Good morning. Today we are going to use a skateboard to introduce the concept of conservation of momentum. Flipping Physics Bo, what is the equation for momentum?
Momentum equals mass times velocity and both momentum and velocity are vectors. Conservation of momentum is true in all isolated systems. In physics, a system is isolated when the net force on the system is equal to zero.
In other words, if you add up all the forces acting on all the objects in the system, they add up to zero. And we know net force equals change in momentum over change in time, where both force and momentum are vectors. And multiplying through by change in time gives us the change in momentum of the system equals zero, and delta, or change in, always equals final minus initial. And we can add the initial momentum of the system to both sides to get the initial momentum of the system equals the final momentum of the system. This concept is called conservation of momentum.
The equation is usually written, the sum of the initial momenta of the system is equal to the sum of the final momenta of the system, where momentum, of course, is a vector. And in an algebra-based physics class like this one, we usually summarize that conservation of momentum is true during all collisions and explosions. I will now use a skateboard and a medicine ball to demonstrate conservation of momentum. Now I know this is not what you typically think of as an explosion. However, it fits the physics definition of explosion and therefore momentum is conserved.
Billy, what is the initial momentum of the system? In other words, what is the momentum of the system right before the explosion? Well, your initial velocity is zero, so your initial momentum is zero. The initial velocity of the ball is also zero, so the initial momentum of the ball is zero. Therefore, the total momentum of the system initial or before the explosion is zero.
Correct. And Bobby, is the momentum of the ball after the explosion positive or negative? After the explosion, the ball is moving to the left. Therefore, the ball has negative momentum.
Therefore, because the final momentum of the ball is negative, in order for the total final momentum to add up to zero, my final momentum after the explosion has to be positive. In other words, I have to move to the right. This is exactly how a rocket works. In the bottom of a rocket, you have a bunch of people throwing medicine balls out the bottom.
That's not right, Mr. P. Okay, you're right. That's not really how a rocket works. Bo, what is that on your desk? This is my rooster.
It's stuffed animal day. My rooster is stuffed. I brought Bill the cat.
Of course it's a c-Okay. Cat. Mr.p.: In a rocket, the rocket fuel is ejected quickly downward and therefore the rocket moves upward.
We can illustrate this using a hose spray nozzle. Again, the water is ejected quickly downward. This means the water has negative momentum. Therefore, because momentum is conserved, the spray nozzle gains positive momentum upward.
Thank you very much for learning with me today. I enjoyed learning with you.