Key concepts
Physics
Collision
Momentum
Energy

Introduction
Did it ever occur to you that tennis, bowling and shopping carts bumping into each other all involve collisions? It is fascinating how just a few rules of physics can predict the outcome of these collisions. You can discover these rules yourself with a fun homemade toy. After creating and playing with the toys in this activity, you will be one step closer to understanding what happens when you hit a tennis ball or go bowling!

Background
Have you ever heard someone say that something "has a lot of momentum?" In everyday language we use "a lot of momentum" to describe things that are hard to stop. In physics an object's momentum depends on its speed—how fast it moves—and its mass—how much stuff it is made off. Momentum also has a direction—the same direction the object is moving. For an object to gain momentum it can gain speed, gain mass or gain both. To give a shopping cart rolling downhill more momentum, you can make it move faster (increase its speed), load more weight (increase its mass) or do both. You probably intuitively know that the shopping cart with the biggest momentum—the fast-moving, heavily loaded cart—is hardest to stop. It also creates the biggest impact when colliding with something.

Physicists discovered that objects transfer momentum when they collide. But even more they observed that the total momentum is conserved during a collision. If you have seen a row of shopping carts creeping away after a fast-moving but empty shopping cart collided into them, you have witnessed conservation of momentum. The fast-moving light cart transferred its momentum to the much heavier row of carts. Its momentum could only make this heavy mass move a little. There is a little more math involved when both objects are moving before the collision, but even then the total momentum is always conserved.

Energy is the other quantity that gets transferred during collisions. More surprisingly the energy associated with the movement of the colliding objects is conserved in collisions, at least in collisions when the colliding objects do not deform, crack or break at all. In real life there is almost always some deformation, and some energy of movement will almost always be converted into other types of energy, such as heat or sound. The fun toy created in this activity will help you get an intuitive feeling of how momentum and energy are conserved during collisions.

Materials

• Two identical balls, half inch to three inches in diameter (large round wooden beads, ping-pong balls, small bouncy balls and round erasers work well)
• At least one more ball (half inch to three inches in diameter) of a different mass (this ball needs to be at least three times as heavy or three times as light as the identical balls)
• Needle and thimble or strong glue
• Thick thread (preferably not twine but a slightly thicker sturdy thread)
• Scissors
• Ruler

Preparation

• Cut the thread into pieces about 30 centimeters long (one for each ball).
• If you are using beads, pull the thread through the hole, and make a knot on one end of the thread big enough so the bead cannot slip over it.
• For any balls you are using, ask an adult to help you hang the balls on threads. Ask them to pierce a threaded needle through the middle of the ball. Make sure they wear a thimble. If needed, pliers can help pull the needle through. Make a knot on one end of the thread so the ball can't fall off. If this is too hard, strong glue can be used to attach a thread to the balls.

Procedure

• Pinch the threads of the two identical balls (or beads) between your thumb and finger, letting the balls hang down. Slide the thread of one ball up or down until the balls are level.
• Pull one ball up 90 degrees, keeping its thread taut so it forms a horizontal line.
• Keep the hand holding the threads steady while you release the ball and observe what happens. Repeat the test a few times. What happens (almost) every time?
• For the second test go through the same procedure, only now jerk the hand holding the threads up about an inch each time the balls move away from each other and move the hand back down when the balls approach each other. You might need to try it a few times before you can keep the balls bouncing. How is this different from the first test? Why would this be the case?
• What do you think will happen if you switch one ball with a heavier or lighter ball?
• Hold the threads of two non-identical balls between your thumb and finger. Repeat the first test. First try releasing the lighter ball and observe, then switch to the heavier ball. How is this similar and how is it different from what you observed when using identical balls?
• Keep the threads of the non-identical balls pinched between your fingers and try the second test. What do you observe now? Can you explain your observations?
• To turn your tests into a toy, first select the combination of balls you like best. Then knot the thread ends farthest from the balls together. Before you tighten the knot adjust the distances between the knot and the balls so these are identical.
• Extra: Test different combinations of balls. What can we learn from a test using same-size balls of different masses? What about balls of equal mass but different sizes?
• Extra: Test the role of the material of the balls. What happens if you use two wooden balls instead of two rubber or two ping-pong balls? Which balls keep bouncing the longest if you keep your hand still? Which ones do you have to jerk more to keep them bouncing? Why would this be the case?
• Extra: If you have more identical small balls (e.g. marbles), you can do another surprising collision test. Place a row of the balls in a crease of an opened book. All balls should touch each other. Softly shoot a ball along the crease into the end of the row of balls and observe what happens. Why would this happen? Is it different if you shoot two balls into the other balls, or if you shoot a heavier ball into the row of balls?

Observations and results
When you tried with the identical balls did you witness the balls exchange speed during the collision? Did you see how jerking the system up makes it possible to keep the balls bouncing? When you tried with non-identical balls did you notice that the collision didn't cause the heavier ball to move as much while the lighter ball was launched off a high speed?

When two balls collide they exchange momentum. For identical balls this means one ball is launched off with the speed of the other ball each time they collide. This explains why the initially motionless ball shot off when bumped by another ball leaving the first ball almost motionless. Before long the ball that was shot off returned and bumped into the first, which shot off returned and so on.

If the second ball shoots off with the same speed, that ball should shoot up to the same height from which you released the first ball. Was that what you observed? Probably not! With each collision some energy goes into moving the tiny particles that make up the balls or particles in the air. We observe this as energy of motion being transformed into heat or sound. As a result the second ball shoots off with a smaller speed than the speed at which it was hit. The difference depends on the material of your balls. Bouncy balls will show a small difference; the speed will decrease only slightly with each collision, and the balls bounce back and forth for a long time. Wooden balls will have a bigger difference and bounce back and forth only a few times before all energy is transformed into heat or sound. Did you notice that when you jerked the system up just after the collision, the balls could keep on going? By doing so you added energy back into the system allowing the balls to keep bouncing.

Non-identical balls also exchange momentum, but if their masses are different, there is more to it than a simple exchange of speed. Did you notice how the motion of the lighter ball was only able to make the heavier ball creep up a little bit? On the other hand when the heavier ball bumped into the lighter ball its momentum could make the lighter ball move a lot. This is because the same momentum can make a lighter ball move way faster than a heavy ball.

More to explore
Momentum and Collision, from Ducksters
Energetic 2-Ball Bounces, from Scientific American
Make Craters with Mini Meteors, from Scientific American
Build a Gauss Rifle!, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies