Newton's Second Law

Newton's First Law of motion predicts the behaviour of objects for which all existing forces are balanced. The First Law states that objects with balanced forces acting on them will stay at rest, or in constant motion.

Newton discovered that objects will continue to do what they are doing until a net, or unbalanced force, acts on the object.

From this we can determine that:

• forces act on objects
• forces cause changes

We can also determine that forces acting on an object can change:

• the shape of the object
• the speed of the object
• the direction in which the object is moving

When an unbalanced force acts on an object Newton discovered that:

• the direction of the object's acceleration is the same as the direction of the unbalanced force
• the magnitude of the object's acceleration varies in direct proportion with the size of the unbalanced force
• the magnitude of the object's acceleration varies inversely with the mass of the object

This is Newton's Second Law.

The unit of force is called the newton (N).

1 newton is defined as that unbalanced force which produces an acceleration of 1 ms^-2 when it acts on a mass of 1 kg. An average sized apple weighs about 1 newton.

Newton's Second Law can be written as the following relationship:

F = ma

F = unbalanced force

m = mass

a = acceleration