Saturday Science: Bouncing Basketballs
One of the most important skills in basketball is dribbling. You have to do it all the time. Sure, baseballs, footballs, even golf balls can bounce, but basketballs are designed specifically to bounce, and if yours isn’t bouncing, you’re gonna have a bad time. What is it about a basketball that makes it so very good at its job? That’s the subject of today’s experiment.
See, the basic reason a basketball bounces is simple: it’s all Newton’s third law of motion. For every action, there is an equal and opposite reaction. When the ball hits the floor, it pushes on the floor, and the floor pushes back on it. These pushes change the shape of the ball a little, compressing the air inside, and an instant later the air pushes back out, returning the ball to its original shape, pushing on the ground again, getting pushed by the ground again, and the ball, uh, bounces.
That’s the simple version, though. Every part of that explanation can change how a ball bounces in the real world. What’s the ball made of? How much air is in it? What kind of floor is underneath you? How far did the ball fall? Heck, what planet are you on? Basketball’s gonna be different on Mars. Just saying. So let’s choose one of those variables and play with it so we can look at the results. Today you will be doing science in one of the most satisfying ways possible: dropping stuff to see what happens.
- A basketball (make sure it’s nice and pumped up with air)
- A yardstick or measuring tape
- A stepladder (optional)
- Multiple different ground surfaces (e.g., concrete, grass, carpet, linoleum, dirt…whatever you have around)
- Paper and pencil (to record your measurements)
- An assistant
- This one’s easy: pick which surface you’re going to test out first.
- Give your assistant a cool name like Igor or Jeeves or Phlebas.
- Have your assistant stand next to you and hold your measuring implement.
- Decide how high you’re going to drop the ball from. Head level? Above your head? If you’re using a stepstool, decide which step to use. Once you make your choice, keep it the same for every single drop going forward.
- Do five drops on your first surface. Keep a close eye on the ball, and instruct Phlebas to do the same. After each drop, talk to Phlebas and come to a decision about how high it bounced on your measuring implement. Write your measurements down.
- Repeat step 5 with all of your other surfaces.
- Once you’ve run out of surfaces, compare your bounce heights across surfaces. Which surfaces bounced higher than others? Can you think of why that might be?
The thing I described up above, with the air compressing and then returning to its original size inside the ball, is happening on every single surface you bounced your ball on, and as far as the ball is concerned, it’s happening the same way. So how come different surfaces bounced higher than others? It all has to do with how energy moves between the ball and the ground.
There are two kinds of energy that are important to this experiment: potential energy and kinetic energy. Potential energy is just what it sounds like: energy stored in something that has potential to make it do something. When you pull a rubber band back, it has elastic potential energy, and when you let it go that potential energy makes it move. When you hold a ball above the ground, it has gravitational potential energy, basically the potential to fall, and when you let it go, well, it falls. This is where kinetic energy comes in. Kinetic energy is the energy of motion. Anything moving has kinetic energy, so when you drop a ball, its potential energy turns into kinetic energy.
When the ball hits the ground, some of that kinetic energy is what’s used to moosh the ball and compress the air, and some is transferred into the ground. Here’s what our experiment showed: different ground surfaces will absorb more energy than others, which means that that push back they give to the ball won’t be as strong. A hard surface, like concrete or hardwood, hardly absorbs any, so most of the kinetic energy of the fall goes into bouncing the ball back up. A soft surface, like grass or carpet, absorbs more energy from the fall, so there’s less left to push the ball back up, and it bounces pretty badly. Pretty obvious now why indoor basketball courts are wood and not carpet, and outdoor ones are asphalt and not astroturf.
Ready to practice your dribbling drills and skills? Take your energy to the Indiana Pacers and Indiana Fever Basketball Experience at the Riley Children's Health Sports Legends Experience this summer!