Now let's proceed to apply Newton's laws to skateboarding.
a) In skateboarding it's important that one's board and body are moving in the desired direction before one jumps up in the air to perform a trick. This is because it's much easier to apply a force with a desired direction and speed while still on the ground. And, one can be confident that inertia, the idea that an object will continue in direction and speed unless another force acts on it, will keep the board moving in the desired manner in the air.
For example, when doing a 180 Ollie, and how much more so a 360 Ollie, it is important to have one's board and body already moving in the right direction before one actually Ollies. This suggest that one should be in the habit of winding up his body in the opposite direction to where he intends his board or body to turn, and to Ollie - that is, become airborne - as late into the unwind as possible.
b) Watch a footballer kick a ball. Notice how he'll follow through with his kick way past the point where his foot intercepts the ball. Why doesn't he stop once the ball is in motion? As mentioned, the force of an object is the combination of its mass and acceleration. The footballer thus wants his leg, which is of much greater mass than the ball and is accelerating quite fast as it swings on his hip socket, to remain in contact with the ball for as long as possible so that the ball receives the full force of his kick. If he was to slow down his leg as soon as it came in contact with the ball he'd fail to transfer the full potential force of his leg.
The same principle underpins most skateboarding tricks. It is most evident in scooping tricks such as 360 shuvits and tre flips where a follow through with the scooping foot adds immense force to the scoop. Yet, it is also important in ollying. When ollying many people perform a quick swipe up the board with their front foot and immediately recoil when the board begins to ascend. If instead they'd extend the swipe way past their point of contact with the board, as though attempting to kick past the board's nose, the force they'd generate and transfer to the board would be remarkably greater and would increase the height or distance of their Ollie.
c) We've all experienced how difficult it is to jump high on a sandy beach. This is because the surface is unstable and gives way to the push of one's feet rather than pushing back. Compare this to the reactive force of a more stable surface such as concrete which affords a relatively high spring. Though one is unlikely to attempt skateboard tricks on sand, the stability of one's stance on the skateboard can either degrade the board into a sand-like surface or solidify it into highly reactive 'concrete'.
For instance, if a person attempts to perform an Ollie with his legs wobbling because of unstable foot positioning on the board, regardless how much force he applies to the board he will not receive the desired reaction - push back - from the board to propel himself upward. In contrast, when foot positioning is stable and one feels he's standing on 'solid ground' the board's reactive force increases, allowing for a much higher Ollie.
Of course this doesn't mean that one should always try to find the most stable foot placement on the board, for most tricks actually require an awkward or off-centred stance for their proper performance. Rather, it means that regardless of the trick being performed one should try to find as stable a stance as possible in the context; the golden mean between the ideal solid stance and the lopsidedness required for the trick. Sometimes the secret is to place one's feet on more stable parts of the board, while at other times it's about attaining the right balance between the feet.