Students can practice the best Class 9 Science MCQ and Class 9 Science Exploration Chapter 6 MCQ Online Test with Answers, How Forces Affect Motion for exam preparation.
Class 9 Science Chapter 6 How Forces Affect Motion MCQ
How Forces Affect Motion MCQ
Class 9 Science Chapter 6 MCQ Online Test
Question 1.
Luggage at the top of a bus are tied using a string to avoid the effect of
A. impulse
B. inertia
C. force
D. None of these
Answer:
(B) Due to inertia of motion, body resists any change in its state of uniform motion. If not tied and driver applies brakes, luggage will drop from the top of bus.
Question 2.
Which of the following statement is not correct for an object moving along a straight path in accelerated motion? (NCERT Exemplar)
A. Its speed keep changing.
B. Its velocity always changes.
C. It always goes away from the earth.
D. A force is always acting on it.
Answer:
(C) For an object moving along a straight path in an accelerated motion, it is not necessary that it always goes away from the Earth.
Question 3.
A water tanker filled upto of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would (Competency Based Question)
A. Move backward
B. Move forward
C. be unaffected
D. rise upwards
Answer:
(B) On the sudden application of brake, the tanker will come in the state of rest hut the water remains in the state of motion, so the water will move forward.
Question 4.
What is the momentum of an object of mass m moving with a velocity y?
A. m2v2
B. mv2
C. \(\frac{1}{2}\) mv2
D. mv
Answer:
(D) The momentum of an object of mass m moving with a velocity y is given by P = mv.
Question 5.
An object of mass 2 kg is sliding with a constant velocity of 4 ms-1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is (NCERT Exemplar)
A. 32 N
B. 0 N
C. 2 N
D. 8 N
Answer:
(B) An object is sliding with a constant velocity on a frictionless horizontal table, the object vill move at constant velocity without any resistance in the state of uniform motion. Force required will be 0 N.
Question 6.
A bullock-cart carrying a block of wood moves on a rough road with frictional force F with constant speed 4 m/s. The cart can he kept moving with constant speed by applying a for (Competency Based Question)
A. 2F
B. \( \frac{F}{2}\)
C. F
D. 0
Answer:
(C) An external force of equal magnitude as friction il force would be required to keep the cart moving with constant speed and the direction of external force will be in opposite direction to frictional force.
Question 7.
Two blocks of masses \(\frac{M}{2} \) and \(\frac{M}{3} \) are shown in the figure, a force of 20 N is applied to right and blocks starts to move with an acceleration 2 m/s2. The value of M will be (Competency-Based Question)
A. 10 kg
B. 14 kg
C. 12 kg
D. 16 kg
Answer:
(C) From second law of motion
F =Ma
Given, F = 20N, a = 2m/s2
20 = M × 2 ⇒ M = 10 kg
M = \( \frac{M}{3}+\frac{M}{2}=\frac{5 M}{6}\) = 10
M= \(\frac{60}{5}\) =12 kg
Question 8.
Two forces F1 and F2 are acting on a body as shown in the figure, then acceleration in the body is Competency Based Que.
A. 23m/s2
B. 3 m/s2
C. 2 m/s2
D. 22 m/s2
Answer:
(B) Resultant force on a body,
F = F2 – F1 = 65 – 50 = 15 N
By Newton’s second law of motion,
F = ma ⇒ 15 = 5a ⇒ a = 3 m/s2
Question 9.
A bullet of 20 g strikes a sand bag at a speed of 200 m/s and gets embedded after travelling 2 cm, then the resistive force exerted by the sand on the bullet is (Competency Based Question)
A. 2 × 103N
B. 2 × 104N
C. 2 × 106N
D. 2 × 105N
Answer:
(B) Given, u = 200 m/s, v =0, s = 0.02 m and m = 20 g = 20 × 10-3kg
From third equation of motion,
v2 =u2 +2 as ⇒ a = \(\frac{v^2-u^2}{2 s} \)
= \(\frac{(0)^2-(200)^2}{2 \times 0.02}\) = -106 m/s2
Negative sign indicates retardation.
∴ Resistive force, F = ma = 20 × 10-3 × 106 = 2 × 104 N
Question 10.
A truck moving with a speed of 54 km/h. Truck driver applied brakes suddenly and brings the truck to rest in 5 s, then the average retarding force on truck, if mass of the truck and driver is 400 kg, will be
A. 1200 N
B. 600N
C. 800N
D. 500N
Answer:
(A) Given, u = 54 km/h = 54 × \(\frac{5}{18}\) m/s =15 m/s,
v = 0 and t = 5s
∴ Retardation, a = \( \frac{v-u}{t}=\frac{0-15}{5}\) = —3 m/s2
Negative sign shows retardation.
Average retarding force, F = ma = 400 × 3=1200 N
Question 11.
