
(A) no energy
(B) electrical energy
(C) kinetic energy
(D) potential energy
Answer:
(D) potential energy
Potential energy is stored energy or the energy of position, so water stored in a dam possesses potential energy.

Question 18.
An airplane is taking off from a field. Find the height at which it should be, so that its potential energy is 1.5 × 10⁸ J. (Take, mass of plane = 10⁵ kg)
(A) 200 m
(B) 150 m
(C) 250 m
(D) 100 m
Answer:
(B) 150 m
Here, mass, m = 10⁵ kg
Potential energy, PE = 15 × 10⁸ J
∴ PE = mgh
h = \(\frac{(\mathrm{PE})}{m g}\)
= \(\frac{1.5 \times 10^8}{10^5 \times 10}\) = 150 m.

Question 19.
A body falling from a height h. After it has fallen a height h/2, it will posses [NCERT Exemplar]
(A) only potential energy
(B) only kinetic energy
(C) half potential and half kinetic energy
(D) more kinetic and less potential energy
Answer:
(C) half potential and half kinetic energy
Velocity when it reaches a height h/2,
From equation of motion,
v² = u² + 2 as

Here, u = 0 m/s,
a = g,
s = \(\frac{h}{2}\)
⇒ v² = gh
Kinetic energy, KE = ½ mv²
= \(\frac{mgh}{2}\) or \(\frac{PE}{2}\)

Question 20.
When a body falls freely towards the Earth, then its total energy [NCERT Exemplar]
(A) increases
(B) decreases
(C) remains constant
(D) first increases and then decreases
Answer:
(C) remains constant
Since, total energy of the system is always conserved, so when a body falls freely towards the Earth, then its total energy remains constant, i.e. the sum of the potential energy and kinetic energy of the body would be same at all points.

Question 21.
A flying bird in the sky possesses which energy?
(A) Only kinetic energy
(B) Only potential energy
(C) Neither potential nor kinetic energy
(D) Both potential and kinetic energy.
Answer:
(D) Both potential and kinetic energy
A flying bird in the sky possesses potential energy by virtue of its position and since its flying with speed, so it also possesses kinetic energy.

Question 22.
In solar cell, light energy is
(A) converted into chemical energy
(B) converted into mechanical energy
(C) converted into electrical energy
(D) converted into gravitational energy
Answer:
(C) converted into electrical energy
In solar cell, light energy is converted into electrical energy.

Question 23.
A horse does 5000 J of work in 100 s. What is its power ?
(A) 50 W
(B) 50 J
(C) 10 W
(D) 10 J
Answer:
(A) 50 W
Given, work done, W = 5000 J
Time taken, t = 100 s
We know that,
Power, P = \(\frac{W}{t}\)
= \(\frac{5000}{100}\) = 50 J/s
Thus power, P = 50 W

Question 24.
If a 15 W bulb and a 10 W bulb are connected to a source of 250 V potential, the bulb glowing brighter is,
(A) 10 W
(B) 15 W
(C) Both Bulb glowing bright equally
(D) cannot be determined
Answer:
(B) 15 W,
the bulb having greater power will be glowing brighter than 10 W bulb.

Question 25.
Mechanical Advantage (MA), load (L) and effort (E) are related as
(A) MA = L × E
(B) MA × E = L
(C) E = MA × L
(D) None of these
Answer:
(B) MA × E = L
Mechanical Advantage (MA) = \(\frac{\text { Load (L) }}{\text { Effort (E) }}\)
⇒ MA × E = L

Question 26.
A woman draws water from a well using a fixed pulley. The mass of the bucket and the water together is 10 kg. The force applied by the woman is 200 N. The mechanical advantage is ( Take, g = 10 m/s²)
(A) 2
(B) 20
(C) 0.05
(D) 0.5
Answer:
(D) 0.5
Given, mass of bucket and water (m) = 10 kg
Force applied (F) = 200 N
Mechanical Advantage (MA) = \(\frac{\text { Load (L) }}{\text { Effort (E) }}\)
= \(\frac{10 \times 10}{200}\)
= \(\frac{1}{2}\) = 0.5

