A pair of equal and opposite forces acting on a body along two different lines of action constitutes a couple. A Couple has a turning effect, but no resultant force acts on a body. When we steer a bicycle round a bend with both our hands on the handlebars, we apple a couple.

A planet revolves around the sun under the influence of a gravitational force which acts towards the sun. This means that the areal velocity of a planet is constant. The is Kepler’s second law of planetary motion which states that the line joining the planet to the sun sweeps out equal areas in equal intervals of time.

According to Newton’s third law, the internal torque on the system due to internal forces is zero because the forces between any two particles are equal and opposite and directed along the line joining the two particles. Hence the total torque is due to external forces only.

A rigid body is one for which the distance between different particles does not change, even though they move. Under the influence of an external force, a rigid body can execute rotational and translational motion.

Spring constant, k = F/x = 10N/1mm = 10N/(10^-3 m)= 10⁴ N/m
Work done in stretching the spring through 40m,
W = 1/2 kx²=1/2×10⁴×(40×10^-3)² = 8J.