Question

Select the incorrect statement about friction:

• A. Static friction force is always equal to μN, where μ is co-efficient of static friction and N is normal force.
• B. Friction is a non-conservative force.
• C. Friction arises from electro-magnetic force.
• D. Friction always opposes relative motion between two surfaces.
• E. Maximum value of static friction is μN, where μ is co-efficient of static friction N is normal force.

Answer 1

The Correct Option is A

Given:

• Five statements (A-E) about friction.

Step 1: Analyze Each Statement

(A) Static friction force is always equal to \(\mu N\).

Incorrect. The static friction force is only equal to \(\mu N\) at its maximum value (when motion is impending). For other cases, static friction adjusts to match the applied force up to this maximum.

(B) Friction is a non-conservative force.

Correct. Friction dissipates energy as heat and depends on the path taken.

(C) Friction arises from electromagnetic force.

Correct. Friction results from microscopic interactions between surface atoms, which are electromagnetic in nature.

(D) Friction always opposes relative motion.

Correct. Kinetic friction opposes sliding, while static friction opposes the tendency to slide.

(E) Maximum static friction is \(\mu N\).

Correct. This is the definition of the maximum static friction force.

Conclusion:

The incorrect statement is (A), as static friction equals \(\mu N\) only at its maximum.

Answer: \(\boxed{A}\)

Answer 2

Let's examine each statement about friction:

(A) Static friction force is always equal to μN, where μ is co-efficient of static friction and N is normal force.

This statement is INCORRECT. The static friction force is equal to μN only at its maximum value, just before the object starts to move. Before that point, the static friction force is equal and opposite to the applied force, and can range from 0 to μN.

(B) Friction is a non-conservative force.

This statement is CORRECT. Friction converts mechanical energy into thermal energy (heat), which is dissipated into the environment and cannot be fully recovered. This energy loss makes friction a non-conservative force.

(C) Friction arises from electro-magnetic force.

This statement is CORRECT. Friction is ultimately caused by electromagnetic interactions between the atoms and molecules at the surfaces in contact.

(D) Friction always opposes relative motion between two surfaces.

This statement is CORRECT. Friction acts in the direction opposite to the direction of relative motion (or the impending direction of relative motion in the case of static friction).

(E) Maximum value of static friction is μN, where μ is co-efficient of static friction and N is normal force.

This statement is CORRECT. The maximum force of static friction that can be exerted before an object starts to move is given by μN.

Final Answer: The incorrect statement is \(\boxed{A}\)