Question 27.
Which one of the following statement is correct?
(A) A machine is used to have more output energy as compared to input energy.
(B) Mechanical advantage of a machine can never be greater than 1.
(C) If a machine gives convenience of direction, then its mechanical advantage should be greater than 1.
(D) For a given design of a machine, increasing the effort arm increases its mechanical advantage.
Answer:
(D) For a given design of a machine, increasing the effort arm increases its mechanical advantage.
For a given lever,
mechanical advantage = \(\frac{\text { effort arm }}{\text { load arm }}\).
If the effort arm is increased while keeping the load arm fixed, the mechanical advantage increases.

Question 28.
A lever for which mechanical advantage is less than 1 has
(A) fulcrum at the mid-point between load and effort.
(B) load between effort and fulcrum.
(C) effort between fulcrum and load.
(D) load and effort acting at the same point.
Answer:
(C) effort between fulcrum and load.
The mechanical advantage of a lever is less than 1, when its effort arm is less than load arm. i.e. The effort is between fulcrum and load.

Question 29.
Class II lever are designed to have
(A) load situated between effort and fulcrum
(B) effort situated load and fulcrum
(C) MA > 1
(D) MA < 1
Answer:
(C) MA > 1
In class II lever, the load is always situated between the fulcrum and effort and the effort arm is always longer than the load arm, so its Mechanical Advantage (MA) is always more than 1 i.e. MA > 1.

Question 30.
The diagram below shows the balanced position of a meter scale.

Which one of the following diagrams shows the correct position of the scale, when it is supported at the centre? ‘
(A)

(B)

(C)

(D)
Answer:
(A)

According to principle of moments,
40 × m₁ = 60 × m₂
\(\frac{m_1}{m_2}=\frac{3}{2}\)
Hence, m₁ > m₁.
Thus, the meter scale will tilt towards m₁ (left) side when supported at the centre.
Hence, option (A) is correct.

Question 31.
If the centre of gravity of a metre scale of mass 80 g lies at the 45 cm mark, then which one of the following diagrams will show the balanced position of the scale?
(A)
(B)
(C)
(D)
Answer:
(A)

Given, mass (m) = 80 g
Centre of gravity (CG) = 45 cm

From the principle of moments, we have
mg (60 – 50) = mg (100 – 60)
⇒ mg × 10 = 10g × 40
⇒ m = 40 gf
Hence, figure (A) represent the balanced position of the scale.

Question 32.
A single fixed pulley is used because it
(A) has mechanical advantage greater than 1.
(B) reduces the load to be lifted.
(C) can be used as a force multiplier.
(D) helps to apply the effort in a convenient direction.
Answer:
(D) helps to apply the effort in a convenient direction.
A single fixed pulley is used because it helps to apply the effort in a convenient direction.
Its mechanical advantage is 1.

Question 33.
In case of negative work, the angle between the force and displacement is [NCERT Exemplar]
(A) 0
(B) 45°
(C) 90°
(D) 180°
Answer:
(D) 180°
In case of negative work, force and displacement are at an angle of 180°.

Question 34.
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 ms^-2) [NCERT Exemplar]
(A) 6 × 10³ J
(B) 6 J
(C) 0.6 J
(D) zero
Answer:
(D) zero
Work done against gravity will be zero as the displacement is at right angle with the gravitational force.

Question 35.
Mukesh drops a ball of 200 g from a tower of height 20 m. What will be its kinetic energy at the height of 5 m? (g = 10 m/s²)
(A) 30 J
(B) 50 J
(C) 60 J
(D) 65 J
Answer:
(A) 30 J
Given,
Mass of the ball, m = 200 g = 0.2 kg
Initial height, h₁ = 20 m
Height at which KE is to be found, h₂ = 5 m
Acceleration due to gravity, g = 10 m/s²
When a body falls freely, the loss in potential energy equals the gain in kinetic energy.
KE = mg (h₁ – h₂)
Substituting values
KE = 0.2 × 10 × (20 – 5)
KE = 0.2 × 10 × 15
= 0.2 × 150 = 30 J
Therefore, the kinetic energy of the ball at a height of 5 m is 30 J.

