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• Hi. It's Mr. Andersen and this AP Physics essentials video 49. It is on momentum which

• is just the product of the mass of an object times its velocity. In 1895 a train was headed

• towards Paris. They were a little bit behind schedule, did not apply the brakes in time

• and it came shooting out through the train station. What was their problem? They had

• too much momentum. And if you were to ask somebody what is momentum they will probably

• come up with a pretty good definition that would fit in physics. Momentum is always going

• to occur if we have an object that is in motion. And if you want to calculate it you simply

• multiply the mass of the object times the velocity of the object. The greater the mass,

• the greater the velocity, the greater the momentum is going to be. And so if we want

• to change our momentum, what could we do? Well we could decrease the mass. We could

• increase the mass. Or we could change the velocity. What is the easiest way to change

• the velocity? It is to apply a net force to that object. If we apply a net force to the

• right to this object, what are we going to see? We are going to see an increase in momentum

• towards the right. Let's say the object is moving towards the right and we change the

• net force on the object, what is going to happen? It does not happen instantaneously,

• but what we will see is a change in momentum that reflects that net force. And so if we

• define momentum, the equation is p=mv, where we are multiplying the mass of an object times

• the velocity of the object. So if we have this cart here, 1.0 kilogram cart, and it

• is moving with a velocity of 2.0 meters per second, figuring out its momentum is really

• easy. You simply multiply 1.0 times 2.0 and so our momentum is going to be 2.0 kilogram

• meters per second. And so if we add velocity to the right we are going to increase that

• momentum. Now one thing that you need to remember is that the net force on an object is going

• to affect that change in p or that change in momentum. And so let's say we have an object

• like this, a cart that is at rest. It has no momentum but we apply a net force to it.

• How would we do that in a physics lab? We are just going to connect it to a weight.

• And so now we are going to apply a net force to the right. What is going to happen to our

• momentum? It is going to increase to the right. What happens if we increase that net force?

• If we double the net force on the object, what is going to happen? We are going to increase

• the momentum more quickly. Let's say we triple it. We are going to see an increase in momentum

• to the right. And so it is a direct relationship between the net force on an object and the

• momentum on that object. But let's make it a little bit trickier. Let's say that object

• is moving now towards the left, but we are applying a net force to the right. What is

• going to happen? Well eventually what is going to happen, let's watch, is it is going to

• slow down. And then it is going to start to accelerate to the right. And so even though

• it was moving to the left, as we apply a net force to it, eventually that momentum is going

• to move in the direction of the net force. Now these are pretty easy problems because

• we are just looking at one dimension. But when you were growing up like me, you probably

• played video games like this. And so if I turn the ship and move it in this direction

• and blow up an asteroid, it is all momentum. But I would find myself playing asteroids

• like this. I would be moving in one direction but I would be applying a force in another

• direction. In other words my momentum is in that direction but my net force is in a totally

• different direction. What is going to happen to that? Well it does not happen instantaneously,

• but what is going to happen is that ship is eventually going to move in the direction

• of the net force. And so did you learn to justify the data we need to collect to show

• that the net force eventually impacts the change in momentum? I hope so. And I hope