A bus of mass 500 kg is moving with a velocity of 5 m/s and is acted upon by a forward force of 500 N due to engine and retarding force of 200 N due to friction. Velocity of bus after 20 s will be
A. 15 m/s
B.17 m/s
C. 19 m/s
D. 21 m/s
Answer:
(B) Given, m = 500 kg, u = 5 m/s and t = 20 s
Resultant force, F = 500 – 200 = 300 N
∴ Acceleration, a = \(\frac{F}{m}=\frac{300}{500} \) = 0.6 m/s
By equation of motion, v = u + at = 5 + 0.6 × 20 = 17 m/s
Question 12.
A force acts on a body of mass 5 kg and changes its velocity from 8 m/s to 12 m/s in 4 s, then magnitude of force is
A. 8N
B. 4N
C. 5N
D. 6N
Answer:
(C) According to Newton’s second law of motion,
Force = Rate of change in momentum
F = \(\frac{m(v-u)}{t}=\frac{5(12-8)}{4} \) = 5 N
Question 13.
Which of the following statements is not correct?
A. Momentum is a vector quantity.
B. Magnitude of force F is a scalar quantity.
C. Inertia of direction is to resist any change in state of uniform motion.
D. Frictional force can be larger/smaller than external force.
Answer:
(C) Inertia of direction is to resist any change in its direction of motion.
Question 14.
A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. Why does the ball slows down to stop?
A. The batsman did not hit the ball hard enough.
B. Velocity is proportional to the force exerted on the ball.
C. There is a force on the ball opposing the motion.
D. There is no unbalanced force on the ball, so the ball would want to come to rest.
Answer:
(C) The ball slows down to stop because the force of friction acting between the ground and the ball acts as an external force which opposes the motion of the ball.
Question 15.
Impulse is equal to
A. Rate of change in momentum
B. rate of change in folLe
C. Change in reaction force
D. Change in momentum
Answer:
D. Change in momentum
Question 16.
A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to (Competency-Based Question)
A. Exert a larger force on the ball.
B. Increase the force exerted by the ball on hands.
C. Increase the rate of change of momentum.
D. Decrease the rate of change of momentum.
Answer:
(D) From Newton’s second law of motion.
F = ma = m \(\frac{(v-u)}{t} \)
The goalkeeper pulls his hands backwards to increase the time of catching, to decrease the force and rate of change of momentum also.
Question 17.
According to the third law of motion, action and reaction (NCERT Exemplar)
A. always act on the same body.
B. Always act on different bodies in opposite directions.
C. Have the same magnitude and directions.
D. Act on either body at normal to each other.
Answer:
(B) According to third law of motion, action and reaction forces are equal in magnitude and opposite in direction. They always act on different objects.
Question 18.
A man pushes a block of wood (w1) to the left, which is attached to another block (w2). If the man is pushing with enough force such that the blocks accelerate to the left, what can be concluded about the magnitudes of the forces on the blocks? (Competency-Based Question)
A. The force on block 2 exerted by block 1 is smaller in magnitude than the force on w1 exerted by w2.
B. The force on w2 exerted by w1, is larger in magnitude than the force on w1 exerted by w2.
C. The force on w2 exerted by w1 is not dependent on the size of the block (w1 ) and vice versa.
D. The force on w2 exerted by w1 can be large/small depending on the size of the acceleration.
Answer:
(C) According to Newton’s third law of motion, force on w2 exerted by w1 and vice versa will be equal in magnitude and opposite in direction, acting on different blocks in the two cases.
Question 19.
Consider two spring balances hooked as shown in the figure below. We pull them in opposite directions. If the reading shown by A is 1.5 N, the reading shown by B will be (Competency-Based Question)
A. 1.5 N
B. 2.5 N
C. 3.0 N
D. zero
Answer:
(A) According to Newton’s third law, every action has equal and opposite reaction. Hence, force (action) exerted byA on B and force exerted by B on A reaction are equal, and reading shown by B will be 1.5 N in both the spring balance.
Question 20.
The inertia of an object tends to cause the object (NCERT Exemplar)
A. to increase its speed
B.to decrease its speed
C. to resist any change in its state of motion
D. to decelerate due to friction
Answer:
C. to resist any change in its state of motion
(C) The inertia of an object tends to cause the object to resist any change in its state of motion or rest.
Question 21.
An object of mass 2 kg is sliding with a constant velocity of 4 ms-1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
A. 32 N
B. 0 N
C. 2 N
D. 8 N
Answer:
B. 0 N
(B) Since the body is already moving with a constant velocity and there is no force present to resist its state of motion, there won’t be any requirement of external force to keep it moving.
Question 22.
The image shows the forces acting on a block. The length of the arrow shows the strength of the force, and the direction of the arrow shows the direction of application of the force. What is the direction of the resultant force?
