Collide Head On with this Toy

Introduction

Did it ever occur to you that tennis, bowling, Newton’s cradle, and cars bumping into each other all revolve around collisions? It is fascinating how a few rules of physics can predict the outcome of these collisions. You can discover these rules yourself with a fun homemade toy. Do the activity, play with the toys you make and be one step closer to understanding what happens when you shoot a marble into another!

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 either gain speed, gain mass, or gain both. To give a truck rolling downhill more momentum, you can make it move faster (increase its speed), load the truck more (increase its mass), or do both. You probably intuitively know that the truck with the biggest momentum – the fast-moving, heavily loaded truck -  is hardest to stop. It also creates the biggest impact when colliding with something.

Physicist 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 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 where 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, like 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

Preparation

Procedure

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?  All of this is expected.

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....

Wait - 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

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

Credits