Do you think you could stop a train moving at 50 miles per hour? No, you probably couldn’t. Now, do you think you could stop a foam ball moving at 50 miles per hour? I think you could, catchers in baseball stop 90 mph fastballs. One thing that would make it hard to stop the train is its momentum. The ball has momentum, too, but much less of it. To find the momentum of an object you multiply its mass by its velocity, or speed. The ball and the train had the same velocity, 50 mph, but their masses were very different. The train’s mass was much larger than the ball’s mass. Momentum can make it very hard to stop an object. We know what happens to an object’s momentum if it has a high mass, but what happens if it has a high velocity? A bullet fired from a gun is a good example of an object with a small mass and a high velocity. The bullet would have a very high amount of momentum.

Your friend, being dumber than you thought, hadn’t worn his seat belt when the car crashed. When your car stopped, his momentum carried him into the windshield. If he had worn his seat belt, it would have connected him to the car. This would make it so when the car stopped, he would have stopped, too.

Conservation of Momentum

If you are playing T-Ball and you swing the bat, the bat will have momentum. As the bat hits the ball, some of that momentum transfers into the ball as the ball begins to move. If there is no friction, or any other outside force, momentum will be conserved. This means that the amount of momentum in the bat before contact is the same as the amount of momentum in the bat and the ball combined after contact. In other words no momentum was lost. In this case, conservation does not mean saving resources, it means an amount is the same before and after an event.

In the waiting room of the emergency wing of the hospital, you find something called a Newtonian Demonstrator. It has five metal balls hanging in a straight line on strings that are tied to a piece of wood six inches off the ground.

You quickly figure out that if you pull one of the balls on one side and let go, it will swing back to where it was and hit the ball it was next to.

The momentum of the first ball transfers into the second ball. Then into the third, and the fourth. The momentum transfers from the fourth ball into the fifth ball.

This last ball then swings out to the side.

When the fifth ball comes back it restarts the chain. The momentum was transferred from one ball to the other. Momentum is conserved.

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All pictures courtesy of NASA unless otherwise noted