1.

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°. (b) The graph is a straight line parallel to the F-axis. (c) The graph is a straight line of slope 45° for small F and a straight line parallel to the F-axis for large F. (d) There is a small kink on the graph.

Answer»

(c) The graph is a straight line of slope 45° for small F and a straight line parallel to the F-axis for large F.
(d) There is a small kink on the graph.

Explanation: 

Until the applied horizontal force F is increased to a Limiting force the block does not move because this horizontal force is balanced by force of friction f. Up to this point the graph is a straight line of slope 45°. Beyond this point the block moves with a uniform velocity for even less than the Limiting force, at this point the graph has a small kink. Now even if the F is increased f remains constant, so now the graph gets parallel to F-axis. Hence options (c) and (d) are correct.



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