Balanced and unbalanced force
As we mentioned in the previous chapter, there is usually more than one force at work on an object at any given time. If the force from one direction is equal to the force from the opposite direction, then they balance. An object is in equilibrium if the forces acting upon it are balanced.
An object with a balanced force exerted upon it:
- does not move (or remains at the same velocity)
- does not change direction; or
- does not change shape
Observe your computer on the desk in front of you. Gravity holds it on the surface of the desk but an equivalent force, the desk itself, is pushing up at the same time. The forces balance, keeping the computer in the same position.
Balanced forces also influence moving objects. When you ride a bicycle, the bicycle holds you up, while gravity keeps you on the bicycle. Riding at a steady velocity, the force you put into pedalling is balanced by the force of the resistance trying to slow you down, so you keep moving at the same speed.
There are a number of ways to upset the balance of forces. Unbalanced force is when one force is greater than another, or when one force is not exerted in the opposite direction to another. The unbalanced force that causes an object to move, change velocity, change direction or change shape is the resultant force. If you knew the strength and direction of all the forces exerted on an object, you could predict what will happen to the object.
Hold out your hand and place a pen on your palm. The force of gravity is balanced against the force of your body pushing it up, keeping the pen at that level. Now, upset the balance by taking away your hand and let the pen fall to the desk or the floor. The pen dropped because gravity exerted a force on it but there was no other force to balance it.
If your computer was sitting on a desk that had a surface at a 45-degree angle, it would slide toward the ground because the force of gravity would be more influential. This is because the force of the desk pushing it up is not exactly opposite to the force of gravity.
You can upset the balance of forces in movement too. Imagine you are riding your bicycle but you exert more force into pedalling than the resistance that is keeping you at the same speed. By accelerating, you create an unbalanced force. If you stopped pedalling, then the resistance would have more influence over the movement of the bicycle. This is also a type of unbalanced force; one that causes deceleration.