Question

A block is placed on a rough floor and a horizontal force F is applied on it. The force of friction $f$ by the floor on the block is measured for different values of $F$ and a graph is plotted between them-
[a] The graph is a straight line of slope ${45}^o$
[b] The graph is straight line parallel to the F axis
[c] The graph is a straight line of slope ${45}^o$ for small F and a straight line parallel to the F-axis for large F.
[d] There is small kink on the graph

• A. c, d
• B. a, d
• C. a, b
• D. a, c

Answer 1

Answer: (A)

c, d

Till the applied force is less than the limiting friction, the friction force will be equal to the applied force. So up to limiting friction value, the graph of force of friction f by the floor on the block versus different values of F is a straight line of slope 45.

After applied force is greater than the limiting friction, the friction force will be constant (Constant Static Friction) on further increasing applied force.
At the limiting friction the coefficient of friction changes from static to kinetic and as the values are different so there will be kink (the kinetic friction force is less than the limiting friction force).