Physics 2a - Resultant Forces

What is a Resultant Force?

Forces always come in pairs. If you have one force, there will always be another opposing it; on the same line. For example, gravity's opposing force is the reaction force.

The resultant force is the calculated overall force from to opposing forces. To calculate resultant forces, you must add or subtract the two opposing forces from each other. By doing this, you will get ONE simplified force from the TWO forces.

Let's explain this further...

How to Calculate Resultant Forces?

Let's look at the car on the left. 

• The car has a a driving force 1500N, right.
• Air resistance act against the car, 700N left.

These two forces are opposing, one is on the right and the other, on the left (not up and left, or down and right). This means we can calculate a resultant force; a simplified version of the two original forces.

1500N right - 700N left = 800N right

By subtracting the forces, we have found out the resultant force. It is essential to understand that whether you draw the forces 1500N right and 700N left, it will be the same as 800N right.. They are the same thing!!

Let's look again at the example with the book on a table.

• The weight of the book is 50N.
• The reaction force acting on the book is also 50N.

The forces are opposing. One force acts downwards (gravity) and the other against it, upwards (reaction force). However, the forces are equal. Let's see what happens if we minus them from each other...

50N up - 50N down = 0N

There is no resultant force; A ZERO RESULTANT. 

What do Resultant Forces do?

If there is a resultant force in the mix, then that means there will be a CHANGE IN VELOCITY. Try and remember these 4 main rules to the effect of resultant forces!

1. If the object is STATIONARY, and there is NO resultant force then the object will stay STATIONARY.
2. If the object is STATIONARY and there IS a resultant force, then the object will ACCELERATE in the direction of the resultant force.
3. If the object is MOVING, and there is NO resultant force, then the object will carry on moving at a STEADY SPEED ( - it will not stop!!)
4. If the object is MOVING, and there IS a resultant force, then the object will either ACCELERATE or DECELERATE. 

Remember point 3! An object does not need a resultant force to make it move. Resultant forces only cause changes in velocity, whether an objecy was originally moving or not. 

If no resultant forces are acting on a moving object, it would be able to move forever (just like in space). However, on Earth, frictional forces will (in most cases) appear.. which explains why objects can't move forever.