Forces are pushes or pulls. They are measured in newtons. Unbalanced forces change the way something is moving. This lecture briefly present on introduction to Forces. A force is an influence on a system or object which, acting alone, will cause the motion of the system or object to change. If a system or object at rest is subjected to a non-zero force it will start to move. Examples: Weight is a force caused (on Earth) by the gravitational attraction of a mass to the Earth’s centre. The force that the string exerts on the object in these types of problems is called tension.