Forces on Objects and Systems

Notes on Forces

A) Note that any single object, call it object A, is affected only by the forces acting directly on it including only contact forces (such as from ropes or table tops)and field forces( such as gravity. This means that only forces from objects in contact with object A or producing a field such as gravitational or electrostatic, which interacts with it, can directly cause object A to accelerate.

For example, without taking into account air resistance and other minor effects, the acceleration of an object hanging from a cable is affected by only two forces, the tension in the cable and gravity. Similarly, the tension in the cable where it is attached to the object can be found by considering only the acceleration of the object and the other force on the object and keeping in mind the respective directions of the forces and of the acceleration.

B) Objects in a system linked together in a way so that they all accelerate and move together can be treated in a sense as a single object with a single mass equal to the sum of the masses in the system and accelerated only by external forces. The forces internal to this system cancel each other as far as accelerating the total mass is concerned. This is because the force of one object in the system on a second object in the system is equal and opposite to the to the force of the second object on the first. Thus they make no contribution to the net force on the system as a whole.

However, once you have you used the external forces on the system and the total mass of the system to help you establish the acceleration of the entire system, and thus each object in the system, go back to Note A above to find the internal forces such as tensions in a cable connecting two objects in the system.