91Ó°ÊÓ

A magnet is hanged in front of an iron cart.

According to Newton's 1st law, an object will remain in motion or remain at the state of rest unless acted on by an external force.

In order for the cart to move, it needs to have a net force acting on it in the forward direction otherwise the cart will stay at rest. The magnet exerts a horizontal force on the cart.

However, according to Newton's $3rd$law, for every action, there is an equal and opposite reaction. So, the cart also exerts a force on the magnet. These forces are equal and opposite (assuming no friction force) hence cancelling each other.

Hence, this system consisting of cart and magnet has no external forces acting on it and the cart will not move.

The magnet is attached with the cart , so both of them become part of a single system on which two internal forces act , cancelling each other , but no external force acts . If magnet is disconnected from the cart and is fixed at some point away from cart , then it can make cart move .