The Physics Of The Ollie!
The physics of an Ollie takes into consideration all of Newton’s first three laws and a few other factors. The skater rides the board which is inert until the rider’s foot pops the tail (acted on by an outside force). The force of the foot explained by the weight of the rider (in my case, 120 lbs) will determine how high the board will rise and the faster it rises. The harder you pop, the higher the board rises. It is a simple and elegant trick invented by a guy named Alan “Ollie” Gelfant in 1977.
Newton’s 1st Law is the law of “inertia.” It simply says the mass or body is either at rest or moves at a constant velocity unless acted upon by an outside force. The skateboard is at rest until the skater starts riding the board. Newton’s 2d law speaks to the force applied equals the mass times acceleration. When 120 pounds are sprung on the rear of the board, the opposite end lifts and leaps over the gap. To stop the left foot from slowing down the lift, it is turned on it’s side and comes in contact with the grip tape on the side of the shoe. At this point it looks like the board is rising on its own. The application of gravity then brings the board back down to earth showing how Newton’s 3d law works to describe that there is an equal and opposite reaction for every action as force meets gravity and the board returns to the ground.
Describing the lift off is the easiest thing to understand about the physics of the board in motion. Lift off is caused by an imbalance of forces. This gives the board upward velocity and is created by the force of the foot pressing downward and the opposite action of the board responding by lifting upward. Upward velocity is what makes the board go up. We then use velocity to find out how high the board goes and where displacement takes place at the peak height and how long it takes to land.
Riding the board level has an inertia where the board is not affected by anything except staying in a level motion until acted on by a force to create a different velocity.
Formulas for the Ollie
1.
Height/ Time = initial velocity (v=d/t)
Board lift height/ time during the pop = .1 m/.05 s = 2 m/s = Vi
2.
Vi= Liftoff velocity
Vf^2 = Vi^2 + 2ad
0^2 = (2 m/s) ^2 + 2(-9.8)(d)
-4 = -19.60d
d= .2 m = maximum height of middle of board
3.
Time of descent = d = ½ at^2
(-.2m) = ½ (-9.81)t^2
(-.2m) = (-4.9)t^2
(.8s) = Time of descent
4.
Landing velocity = Vf^2 = Vi^2 + 2ad
Vf^2 = 0^2 + 2(-9.8) (.2m)
2.0 m/s = Landing velocity
5.
Impulse = Force of landing
(estimated time of collision with ground=0.05s)
F x t = m x ▲ V Delta V = final velocity
F x (.05) = 54 (2.0)
F(.05) = 108
F= 2160N
So what does this all mean you ask? Well, taking that I used .1 m for my lift off height, and .5 s for my time to pop the tail, I got a 2 m/s upward velocity, aka initial velocity (Vi). Then I took the 2 m/s Vi, and put it in the force equals mass times accerleration equation, which is Newton’s 2nd Law. My result was .2 m as my maximum height reached. Then, putting that .2 into the time of descent, meaning how long it takes for me to go from my peak height to the ground, I got .8 s as my time it took to descend from my peak. Calculating the .8 seconds into the landing velocity equation, I got 2 m/s as my velcoity when riding away. Then I estimated the impulse force, which is the force I felt when landing. I got 2,160 N which is about 4 times my weight which is reasonable when thinking abuot the 3rd law being for every reaction, there is an equal, and oppsite reaction.
Hopefully this helped a few people learn a thing or two about not only physics, but also what’s happening when you’re popping an ollie!
