Newton's 1st Law of Motion
Newton's First Law of Motion
A body continues in a state of uniform motion unless acted on by a net external force.
There are several points to be noted.
Uniform motion here means a constant speed in a straight line. Motion in a circle is not uniform motion, even if the speed is the same.
A body in a state of uniform motion is not accelerating, as there is no net external force acting on the body. A body moving in a circle is accelerating, as the direction in which the body is moving is constantly changing.
Internal forces can be ignored. Internal forces cannot act to produce an acceleration since all internal forces are produced in pairs, equal in size an opposite in direction to each other, so internal forces cannot combine to produce a net force. This means for example that a body may change shape, as when an ice skater moves her arms and legs, but if as in this example, a changing shape is as a result of internal forces, no acceleration will be produced. This point applies even if the body in question breaks into smaller pieces – we still consider motion of the pieces as a whole and not of the individual pieces.
Though there may be several forces acting on a body, we may add them as vectors, to express them as one force. This means that any calculations to find acceleration in a line can be done using this one force.
In practice it is often very hard to produce a situation in which no net forces act on a body. Friction is ever present in one form or another, and usually acts to slow down the velocity of a body.
Being in a 'State of Equilibrium' is not the same as being in a 'State of Uniform Motion.' A state of equilibrium implies no motion. A body in equilibrium is not moving an experiences no external force, but a body in a state of uniform motion merely experiences np external force.
The external force can be friction, a tension in a cable, a contact force, gravitational, electrical etc. The body exerting the force does not have to be in contact with the body in question – it can be action at a distance, as gravitational and electrical forces usually are.