Question

The coefficient of friction between object and substance, if we need to move an object of weight \(150\ \text{N}\) on a horizontal surface with a force of \(75\ \text{N}\), is

• A. \(0.8\)
• B. \(0.5\)
• C. \(0.7\)
• D. \(0.9\)

Hint:

For a body on a horizontal surface: \[ \text{Limiting friction}= \mu N \] and \[ N=W \] Always use the normal reaction while calculating frictional force.

Answer 1

The Correct Option is B

Step 1: Understand the physics of threshold motion on a horizontal surface.
To move an object at the threshold (just starts to slide), the applied force equals the limiting static friction. At this point, the net horizontal force is zero.
Step 2: Identify forces acting on the object.
Weight $W = 150$ N acts downward; normal reaction $N$ acts upward. On a horizontal surface: $N = W = 150$ N.
Step 3: Write the friction equation.
Limiting friction: $F = \mu N$. Given $F = 75$ N.
Step 4: Solve for $\mu$.
\[ \mu = \frac{F}{N} = \frac{75}{150} = 0.5 \]
Step 5: Dimensional check and reasonableness.
$\mu$ is dimensionless. The value 0.5 is physically typical for many solid-on-solid surfaces.
Step 6: State the final answer.
\[ \boxed{\mu = 0.5} \]