Why do balls bounce?

Nearly 200 students at our school are spending their time after the last bell rings for the day learning, training, competing and representing our pack as sporty wolves in the gym. They are playing basketball, volleyball and soccer, and are going to play rugby and badminton in the future.

With nearly 100 balls living on our campus (I did not count them), in different shapes, forms, ages and functions, I thought it was time for us to understand how they work and why they work the way they do. More specifically, as this season has been pretty bouncy with basketball being played by the U15 teams, and volleyball being played by the varsity teams, I thought it would be an appropriate time to understand the ability of our balls to bounce and the physics behind it.

(‘Cause why not!)

A ball’s ability to bounce is based on its elasticity and its mass. Elasticity is the ability of an object to deform by force and return back to its original shape and size after the force is removed (Education.com). But how does a ball get deformed, and how does mass affect its bounce? To understand why this is so, we need to travel with a bouncing ball. 

To start with, imagine holding a ball above the ground.  As the ball is above the ground on earth, we know that it is affected by gravity. Gravity is the force that pulls any object, in this case, the ball, closer to the middle of the earth. It is a constant force, with a constant acceleration of 9.8ms-1 (Carlsen). That was pretty simple and straightforward. However, there is more to it. 

A ball held high will also store potential energy or the energy stored by objects at rest (Carlsen). In this case, the Potential energy is directly proportional to the height of the ball, the mass of the ball and gravity acting on it (Carlsen). 

When we release the ball, the potential energy converts into kinetic energy or the energy of a moving object [see figure 1]. But when the ball reaches the ground it is no longer moving, does that mean it has 0 kinetic energy? Yes, but that doesn’t mean the energy is used up or lost (Carlsen). The law of conservation of energy states that “energy can neither be created nor destroyed”. This means that the kinetic energy of the ball needs to have been converted into another energy, and here is how a ball bounces.

When the ball hits a surface it pushes against the matter on the surface (Carlsen). According to Newton’s third law of motion, since every action has an opposite and equal reaction, the matter of the surface pushes back on the ball and causes it to deform (Carlsen). [refer to figure 2]

figure 2

The deformation proves that the kinetic energy is now converted into elastic energy (Carlsen). However, as the elastic energy needs to be released, it converts back into kinetic energy and bounces back (Carlsen). But why would the object have enough energy to bounce after it reforms its original shape and size? Now, this leads to the importance of the stiffness of the object. The stiffer the ball is, the less it physically deforms, while more elastic energy is applied to it. Therefore, when it reforms there is extra energy to bounce up. That is why a ball bounces back up, while a stress ball which is also elastic doesn’t. [refer to figure 3]

figure 3

Another scenario to consider is what makes a volleyball bounce higher than a soccer ball. This time, the answer to this lies in the difference in mass between these two objects.

Now, a soccer ball is approximately 400 to 450 grams, while a volleyball is approximately 260 to 280 grams (Art). Since potential energy is directly proportional to mass, the soccer ball will have greater potential energy than a volleyball will (Murthi). However, if you might know or remember Galileo’s experiment on the leaning tower of Pisa you might know that gravity’s acceleration on it will not change, but be the same at 9.8m/s-2.

To those who have no idea what Galileo’s experiment was, here is a very quick crash course. So our famous nerd boy Galileo, who can be blamed for why our physics and math books are the height of Mount Everest, decided to go to the roof of the tower of Pisa, and drop two balls of significant different masses. When the balls hit the ground at the same time, he proved that the effect of gravity is the same on all objects regardless of their mass (Cox).

Now, back to bouncing balls.

Since the effect of gravity will be the same for both the soccer ball and volleyball, the amount of potential energy converted into kinetic energy will be the same. Therefore, the same amount of kinetic energy will be converted into elastic energy and then, again, kinetic energy for it to bounce.

However, since the soccer ball is heavier than the volleyball, the effect of the kinetic energy will lead to a greater bounce on the volleyball than the soccer ball.

To conclude, a ball’s ability to bounce greatly depends on its elasticity and the effect of the elastic energy is based on its mass. The more elasticity, the lesser the mass, and the more the ball bounces. 

That is why the balls used for many sports are filled with air. This allows them to be lightweight and bounce better. But this conclusion also raises the question: do any light objects bounce more than any heavy object? Do needles bounce better than soccer balls which are significantly heavier? Of course not, anyone who has not been living under a cave would know that. But why? And to answer this question comes another aspect that affects a ball’s ability to bounce: shape. 

Every ball is spherical. In fact, an object can only be called a ball if it is spherical. But why? As we discussed earlier, the elasticity of a ball is what allows a ball to bounce. When a spherical object hits the ground, only a small part of its surface makes contact with the ground. The area around this point is also equally affected by the fall, and so will try to stretch downwards. This allows for the conversion from kinetic to elastic energy. 

figure 4

There are many more aspects that involve understanding the manufacturing and the different layers of different balls themselves that affect their ability to bounce, but that will be something for the young physicists of our community to dig deeper into!

But for now, you know why volleyballs are used to play volleyball (a bouncy sport) and why soccer balls are not! As our sports season is at its hottest period with league games beginning, good luck to our sporty wolves! To all KIS students, please go and support our teams, cause though they use the right balls, they need the energy to hit, slap, dribble, and kick them to win games, which we can give through our love. GO WOLVES!

Works Cited

Art. “Soccer Ball vs. Volleyball (Can You Use Them Interchangeably?).” Penalty File, 4 Sept. 2021, penaltyfile.com/soccer-ball-vs-volleyball/#:~:text=But%20according%20to%20their%20official. Accessed 24 Sept. 2022.

Carlsen, Matthew. “Why Do Some Balls Bounce and Others Don’t? || (PHYSICS EXPLAINED).” Www.youtube.com, 4 May 2020, youtu.be/71rdfZr_PgY. Accessed 16 Sept. 2022.

Cox, Brian. “Brian Cox Visits the World’s Biggest Vacuum | Human Universe – BBC.” YouTube, 24 Oct. 2014, youtu.be/E43-CfukEgs.

Education.com. “Education.com.” Education.com, Education.com, 9 Sept. 2013, http://www.education.com/science-fair/article/ball-bounce-higher-dropped-greater-height/.

Murthi, Keerthi. “Does Mass Affect Potential Energy:Detailed Facts,Examples and FAQs.” Lambdageeks.com, 18 Dec. 2021, lambdageeks.com/does-mass-affect-potential-energy/#:~:text=How%20does%20mass%20affect%20gravitational.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s