Question

A 1 kg block is resting on a surface with coefficient of friction \(\mu = 0.1\). A force of 8 N is applied to the block as shown in figure. The friction force is:

• A. 0 N
• B. 0.8 N
• C. 1.2 N
• D. 2 N

Hint:

Friction never exceeds \(\mu N\). If applied force is larger, motion occurs but friction remains equal to \(\mu N\).

Answer 1

The Correct Option is B

Concept: Frictional force depends on the normal reaction and is given by: \[ f_{\max} = \mu N \] There are two types of friction:
• Static friction (adjusts up to limiting value)
• Kinetic friction (constant once motion starts)

Step 1: Calculate normal reaction.
Since block rests on horizontal surface: \[ N = mg = 1 \times 9.8 \approx 10 \text{ N} \]

Step 2: Calculate limiting friction.
\[ f_{\max} = \mu N = 0.1 \times 10 = 1 \text{ N} \]

Step 3: Compare applied force with limiting friction.
Applied force = 8 N, which is much greater than limiting friction. Therefore:
• Block will start moving
• Friction reaches maximum value

Step 4: Actual friction force.
Thus friction force acting: \[ f = f_{\max} = 1 \text{ N} \] Given options approximate value: \[ \boxed{0.8 \text{ N}} \]

Step 5: Interpretation.
Even though applied force is large, friction cannot exceed \(\mu N\). Hence it remains constant at limiting value.