Can Physics Repair My Ping Pong Ball?

By Matthew Allen

Often when trying to solve a hard scientific problem, I sit down with a hot cup of tea to help me relax and hopefully solve the problem. However, what if the cup of tea itself was the solution!

When you’re a master ping pong player like myself, sometimes you hit the ball so hard that it puts a dent in the ball! OK, so I’m lying, I actually dented the ping pong ball in our office by accidentally standing on it; but the point stands, our ongoing tournament of office ping pong has been put on hold by my clumsiness. After thinking over the problem of how to fix the ball for a few minutes, I finally had a brain wave. I am a scientist! I can fix my dented ping pong ball using my knowledge of physics, pressure and a hot cup of tea!


Under pressure

Have you ever noticed that the bag of crisps that’s in your luggage on a plane seems to inflate as the plane goes to higher altitudes? This is due to a relationship between three physical parameters: Pressure, Volume and Temperature. Émile Clapeyron first noticed this relationship back in 1834 and named it the Ideal Gas Law. It states that:

Pressure x Volume is proportional to Temperature

Let’s go back to our aeroplane crisp packet example. The crisp packet contains air (as well as delicious crisps!). As the plane ascends, the pressure inside the plane decreases, as the pressure outside the plane also decreases. The equation above says that if you halve the Pressure of the air in the crisp packet, then either the Temperature of the air must halve or the Volume must double, to make both sides of the equation remain equal (or some combination of the two). In the case of our crisp packet, it is mainly the Volume of the air in the bag which increases, which is why the crisp packet inflates itself.


A nice hot cuppa

But what does all of this have to do with my ping pong ball? If you had a dent in a sheet of metal, such as on the door panel of your car, you would knock the other side of the metal with a hammer until you flattened it back out. If we could get inside the ping pong ball and push against the inside of the dent, we could pop it back to the right shape; but we can’t get inside the ping pong ball. However, we can use the air inside to push against it.

Using our equation above (Pressure x Volumes is proportional to Temperature), if we were to increase the temperature of the air inside the ping pong ball, then either the Pressure or Volume of the gas would increase. My hypothesis, is that the pressure inside would increase, until it was too much for the dent to handle, popping the dent back out. This would increase the volume of gas inside the ping pong ball, lower the pressure back down, and hence our equation would be balanced again.

If I therefore put my ping pong ball into a hot cup of water (or even a cup of tea), then the air inside should expand, pushing against the inside of the dent and repair my ping pong ball. I performed my experiment and you can see the step by step process in the images below

Step 1 – Get your dented Ping Pong Ball

Step 2 – You’ll need a spoon and hot water

Step 3 – Put the ping pong ball in to the hot cup of water. Be careful!

Step 4 – Use the spoon to push and hold the ball under water.

Step 5 – Take the ping pong ball out of the water using the spoon and leave it to cool. Ta da! A repaired ping pong ball!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

So there you go, I managed to fix my dented ping pong ball using my knowledge of physics, pressure and the ideal gas law. Now if only I could use a similar technique to make me good and ping pong!


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