A simple pendulum with a bob of mass m swings with an angular amplitude of $$40^\circ $$. When its angular displacement is $$20^\circ $$, the tension in the string is greater than mg cos$$20^\circ $$.

When a person walks on a rough surface, the frictional force exerted by the surface on the person is opposite to the direction of his motion.

A rocket moves forward by pushing the surrounding air backwards.