A force causes an object to undergo a specific change. Unbalanced forces cause changes in speed, shape or direction. Gravitational forces act between masses. Friction opposes motion.

In his **laws of motion**, Isaac Newton made statements about the effect of balanced and unbalanced forces.

Newton’s **First Law of Motion** states that objects with **balanced forces** acting on them will stay at rest or stay in constant motion.

Newton discovered that objects will continue to do what they are doing until an 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, and the **direction** in which the object is moving.

Newton’s **Second Law of Motion** states that when an **unbalanced force** acts on an object:

- 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

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

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

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

**F = ma**

where:

**F** = unbalanced force

**m** = mass

**a** = acceleration

Note that when you use this relationship, F always stands for unbalanced force.