Assertion (A): A block of weight \(10 \text{ N}\) is pushed against a vertical wall by a horizontal force of \(15 \text{ N}\). The coefficient of friction between the wall and the block is \(0.6\). Then the magnitude of maximum frictional force is \(9 \text{ N}\).
Reason (R): For given system block will remain stationary.
1. (1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. (2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (3) (A) is true but (R) is false
4. (4) Both (A) and (R) are false
View Answer
Normal force \(N = 15 \text{ N}\). Maximum static friction \(f_{\text{max}} = \mu_{\text{s}} N = 0.6 \times 15 = 9 \text{ N}\). So (A) is true. Weight of block is \(W = 10 \text{ N}\). Since \(W > f_{\text{max}}\), the block will slide down. So (R) is false.
Assertion (A): Due to frictional force acting on a body, the body is always retarded by friction.
Reason (R): Friction force opposes the motion of object.
1. (1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. (2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (3) (A) is true but (R) is false
4. (4) (A) is false but (R) is true
View Answer
Friction can cause motion (e.g., walking, car acceleration), so (A) is false. Friction opposes *relative* motion, not necessarily the overall motion of the object. Hence, (R) is also false.
Assertion (A): A man standing in a lift which is moving upward, will feel his weight to be greater than when the lift was at rest.
Reason (R): If the acceleration of the lift is ‘a’ upward, then the man of mass m shall feel his weight to be equal to normal reaction (N) exerted by the lift given by \(N = m(g – a)\) (where \(g\) is acceleration due to gravity)
1. (1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. (2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (3) (A) is true but (R) is false
4. (4) Both (A) and (R) are false
View Answer
When a lift accelerates upwards, apparent weight is \(N = m(g + a)\), which is greater than \(mg\). So (A) is true. Reason (R) provides the formula for downward acceleration, thus (R) is false.
A man of mass \(80 \text{ kg}\) pushes a box of mass \(20 \text{ kg}\) horizontally. The man moves the box with a constant acceleration of \(2 \text{ m/s}^2\) but his foot does not slip on the ground. There is no friction between the box and the ground, whereas there is sufficient friction between the man’s foot and the ground to prevent him from slipping.
Assertion (A): The force applied by the man on the box is equal and opposite to the force applied by the box on the man.
Reason (R): Friction force applied by the ground on the man is \(200 \text{ N}\).
1. (1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. (2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (3) (A) is true but (R) is false
4. (4) Both (A) and (R) are false
View Answer
Assertion (A) is Newton's Third Law, which is true. For the box: \(F_{\text{man-box}} = m_{\text{box}}a = 20 \text{ kg} \times 2 \text{ m/s}^2 = 40 \text{ N}\). By action-reaction, \(F_{\text{box-on-man}} = 40 \text{ N}\). For the man: \(F_{\text{friction}} - F_{\text{box-on-man}} = m_{\text{man}}a). So, \(F_{\text{friction}} - 40 \text{ N} = 80 \text{ kg} \times 2 \text{ m/s}^2 = 160 \text{ N}\). Thus, \(F_{\text{friction}} = 200 \text{ N}\). So (R) is true. But (R) does not explain (A).
Assertion (A): When a man climbs the rope friction force acts in downward direction.
Reason (R): Friction force opposes relative motion or tendency of relative motion between two contact surfaces.
1. (1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. (2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (3) (A) is true but (R) is false
4. (4) Both (A) and (R) are false
View Answer
When a man climbs up, his hands exert a downward friction force on the rope. So (A) is true (referring to friction on the rope). (R) is the correct definition of friction and explains why this force acts.
If a cloth covers a table and some dishes are kept on it, then the cloth can be pulled out without dislodging the dishes from the table by pulling the cloth suddenly. The dishes will remain on table due to:
1. Work-energy theorem
2. For every action, there is an equal and opposite reaction
3. Inertia of motion
4. Inertia of rest
View Answer
According to Newton's first law of motion, the dishes tend to remain at rest due to their inertia of rest when the cloth is suddenly pulled.
Read the statements marked as assertion (A) and reason (R) and choose the correct option.
Assertion (A): If no external force acts on a system, the velocity of the centre of mass remains constant.
Reason (R): If there is no external force on system, then momentum of system is conserved.
1. Both (A) and (R) are true and (R) is the correct explanation of (A)
2. Both (A) and (R) are true but (R) is not the correct explanation of (A)
3. (A) is true but (R) is false
4. (A) is false but (R) is true
View Answer
When no external force acts on the system, the acceleration of the center of mass is zero, hence its velocity is constant. This also implies the conservation of total linear momentum of the system.