Assertion (A): The contact force is the net force applied by the surface on the body kept on it.
Reason (R): When a body is at rest on a horizontal surface then the contact force on the body by the surface must be equal to the weight of body.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: contact force is the resultant of normal and friction forces from the surface. Reason (R) is true: for a body at rest on a horizontal surface, contact force magnitude is equal to \(mg\). (R) is a specific case, not a general explanation of (A)'s definition.
Assertion (A): A block is lying at rest on horizontal rough surface. A person moving with acceleration \(a\) in forward direction will observe a friction force acting on the block.
Reason (R): When there is relative motion between the two surface then only static friction acts between them.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
In an accelerating (non-inertial) frame, a pseudo-force acts on the block, requiring a forward friction force. So, A is true. Static friction acts when there is no relative motion; kinetic friction acts when there is relative motion. Thus, R is false.
Assertion (A): An insect is climbing up a vertical wall with constant speed then the force applied by the wall on the insect is vertically upwards and equal to its weight.
Reason (R): Friction is a self-adjusting force.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
For an insect climbing at constant speed, the upward friction force component from the wall must balance its weight. Static friction is indeed a self-adjusting force. Both A and R are true statements, and R explains why the friction force can adjust to balance the weight.
Assertion (A): A body is lying at rest on a rough horizontal surface. A person accelerating with acceleration \(a\) (where \(a\) is positive constant and \(\hat{i}\) is a unit vector in horizontal direction) observes the body. With respect to him, the block experiences a kinetic friction.
Reason (R): There is relative motion between the block and surface in person’s frame of reference.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
In the accelerating person's non-inertial frame, a pseudo-force acts on the block. If this pseudo-force causes relative motion, kinetic friction acts. A is true. Kinetic friction by definition arises due to relative motion between surfaces. So, R is true and correctly explains A.
Assertion (A): The acceleration of a body moving down on a rough inclined plane is greater than the acceleration due to gravity.
Reason (R): The body is able to slide on an inclined plane only when its acceleration is greater than acceleration due to gravity.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is false. The acceleration of a body on a rough inclined plane is \(a = g(\sin\theta - \mu\cos\theta)\), which is always less than \(g\).
Reason (R) is also false. A body slides when the component of gravity along the plane overcomes static friction, not when its acceleration is greater than \(g\). Thus, both (A) and (R) are false.
Assertion (A): A man starts walking towards west. Friction force on him acts towards east.
Reason (R): Friction opposes relative motion.
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 false; when walking west, the static friction force on the man acts west, in the direction of motion. Reason (R) is true as friction fundamentally opposes relative motion. Thus, (A) is false and (R) is true. Option (4) is selected as it correctly identifies (A) as false.
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.
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.