Question 36.
A car of mass 1000 kg is moving with a constant velocity of 20 m/s. The driver applies brakes and brings it to rest in 5 s. Find the work done by the brakes.
(A) + 2 × 10⁵ J
(B) 1 × 10⁵ J
(C) – 2 × 10⁵ J
(D) – 1 × 10⁴ J
Answer:
(C) – 2 × 10⁵ J
Given, m = 1000 kg,
u = 20 m/s,
v = 0
Work done by brakes = Loss in kinetic energy
W = ½ m (v² – u²)
W = ½ × 1000 × (0 – 20²) = – 200000 J
Negative sign shows that work is done against the direction of motion.
Hence, correct option is (C) – 2 × 10⁵ J.

Question 37.
Which of the following is unit of power?
(A) kilowatt-hour
(B) horsepower
(C) watt hour
(D) All of these
Answer:
(B) horsepower
Horsepower (hp) is unit of power. Kilowatt-hour and watt-hour are the units of energy consumed.

Question 38.
A fulcrum divides a crow-bar in the ratio of 3 : 1. What weight will be lifted, if an effort of 100 N is applied at the end of its longer arm?
(A) 400 N
(B) 100 N
(C) 300 N
(D) 500 N
Answer:
(C) 300 N
As we know,
Load × Load arm = Effort × Effort arm
Let load arm =x
As, given, effort =100 N,
effort arm = 3x
Hence, 3x × 100 = x × load
Load = \(\frac{{3 x} \times 1000}{x}\) = 300 N

Question 39.
The MA of sugar tongs is
(A) < 1
(B) = 1
(C) > 1
(D) None of these
Answer:
(A) < 1
Since we know that,
MA = \(\frac{\text { Load }}{\text { Effort }}\)
In case of sugar tongs, its a III class lever, effort lies between fulcrum and load, hence effort arm is always shorter than load arm.
And Mechanical advantage MA = \(\frac{\text { Effort arm }}{\text { Load arm }}\)
Hence, MA < 1.

Question 40.
If FA = 40 cm, AB = 60 cm, find MA of the lever.
(A) 1
(B) 1.5
(C) 2
(D) 2.5
Answer:
(D) 2.5
Given, load L = 100 N,
FA = 40 cm (load arm),
AB = 60 cm

Hence, FB = FA + AB
= 40 + 60
= 100 cm (effort arm)
As, we know,
load × load arm = effort × effort arm
100 × 40 = E × 100
⇒ E = 40 N
Hence, mechanical advantage = \(\frac{\text { Load }}{\text { Effort }}\)
= \(\frac{100}{40}\) = 2.5

Work Energy and Simple Machines Class 9 Assertion and Reason Questions

Directions (Question Nos. 1 – 15): In each of the following questions, a statement of Assertion is given by the corresponding statement of Reason. Of the statements, mark the correct answer as
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(B) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
(C) If Assertion is true, but Reason is false.
(D) If Assertion is false, but Reason is true.

Question 1.
Assertion (A): When the force opposes the motion of a body, the work done is zero.
Reason (R): The angle between the force and displacement is 180°.
Answer:
(D) If Assertion is false, but Reason is true.
The work done is negative, when force opposes the motion of a body and the angle between them is 180°.

Question 2.
Assertion (A): A moving hammer drives a nail into the wall of house.
Reason (R): Work done in driving the nail is positive.
Answer:
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
The nail displaces in the direction of force applied. So, the work done is positive.

Question 3.
Assertion (A): A person driving the motorcycle increases the speed on approaching a hilly road.
Reason (R): To provide more potential energy to the motorcycle. So, that it may go up against gravity of earth.
Answer:
(C) If Assertion is true, but Reason is false.
The person increases the speed, to provide more kinetic energy to the motorcycle, which gets converted to potential energy as it drives up on hilly road.

Question 4.
Assertion (A): The kinetic energy of a body with any reference, must always be positive.
Reason (R): The potential energy of a body can be positive/negative with the chosen reference.
Answer:
(B) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
Expression for kinetic energy,
KE = ½ m²
m and v² can’t be negative in any reference.
Potential energy depends on the chosen reference level. So, it can be positive or negative.

