Although it is said Isaac Newton didn't really get hit in the head with an apple let's suppose he did. When people talk about this scenario they usually talk about , why that apple fell out of the tree. let's instead look at why all the other apples didn't fall as well.

An apple has a mass of about .18kg And the branch which holds the apple up acts as a tension. When the apple is hanging there it is in static equilibrium. This means when you add up all the net forces the equal zero. As a result the two forces acting upon this apple must cancel out. So mg= T The force in which is holding the apple up must equal the force which is pulling the apple down. (to see the free body diagram of this situation see the attachment)

fbd of an apple.doc

We can solve for the force pulling the apple down by using mg. The mass of the apple (.18kg) x gravity (9.81m/s²) is equal to1.8N We now know that this is the force pulling the apple down so the force pulling the apple up is also 1.8N however this force is acting in the opposite direction.

In order for the apple to fall to hit newton on the head the force forces could not equal each other. We know that gravity does not change so this wouldn't be the cause of a falling apple. However if the apple grew (increasing its mass to lets say 20kg) the force which the branch exerts on the apple would have to increase from 1.8N to 2N. If this did not occur than the force pulling down would exceed the force holding the apple up causing the apple to fall. Just think about it every single apple hanging from a tree is in a state of static equilibrium.