CBSE Class 11 Physics Newton laws of Motion Assignment

Comprehension-1

Newton’s Laws for a System of Particles
The first and second laws of motion for a system of particles are modified as:
First Law: The centre of mass of an isolated system is at rest or moves with constant velocity.
Second Law: The neT external force acting on a system of total mass M is related to the acceleration of centre of mass of the system, i.e., F_ext = M . a_cm.

1. A non zero external force acts on a system of particles. The velocity and the acceleration of the centre of mass are found to be vc and ac at any instant t. It is possible that
(a) v_c = 0, a_c = 0 (b) v_c = 0, a_c ≠ 0 (c) v_c ≠ 0, a_c = 0 (d) v_c ≠ 0, a_c ≠ 0

2. Two particles of masses m₁ and m₂ (m₁ > m₂) attract each other with a force inversely proportional to the square of the distance between them. The particles are initially held at rest and then released. Which one is correct
(a) the CM moves towards m₁
(b) the CM moves towards m₂
(c) the CM remains at rest
(d) CM moves at right angles to the line joining m₁ & m₂.

Comprehension-2

It is found that when two surface comes in contact with each other contact force comes into effect i.e., one surface applies contact force on the other surfaces. This force is adjustable in nature and tries to oppose the relative motion between the contact surfaces. It is found that force depends on contact area (A), relative velocity (v) between the surfaces, such that f = CAv. Where C is a positive constant. (C = 1 N/m³ sec)

3. A cube of side length l is placed on a plane surface and given velocity v0 along the plane such that one of its face is perpendicular to its velocity. The distance travelled by cube, till its velocity becomes half of the initial velocity is (take mass of cube is 1 kg)
(a) l. ln2 (b) 2ln/ l² (c) v₀/2l (d) V₀/2l²

4. Velocity of cube will become zero in the time interval
(a) 2ln/l² (b) 2l/V₀  (c) infinite (d) data is insufficient.

5. Contact force between the two surfaces depends on
(a) Normal force between the surfaces (b) Area of contact of surface
(c) relative velocity between the body (d) none of these

Each of the questions given below consists of two statements, an assertion (A) and reason (R).
Select the number corresponding to the appropriate alternative as follows
(a) If both A and R are true and R is the correct explanation of A
(b) If both A and R are true but R is not the correct explanation of A
(c) If A is true but R is false
(d) If A is false but R is true

6. A: If a book is placed on table at rest then force exerted by table on the book and the weight of the book form action reaction pairs according to Newton 3rd law of motion
R: Since both are equal in magnitude and opposite in directions.

7. A: The mass of a body can be regarded as a quantitative measure of the resistance of a body to acceleration by a given force
R: The acceleration produced by a given force is inversely proportional to mass being accelerated.

8. A: While conserving the linear momentum of the system we must specify the reference frame.
R: Like velocity, momentum also depends on the reference from of observer.

9. A: The law of conservation of linear momentum, as applied to a single particle, is equivalent to Newton’s first law of motion.
R: As Newton’s first law states in the absence of external force state of motion of a body does not change.

10. A: The impulse of a force can be zero even if force is not zero.
R: The impulse of a force is equal to change in momentum of a body.

11. A: Frictional forces are non-conservative forces.
R: Potential energy cannot be associated with frictional forces.

12. A: The driver in a vehicle moving with a constant speed on a straight road is in a non inertial frame of reference.
R: A reference frame in which Newton’s law are applicable is non-inerted.

13. A. Direction of force acting on a body and direction of velocity of the body are always same.
R. Velocity is due to acceleration, produced by a force, and the direction of velocity is always same as that of acceleration.

14. A. When a person walks on a rough surface, the frictional force exerted by the surface on the person is in the direction of his motion.
R. Frictional forces opposes the motion of a body

15. A. Coefficient of friction is always less than 1.
R. The surfaces, in general, are so smooth that coefficient of friction is always less than 1.

16. A. It is easier to pull a lawn-mover than to push it on a ground.
R. Frictional force is smaller for pulling action than for pushing action.

17. A. Work done by frictional force is negative.
R. Frictional force is a conservative force.

18. A. A person feels lighter in a lift falling with uniform acceleration.
R. Apparent weight of a body in a lift moving down with uniform acceleration is greater than its real weight.

Miscellaneous Assignment

1. (c)     2. (c)     3. (d)     4. (c)     5. (b)    6. (d)   7. (a)   8. (a)   9. (a)   10. (d)    11. (a)   12. (d)   13. (d)   14. (a)   15. (d)  16. (a)  17. (c)  18. (c)