Balanced Forces

When we push or pull an object we can make it move. We have said that if we use a greater force we can make it speed up (accelerate). But what Forces are at work when an object is stationary or is moving at constant speed in a straight line?

First, let us look at the stationary object. A good model for this is to think of a tug-o-war. If the force exerted by the two teams is equal it is clear that neither will win and we say the forces are balanced. Similarly if a toy car is on the living room floor and it is not moving we can say that all the forces acting upon it are balanced. If we push the car we make it move because our push is greater than the other forces acting upon it.

If, on the other hand, an object is moving at a constant speed in a straight line then again the forces are said to be balanced.

If we think about this in the context of a skater it makes it easier to understand. We know that one force which causes objects to slow down or stop is friction. In the case of the skater the friction between the skate and the ice is reduced by the nature of the two materials. This enables the skater to keep moving for a longer time than, say, using roller skates on a road surface. If there was no friction at all then the skater would be able to keep s steady speed in a straight line ad infinitum.

The Law of Conservation of Momentum

The total momentum of any group of objects that are not subject to external unbalanced forces is constant.


Balanced forces also mean that an object will keep its shape. If this were not so then we could find objects changing shape in front of our eyes for no apparent reason.

However, if the forces are unbalanced we can see a different effect. If we refer back to our tug-o-war and one side exerts greater force than the other then the forces are unbalanced. Another example would be if we think about an empty plastic bottle. Under normal circumstances it will keep it shape because the air pressure outside is equal to that inside BUT if we exert a greater external force on it than the force of the air pressure inside it, then the bottle will squash.


Measuring Force, Mass, Speed
Velocity, Acceleration
Forces that oppose motion
What children might say or think about forces
Self assessment