Question 5.
Assertion (A): Total energy of freely falling body is constant at each point.
Reason (R): Kinetic energy of freely falling body is minimum, when it reaches at ground.
Answer:
(C) If Assertion is true, but Reason is false.
According to law of conservation of energy, total energy of freely falling body remains conserved. Kinetic energy is maximum when body reaches at ground. Hence, Assertion is true, but Reason is false.

Question 6.
Assertion (A): Electric motor converts electrical energy into mechanical energy.
Reason (R): Mechanical energy is equal to kinetic energy.
Answer:
(C) If Assertion is true, but Reason is false.
Electric motor converts electrical energy into mechanical energy. Mechanical energy is equal to sum of kinetic energy and potential energy.
Hence, Assertion is true but Reason is false.

Question 7.
Assertion (A): A truck and a bus does the same amount of work in 40 s and 60 s respectively. Truck is more powerful than the bus.
Reason (R): Power = \(\frac{\text { Work done }}{\text { Time taken }}\)
Answer:
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Power ∝ \(\frac{1}{\text { Time taken }}\)
(As work done is same in both cases)
i. e., truck is more powerful than bus.

Question 8.
Assertion (A): An electric bulb consumes 200 J of electrical energy in 10 s. Then, its power is 20 W.
Reason (R): To get power, energy consumed is divided by the time taken.
Answer:
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Power of bulb = \(\frac{\text { (Energy consumed) }}{\text { (Time taken) }}\)
P = \(\frac{200}{10}\) = 20 W.

Question 9.
Assertion (A): For class I lever the mechanical advantage can have any value either greater than 1 or equal to 1 or less than 1.
Reason (R): In class I lever, fulcrum lies in between the load and the effort.
Answer:
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Mechanical advantage,
MA = \(\frac{\text { Effort arm (L) }}{\text { Load arm (E) }}\)

In a class 1 lever, the fulcrum’s position can vary. Changing the position changes the lengths of the arms, thus changing the mechanical advantage.

Question 10.
Assertion (A): Nut cracker is a class III lever.
Reason (R): In nut cracker, the load is situated between fulcrum and effort.
Answer:
(D) Assertion is false, but Reason is true.
A nut cracker is a class II lever and the load is placed between the fulcrum and the effort. Thus, A is false but R is true.

Question 11.
Assertion (A): Inclined plane is a simple machine.
Reason (R): It changes only the direction of force.
Answer:
(C) Assertion is true, but Reason is false.
Inclined plane reduces effort, not mainly changes direction.

Question 12.
Assertion (A): Work done by a force is zero only when displacement produced by the force is zero.
Reason (R): Work done by a force is negative, when displacement occurs in the opposite direction of applied force.
Answer:
(D) Assertion is false, but Reason is true.
Work done by a force zero not only when displacement is zero, but also when the angle between force and displacement is 90°. Work is negative when the displacement and force applied are in opposite directions.
Hence, assertion is false and reason is true.

Question 13.
Assertion (A): In a stretched bow, potential energy is stored.
Reason (R): Mechanical energy of a moving body remains conserved.
Answer:
(B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
In a stretched bow, potential energy is stored in the bent limbs (elastic part) of the bow. According to law of conservation of energy, mechanical energy of a moving body always remains conserved.
Hence, assertion and reason both are true but reason is not the explanation of assertion.

Question 14.
Assertion (A): Steam engine converts heat energy into kinetic energy.
Reason (R): Steam engine works on the principle of conservation of energy.
Answer:
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Steam engine works on the principle of conservation of energy as it converts heat energy into kinetic energy.

Question 15.
Assertion (A): The weight of single pulley itself does not affect its mechanical advantage.
Reason (R): For single fixed pulley, the effort and load are equal in magnitude. So, its mechanical advantage is 1.
Answer:
(A) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
A single fixed pulley only changes direction of force.
Since effort equals load, MA = 1, regardless of the pulley’s own weight.