BALANCED AND UNBALANCED FORCES

  Let’s start with the most important point:

FORCES TRY TO CHANGE AN OBJECT’S MOTION

  A single force makes an object speed up, slow down or change direction.

  In the real world, it is very rare that there is only one force on an object, as other forces such as friction, drag and weight will almost always be present. To determine the effect of multiple forces, you must compare the total force acting in each direction. The largest total force will win.

UNBALANCED FORCES

  If the forces in one direction are bigger than the opposing forces then the larger force will win.

  If the larger force is trying to make the object speed up, then it speeds up. If it’s trying to slow the object down, then it slows down.

  If you have a force acting sideways to the motion and a smaller force (or nothing) opposing it, then this force wins and the object changes direction.

 The difference between the forces is the important quantity. This difference determines by how much one force wins over the other and is called the unbalanced (or resultant) force. 

  The resultant force in the second case is twice as large as the first. If the blocks are identical (same masses) then the second block will change its motion (accelerate) twice as fast as the first.

BALANCED FORCES

  If there are equal amounts of force acting in opposite directions, then neither wins. The forces balance and cancel each other.

  The force of 300N trying to make it speed up is equal to that trying to make it slow down. The two effects cancel and the object will neither speed up nor slow down. The object simply keeps moving at whatever speed it is travelling (in a straight line).

  “Sitting still” is a special case where the steady speed happens to equal zero.