If you’ve ever watched a football game, you may have wondered: When two players collide at the same speed, why is it possible for one player to knock the other player back? The reason has to do with the mass of each player, and an important physical concept called inertia.
In this experiment, you will use a golf ball and a lead fishing weight that is roughly twice as large as the golf ball. When you send the items swinging toward one another, what do you think will happen? Which will win—the lighter golf ball, or the heavier lead fishing weight?
What will happen when two objects collide at the same speed and one has a greater mass than the other?
- Lead fishing weight (about 2800 grams)
- Golf ball
- Heavyweight fishing line
- Paper clips or thumb tacks from which to suspend the two objects
- Super glue
- Notebook and pen
- Cut two equal lengths of fishing line.
- Use the super glue to affix a paper clip to the golf ball so that you have a loop you can thread your string through.
- Thread your first piece of fishing line through the paper clip and tie a knot.
- Take the other piece of fishing line and tie it to the metal loop on the end of the lead fishing weight.
- Ask a parent to hang each object from the ceiling or a table. Make sure they’re secured. The two objects should hang side by side so that they are barely touching, but not leaning on each other.
- What do you think will happen when the moving lead weight strikes the stationary golf ball? Write down your hypothesis, or best guess about what you think will happen, in your notebook.
- Grab the fishing weight and take five steps backwards. Lift the weight up and then let it go so that it swings and strikes the golf ball. Record what happens in your notebook. Did the golf ball move? How did it move? Did the weight slow down when it hit the stationary golf ball?
- Stop the movement of both objects.
- What do you think will happen when the moving golf ball strikes the stationary lead weight? Write down your hypothesis in your notebook.
- Move to the other side and take hold of the golf ball. Take five steps back. Lift the golf ball up and let it go. Record what happens.
- What do you think will happen when both objects swing towards each other and collide? Write down your hypothesis in your notebook.
- For this third trial, find a friend or parent to help. Face one another. Each of you will take hold of one of the objects. Take five steps backward, lift the objects and let them go on the count of three. Observe what happens and record your observations.
The lead weight will have a more noticeable change on the course of the golf ball in all three experiments—even when the lead weight sits still, and the golf ball slams into it. In all three experiments, the path and position of the lead weight will change only slightly when it comes in contact with the golf ball.
This project is a great demonstration of Newton’s first law: An object at rest remains at rest unless acted upon by an outside force, and an object in motion remains in motion and travels at a constant speed unless acted upon by an outside force. We applied forces to our objects in each of our experiments, and we understand why each moved—but in each instance, the movement of the golf ball was more dramatic than the movement of the lead weight. Why? There are two big ideas we have to keep in mind here.
The first comes from Newton’s third law: when one object exerts a force on another object, that second object exerts a force back (we saw this law in action every time we saw our objects slam into each other). This force is always equal and exerted in the opposite direction of the original force. But wait! If an equal force is applied to both objects, then why does the golf ball get launched so much farther than the lead weight?
The heavier or more massive an object is, the more inertia it has, and the more inertia an object has, the harder it is to move. If a really massive object is already moving, inertia makes it just as hard (if not harder) for an outside force to change how that object moves. Our lead weight was so much more massive than the golf ball that its motion hardly changed at all during each of our three experiments, even though an equal force was always exerted on the golf ball.
Ever wondered why it’s so much harder to throw a shot-put than it is to throw a baseball? Well, now you have your answer!