Unbalanced Forces

An object at rest stays at rest and an object in motion stays in motion, with the same speed and in the same direction unless acted upon by an unbalanced force.

When we have unbalanced forces acting on an object:

  • A moving object will change speed or direction
  • A stationary object (an object that is not moving) will start to move

When the two forces acting on an object are not equal in size, we refer to them as unbalanced forces.

In this example, to determine the resultant (overall) force acting on the truck, we need to find the difference between the two forces.

100N 60N = 40N

The resultant force on this object is 40 N, and it is acting towards the right.

If the driver increases the acceleration, there will be a greater force (in N) propelling the car forward, relative to the friction. This means that the truck will increase in speed. If the driver eases off the acceleration, there will be a smaller force (in N) propelling the truck forward, compared to the friction. As a result, the truck’s speed will decrease.

When the forces are balanced, the force forward from the engine is equal to the friction and air resistance pushing the truck backwards. This doesn’t mean that objects won’t move; rather, it means they will move at a constant speed, neither accelerating nor decelerating. In this context, it means that the truck is moving at a steady pace.

The change in the motion of an object is dependent on:

  • The direction of the resultant force
  • The size of the resultant force

Below is an example illustrating all the opposing forces acting on a moving car.