Momentum and Forces Demo

We're going to talk about something physicists call momentum. Who has heard of momentum before? (usually someone knows that momentum has to do with moving. That's good enough for now.). We're going to use a pair of skateboards to explore linear momentum. Linear momentum has to do with motion in a straight line.

[This experiment may be skipped for older students] Now we need two volunteers. (Choose two students of roughly the same size, instruct them to sit on the skateboards, facing each other, with their hands together). Now we are going to try an experiment. If the two volunteers push on each other's hands, what will happen? (get an answer from the audience, this is an easy one). (Do the experiment, and make sure you have a person ready on each side to catch the skateboards). Physicists say that an object has momentum in the direction it’s moving. The faster it goes, the more momentum it has. An important rule that physicists discovered by doing lots of experiments is that if you add up the momentum of everything moving, the sum always stays the same, no matter what happens. This is called the conservation of momentum. In this case both skateboards started out stopped, so they had no momentum. After the volunteers (use their names) pushed off each other, they were both moving, so they each had some momentum. The trick here is that direction is very important when adding up momentum, and the skateboards were moving in opposite directions, so their momentum cancels when you add it together, so the total momentum of both skateboards added together is still zero. (older students may understand this canceling as adding a positive and negative number).

So we just saw that when two people push against each other, they move away from each other. That experiment was pretty simple, but now I want to try something more interesting. What if only one person pushes and the other sits still while they're both on the skateboards? (Get one of the volunteers to turn around on his/her skateboard) What will happen different than before? (get a few answers from the audience) Raise your hand if you think the one getting pushed will move? Raise your hand if you think the one doing the pushing will move? Raise your hand if you think both will move? Since we're scientists, whenever we don't know the answer to a question, we do the experiment to find out. (Do the experiment, having only one person push). Wow! Both skateboards rolled backwards. Maybe we should try having the other person push to see what happens. (Have both volunteers to turn around and repeat the experiment with the other volunteer pushing, if time permits). Very interesting! It doesn't seem to matter who does the pushing, both skateboards move away from each other in every experiment. But if we remember that total momentum has to say the same, we should have expected this. Since there is zero total momentum before the push, there has to be zero total momentum after the push. So if the person getting pushed starts moving, the person doing the pushing has to move in the opposite direction to cancel out the other person’s momentum.

[for older students who have learned Newton's laws ~5th grade and up] The last experiment was a good example of Newton's 3rd law: for every action there is an equal and opposite reaction. This means that if I push on something, it pushes back on me with the same force. We saw that happen when only one person pushed on the skateboards. Even though only one person made the effort to push, the other person, just by sitting there and being pushed on, was pushing back with the same force. We can understand what happened in the last experiment either using momentum or using forces, because momentum and forces are very closely related, and are really just different ways of explaining the same physics.

Let’s experiment with another aspect of momentum. We're going to need two more volunteers (this time pick two very different sized people, preferably a student and a teacher, and have them sit on the skateboards like before). Now one person weighs more than the other, so physicists say they have more mass. If they push off each other, are they going to move back the same like before, or is one person going to move faster than the other? Raise your hand if you think the big person will move faster? Who thinks the little person will move faster? Who thinks they'll both move at the same speed? Let's do the experiment (do it). We see the little person moves faster. So our experiment just showed us that mass has something to do with momentum. Remember since the total momentum must always stay the same, both of the skateboards have to have the same amount of momentum but in opposite directions. The smaller person has a lot of momentum because it is moving faster, so the extra mass of the big person must be contributing to his/her momentum. Now we know that the faster something is moving, the more momentum it has, and the more mass the moving object has, the more momentum it has.

[Forces] Newton's laws say the amount of force on each person is the same, but in opposite directions. So we saw the same amount of force made the lighter person move faster than the heavy person.

When physicists discovered that momentum is conserved for motion in lines, they thought it might apply to other kinds of motion, like motion in circles. We can do some experiments to find out.

Who knows how to ride a bike? Who can tell me one of the most important things you need to learn in order to ride a bike? (you have to keep peddling). If you try to just sit on a bike without peddling, you fall over (Try to balance a bicycle wheel on the ground, and show that it falls over). Once you start peddling and get going pretty fast, it’s a lot easier to keep your balance (roll bicycle wheel to partner across stage to show that the wheel will stay up when it’s rolling). This is because once the wheel starts spinning it wants to keep spinning in the same direction. What would happen if the wheel tipped over? (it would change the direction the wheel was spinning ). Just like the skateboards had linear momentum, the bicycle wheel has angular momentum. The faster it’s spinning, the more angular momentum it has.

Now we need another volunteer to experiment with angular momentum. Here I have a spinning stool that we’re going to use for this experiment. (Pick a strong-looking volunteer if the students are young. Have the volunteer sit on the stool, with feet on the bar). Now I’m going to spin up this bicycle wheel and hand it to you. Then I’m going to ask you to try to turn the wheel to the side (show motions, first with the wheel upright, then turn it 90 degrees so it is spinning horizontally). (Hand the wheel to the kid, and watch them start spinning. They may struggle with the wheel because it is harder to turn than they expect. After they have been spinning for a few seconds, ask them to flip the wheel completely over, so it’s spinning the opposite direction). (For the larger kids, you may have the problem that the wheel doesn’t transfer enough force to overcome static friction. In that case, start with the wheel spinning horizontally, then ask the kid to flip it 180 degrees to get double the momentum boost. You will then have to modify your discussion since there is a net angular momentum to the system). Did the stool spin in the same direction or the opposite direction as the wheel? (opposite) Why would it do that? Remember when two people on skateboards pushed off each other they went in opposite directions. The same thing is happening here, only with spinning motion instead of linear motion. By turning the wheel horizontally, the volunteer is pushing on the axle—or doing the work to spin up the wheel horizontally, so the wheel has to push back by spinning up the person and stool in the opposite direction. Just like with linear momentum, if we add all the angular momentum of everything moving in circles, the total always stays the same.

Now we’ve seen that total angular momentum stays the same just like linear momentum. I bet mass has something to do with angular momentum too. You probably already know that something heavy is harder to spin up than something light, so I don’t have to show you that. There is something interesting with mass and angular momentum I do want to show you, so I’m going to need another volunteer. (have the volunteer sit on the stool like before, but now you’re going to hand them a pair of weights. We have 4 pound weights for bigger students and 2 pound weights for smaller students. Hand the weights to the volunteer and have them hold their arms straight out to their sides. Tell them you’re going to spin them up, and once they get going they should pull their arms in to their chest. Go through all the motions so the volunteer understands). What is going to happen when the volunteer pulls their arms in? (get a few answers, then do the experiment. The volunteer speeds up when they pull their arms in). This is interesting, we didn’t see anything like this happen with linear momentum. When the volunteers arms moved in, it was almost like they got lighter so they would spin faster. In fact, mass does work a little bit differently when we’re dealing with angular momentum. Now it matters where the mass is. When the mass is really far out from the axis of rotation (arms out), it counts a whole lot more than when it is close to the axis of rotation (arms in). This may seem kind of weird, but think of how far the weights have to go around. When the weights are far out, they are moving in big circles (show motion), but when they are close in, they are moving in small circles (show motion). So if they have the same speed, it takes more time for the weights to move around the big circles than it does for them to move around the small circles. If the farther out weights take longer to get around the circle, the rotation is slower, so it must be that the weights counts more towards the momentum when they are farther out.