Question

If the acceleration of a system is zero, are no external forces acting on it? What about internal forces? Explain your answers.

Short answer

The sum of the external forces acting on a system will be zero when the acceleration of the system is zero. The internal forces on a system do not depend on whether a system is under acceleration or not.

Step-by-step solution

Concept of Newton’s second law of motion

Newton’s second law of motion states that the acceleration of a system is directly proportional to the net external force acting on the system and is inversely proportional to the mass of the system. Mathematically,

$F_{net}=ma$

Here$F_{net}$is the net force,${}^m$is the mass, and${}^a$is the acceleration.

Explanation for a system with no external forces has zero acceleration

The sum of the external forces acting on a system will be zero when the acceleration of the system is zero. This does not mean that there are no external forces acting on it, but the sum of all the external forces acting on it must be zero.

The internal forces on a system do not depend on whether a system is under acceleration or not. An internal force is considered when any part of a system exerts a certain force on another part of the same system.

According to Newton's Third Law of Motion, the other part of the system exerts an equal and opposite force on that part. Since they are equal and opposite forces, they cancel each other, and hence the net force is zero.