Newton's First Law of Motion

NEWTON'S FIRST LAW - Balanced/Unbalanced Forces

1, BALANCED FORCES

An object's motion will stay the same (stopped or constant speed) if:

1. There are no forces acting upon it, or
2. The forces acting upon it are balanced.

What do we mean by balanced forces? 

If the size of the forces are the same size but in the opposite direction, we say they are balanced.

Here are some common pairs of forces that usually balance each other out:

For something that flies, like an aircraft (for example), the upward force is called lift.

2, UNBALANCED FORCES

If the forces are not balanced, the motion of the object will change:

1. its speed (starts moving, accelerates or decelerates), and/or
2. its direction

Imagine skydiving:

During free fall, the skydiver accelerates. This is because the forces are unbalanced.

As he falls faster, there is more friction (air resistance). His weight stays the same throughout the fall.

Eventually, the friction and weight forces are balanced and he falls at a constant speed - the speed he is travelling at now is called terminal velocity.

When the parachute is opened, there is a lot more friction (air resistance). The forces are unbalanced again. This time, the unbalanced force decelerates the skydiver, so he can land safely.