CBSE Class 11 Laws of Motion Worksheet D

Assertion and Reason

Note: Read the Assertion (A) and Reason (R) carefully to mark the correct option out of the options given below:
a. If both assertion and reason are true and the reason is the correct explanation of the assertion.
b. If both assertion and reason are true but reason is not the correct explanation of the assertion.
c. If assertion is true but reason is false.
d. If the assertion and reason both are false.
e. If assertion is false but reason is true.

Question. Assertion: A truck moving on a horizontal surface with a uniform speed u is carrying sand. If a mass Δm of the sand ‘leaks’ from the truck in time Δt , the force needed to keep the truck moving at its uniform speed is u Δm/ Δt .
Reason: Force = rate of change of momentum.
Answer : D

Question. Assertion: A cloth covers a table. Some dishes are kept on it. The cloth can be pulled out without dislodging the dishes from the table.
Reason: For every action there is an equal and opposite reaction.
Answer : B

Question. Assertion: Linear momentum of a body changes even when it is moving uniformly in a circle.
Reason: Force required to move a body uniformly along a straight line is zero.
Answer : B

Question. Assertion: The maximum value of force F such that the block shown in fig. 5 does not move is m mg / cosθ , where μ is the coefficient of friction between the block and the horizontal surface.

Reason: Frictional force = coefficient of friction X normal reaction.
Answer : C

Question. Assertion: A rocket works on the principle of conservation of linear momentum.
Reason: Whenever there is a change in momentum of one body, the same change occurs in the momentum of second body of the same system but in opposite direction.
Answer : A

Question. Assertion: When a ball dropped from a certain height hits the floor, it exerts a force equal to the rate of change of momentum.
Reason: The floor does not move because the action and reaction forces, being equal and opposite, cancel each other.
Answer : D

Question. Assertion: No force is required by the body to remain in any value.
Reason: In uniform linear motion, acceleration has a finite value.
Answer : C

Question. Assertion: The work done in bringing a body down from the top to the base along a frictionless incline plane is the same as the work done in bringing it down the vertical side.
Reason: The gravitational force on the body along the inclined plane is the same as that along the vertical side.
Answer : C

Question. Assertion: Two blocks of masses m and M are placed on horizontal surface as shown in Fig.

The coefficient of friction between the two blocks is μ₁ and that between the block M and the horizontal surface is μ₂ . The maximum force that can be applied to block M so that the two blocks move without slipping is F = (μ₁ + μ₂) (M + m) g.
Reason: Maximum force = total mass X maximum acceleration.
Answer : C

Question. Assertion: A shell of mass m is at rest initally. It explodes into three fragments having masses in the ratio 2:2:1, The fragments having equal masses fly off along mutually perpendicular directions with speed v. The speed of the third (lighter) fragment will be 2√2v
Reason: The momentum of system of particles is conserved if no-external force acts on it.
Answer : C

Question. Assertion: Newton’s third law of motion is applicable only when bodies are in motion.
Reason: Newton’s third law applies to all types of forces, e.g., gravitational electric or magnetic forces, etc.
Answer : C

Question. Assertion: The acceleration produced by a force in the motion of a body depends only upon its mass.
Reason: Larger is the mass of the body, lesser will be the acceleration produced.
Answer : B

Question. Assertion: A body subjected to three concurrent forces cannot be in equilibrium.
Reason: If large number of concurrent forces are acting on the same point, then the point will be in equilibrium, if sum of all the forces is equal to zero.
Answer : E

Comprehension Based

Paragraph –I

A man of mass m climbs up the rope attached with a mass M as shown in figure passing over a pulley

Question. The acceleration of M when man is at rest on the rope is:
a. Mg/M + m
b. mg/M + m
c. (M - m)g/M + m
d. Zero
Answer : C

Question. If man climbs up the rope by acceleration a, then the tension in the rope is:
a. M mg/M + m
b. 2M mg/M + m
c. Mm(g + a)/M + m
d. Mm(g + a)/M + m
Answer : D

Question. If man climbs down the acceleration of system will be:
a. g − a
b. M(g - a)/M + m
c. m(g - a)/M + m
d. m(g + a)/M + m
Answer : A

Paragraph –II

A spring is attached between two masses 1 m and 2 m as shown in figure. Both masses are pulled by forces f₁ and f₂ where f₂ > f₁ .

Question. The system will move with acceleration a given by:
a. f₂ - f₁/m₁ + m₂
b. f₁ + f₂/m₁ + m₂
c. f₁ - f₂/m₁ + m₂
d. f₁ + f₂/m₁ - m₂
Answer : A

Question. The tension in the spring will be T equal to:
a. f₁ + f₂
b. f₁ - f₂
c. m₁f₂ + m₂f₁/m₁ + m₂
d. m₁f₂ - m₂f₁/m₁ - m₂
Answer : C

Question. If two masses are equal but pulling forces are different then tension in spring is:
a. independent of masses attached to spring
b. more is sum of masses of system more is tension
c. more is sum of masses of system less is tension
d. tension depends on nature of spring selected
Answer : A

Integer

Question. A Ship of mass 3×10⁷ kg initially at rest is pulled by a force of 5×10⁶ N through a distance of 3 m. If resistance due to water is negligible, the speed of the ship would be:
Answer : 1

Question. An open knife edge of mass 200 g is dropped from a height of 10 m on a cardboard. If the knife edge penetrates a distance of 4 m into the cardboard, average resistance offered by the cardboard to the knife edge is: (neglect air resistance and take g = 10 m/s²).
Answer : 5

Question. A block is released from top of a smooth inclined plane. It reaches the bottom of the plane in √2s ⋅ The time taken by the body to cover 1^st half of inclined plane is:
Answer : 1

Question. Two bodies of masses 3 kg and 2 kg are connected by a spring balance; fig. two forces of 10N and 5N are applied on the blocks as shown. The reading of the spring balance would be:

Answer : 7

Question. Three identical blocks each of mass 2 kg are drawn by a force of 10.2 N with an acceleration of 0.6 m/s² on a frictionless surface, fig. What is the tension in the string between blocks A and B?

Answer : 9