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Jimpulse and Rhomentum


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This week we did an independent unit on Impulse and Momentum (I like to call them Jimpulse, cause Impulse's symbol is a J, and Rhomentum, because Momentum's symbol is a Rho which looks like a p). In other physics classes, we were told that impulse equals momentum, but in Mr. Fullerton's Physics AP-C class we did a lab and a few worksheets that proved just that!

In a lab we did this week we used a force and velocity sensor, and pushed a cart on a track connected by rubber band to the force sensor. Using logger pro, we obtained the force vs. time graph and position vs time graph. When we took the area of the force vs. time graph (the definite integral) it was the same number as the slope (the derivative) of the position vs time graph multiplied by the mass of the cart. And there you have it. The integral of Force with respect to time is Impulse (since png.latex? $$J=F\Delta t) and mass times velocity is Momentum (Since png.latex?\rho =m\Delta V , and the initial velocity was zero). Since those two numbers were the same no matter how hard we pushed the cart, we proved that png.latex? J=\Delta p or png.latex? m\Delta V = F\Delta t!

When you get a fairly well understanding of Impulse and Momentum, you start to look at everyday things you might see differently, and some things might make sense all of the sudden. For example - skateboarding :cool:.

The "Ollie" is the fundamental trick of skateboarding. It takes years for some to master, but it is essential if you want to skateboard. An ollie is a trick that allows the skateboarder to jump with the skateboard. His feet are not attached to the skateboard, so it would seem difficult to jump and have the board follow behind! But, there is a way, shown here:

The ollie! The secret about it is not to jump first. Before jumping you need to stomp down on the tail of the board, causing it to hit the ground. The more force you put on the tail of the board, the higher you will potentially be able to ollie. When the tail of the board strikes the ground, you get an impulse. The board comes in contact with the ground for a certain time, and when you increase the force, you increase the impulse. Since every action has an equal and opposite reaction, (New told me so!!), the impulse from the board to the ground equals the impulse from the ground to the board. Impluse = Momentum, so png.latex? F\Delta t = m\Delta V. Since the ground has a lot of mass, the velocity is very slow (arbitrarily close to 0 :P) but the skateboard and boarder have a smaller mass, so the velocity is enough to propell the boarder upwards!

At that exact moment that impulse acts, the skateboarder jumps and the skateboard follows. The skateboarder guides the side of his foot up the grip tape, and the friction pulls the board up and levels it out.

Physics can explain why simple, or not so simple, things in our world work.

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Easy enough to fix up... to embed a movie, all you have to do is copy the regular youtube link (not the embed link). Then, in the APlusPhysics editor, click the movie icon and paste your youtube link in there. The editor will take care of the rest! (http://www.aplusphysics.com/forums/showthread.php?16-Embedding-videos-into-your-posts)

(To make your equations work, I just added the tex and /tex tags around either side of your code. Details available here: http://www.aplusphysics.com/forums/showthread.php?15-Embedding-formulas-into-your-posts)

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Thanks for explaining that real life example! I never understood how an ollie worked, it seemed impossible to do from my perspective. Jumping over a lot of skateboards is even more impressive! With the formulas you explained, I now understand the physics behind the move and it makes sense with the impulse. After completing the MC packet we were assigned, I also learned that in the example of a kid on a skateboard, walking from one end to the other, the center of mass is unchanged, along with the center of velocity.

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When we used to skate all the time, I remember there being this specific ollie, I forget the name, where the goal was to not let the board scrape the ground. I'm wondering what else, like torque, is involved to compensate when the tail doesn't touch the ground. Friction is definitely a major factor considering the rough grip tape and the rubber skate shoes.

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Oh yeah!!!! Ummmmm. A "Pressure Ollie"? I think that was it, B. A Pressure ollie (i think that is what it is called) is when you press down on the tail slightly, so that the board gets close to touching the ground but doesn't, and then you proceed to steps two and three of the ollie, jumping and scraping your foot up the board. You could only get a few inches of air this way. And yeah, i would assume that the major factor is the friction of your shoe scraping up the grip tape. There is no contact to the ground, so it cant have to do with impulse or momentum.

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