A block is placed on a rough surface and an external horizontal force ...
Explanation:
The force of friction f exerted by the floor on the block is measured for different values of F. The graph between F and f is given below.
The relationship between F and f can be explained by the following two cases:
Case 1: When F is small
When F is small, the force of friction f is equal to the product of the coefficient of static friction and the normal force between the block and the surface. Therefore, f ∝ N. As the normal force N is equal to the weight of the block, which is constant, the force of friction f is also constant and independent of the applied force F. Hence, the graph between F and f is a straight line parallel to the F-axis.
Case 2: When F is large
When F is large, the force of friction f is equal to the product of the coefficient of kinetic friction and the normal force between the block and the surface. Therefore, f ∝ N. As the normal force N is equal to the weight of the block, which is constant, the force of friction f is also constant and independent of the applied force F. Hence, the graph between F and f is a straight line parallel to the F-axis.
Therefore, the graph between F and f is a straight line inclined at 45o for small values of F and a straight line parallel to the F-axis for large F. This is because the force of friction changes from static friction to kinetic friction as the applied force increases beyond a certain value (known as the limiting friction). At the limiting friction, the block just begins to move, and the force of friction becomes kinetic friction.
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