We use words like acceleration, constant speed and stationary to describe motion. Forces acting on a body cause the motion of that body to change. Newton's First Law and Second Law explain the different types of motion.
The motion of all objects that are speeding up, slowing down or changing direction is governed by Newton's Second Law of Motion. In Physics, we use Newton's Second Law to explain different types of motion and to do calculations involving force, mass and acceleration.
'Unchanging motion' is when the body is at rest or is moving with a steady speed in a straight line. Balanced forces are responsible for unchanging motion.
Balanced forces are forces where the effect of one force is cancelled out by another. A tug of war, where each team is pulling equally on the rope, is an example of balanced forces. The rope will have an acceleration of zero under the action of these balanced forces. It will therefore remain stationary.
'Changing motion' includes movement where the body is speeding up or slowing down. It also includes motion where the direction in which the body moves is changing, ie the body is moving in a curved path. 'Changing motion' is caused by unbalanced forces.
The lift-off of a space shuttle is an example of an unbalanced force in action. The space shuttle accelerates upwards from its launch pad. The thrust \(T\) from the rocket engines is greater than the weight \(W\) of the rocket system. Since force \(T\) upwards is greater than the force \(W\) downwards, the effect of one force does not cancel that of the other. The forces acting are unbalanced.