# The contact force exerted by a body A on another body is equal to the normal force between the bodies, we conclude that 1) surface must be smooth 2)force of friction between two bodies may be equal to zero 3) magnitude of normal reaction is equal to that of friction 4) bodies may be rough

$\begin{array}{1 1} 2\;and\;4 \\ 1\;and\;2 \\ 3\;and\;4 \\ 1\;and\;4 \end{array}$

When no frictional force acts between the bodies then normal reaction N is equal to the contact force(F)
The bodies may also be rough
Hence a is the correct answer.

edited Feb 10, 2014 by meena.p