A. Downwards to D
B. Upwards to C
C.Towards left to A
D. Towards right to B
Answer:
D. Towards right to B
(D) FC and FD are equal in magnitude but opposite in directìon so they will cancel out each other. FB is greater in magnitude than FA, and they are Opposite in direction, so their net resultant will be towards right to B.
Question 23.
Trolleys X and Y are joined with stretched spring. Trolley X has twice the mass of trolley Y.
When the trolleys are released, the acceleration of X is 2 m/s2 to the right. What is the initial acceleration of trolley Y to the left? (Competency Based Question)
A. 1 m/s2
B. 2 m/s2
C. 3 m/s2
D. 4 m/s2
Answer:
D. 4 m/s2
(D) By Newton’s third law of motion, both X and Y will exert equal and opposite force on each other. So the magnitudes of forces causing acceleration in both trolleys will be equal.
For constant force F a ∝ \( \frac{1}{m}\) i.e.,
aXMX = aY MY
⇒ 2MX = aY × \(\frac{M_X}{2} \) (∵ MX = 2 MY)
⇒ aY = 4 m/s2
Question 24.
Two Bodies have masses in the ratio 3: 4 when a force is applied on the first body, it moves with a acceleration of 6 m/s2. How much acceleration will the same force produce in the other body.
A. 3 m/s2
B. 4.5 m/s2
C. 15 m/s2
D. 45m/s2
Answer:
B. 4.5 m/s2
(B) As we know, if force applied is same, then
a1M1 = a2M2 ⇒ \(\frac{a_1}{a_2}=\frac{M_2}{M_1} \)
⇒ \(\frac{6}{a_2}=\frac{4}{3} \) (∵ \(\frac{M_1}{M_2}=\frac{3}{4} \) )
⇒ a2 = 4.5 m/s2
How Forces Affect Motion Class 9 Assertion and Reason Questions
Directions (Q.Nos. 1- 8) 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): If a balanced force is applied on a table, it will move.
Reason (R): A balanced force doesn’t change the state of rest or uniform motion.
Answer:
(D) When a unbalanced force is applied on table, then it will move. Balanced force can change the shape of object only.
Question 2.
Assertion (A): To keep moving with uniform velocity, a cycle has to be pedalled continuously.
Reason (R): To balance the force of friction due to ground on cycle, an external force in the form of pedaling is applied.
Answer:
(A) If pedalling of cycle is stopped, cycle will stop after some time due to friction acting in the opposite direction of motion.
Question 3.
Assertion (A) A 1 kg book has more inertia than that of 1 kg cotton.
Reason (R): Inertia depends upon the mass.
Answer:
(D) Both the 1 kg book and 1 kg of cotton will have same inertia as it depends upon the mass.
Question 4.
Assertion (A): Road accidents occurring due to high speeds are much worse than accidents due to low speeds of vehicles.
Reason (R): Momentum of high-speed vehicles is more than that of low-speed vehicles.
Answer:
(A) Road accidents due to high speeds are much worse than accidents due to low speeds of vehicles.
This is because the momentum of high-speed vehicles is more than that of the low-speed vehicles, as momentum is the product of mass and velocity.
Question 5.
Assertion (A): If two persons walking or running in opposite directions collide with each Other, then both feel hurt.
Reason (R): To every action, there is an equal and opposite reaction.
Answer:
(A) According to Newton’s third law of motion, every action has an equal and opposite reaction, but always act on two different bodies. Therefore, both persons feel hurt after collision.
Question 6.
Assertion (A): Rate of change of momentum of a body is equal to resultant force applied on the body.
Reason (R): Change in momentum is equal to impulse.
Answer:
(B) According to second law of motion, rate of change of momentum is equal to the resultant force applied on the body.
Impulse is defined as the change in momentum of a body when a force is applied for a very short duration of time. Hence, Assertion and Reason both are true, but Reason is not the correct explanation of Assertion.
Question 7.
Assertion (A): Newton’s third law of motion even apply to a system where bodies do not actually touch each other.
Reason (R): Action and reaction forces act on only first body.
Answer:
(C) Newton’s third law of motion applies to every system irrespective of presence of contact or non-contact forces. For example, two bodies attract each other by gravitational force of equal magnitude even if they are far apart. Action and reaction forces act on different bodies. Hence, Assertion is true, but Reason is false.
Question 8.
Assertion (A): We should wear a seat belt while driving a car.
Reason (R): Seat belts prevents us from falling forward due to inertia when sudden brakes are applied.
Answer:
(A) When sudden brakes are applied, the car stops suddenly, but due to inertia of motion, our body keeps moving forward, which can cause serious injuries. Seat belts prevents us from moving forward in such situations and keeps us safe from injuries. Hence, both Assertion and Reason are true, and Reason is correct explanation of Assertion.