Physics and a Tennis Ball

Fact: Tennis balls float in the sea. 

Due to being on a cricket team whenever I go anywhere on holiday for an extended period of time I make sure I pack a ball (be it a cricket ball or a tennis ball) in my suitcase to practice catching or bowling whilst away, but after taking my cricket ball to Germany in my hand luggage and thus having my baggage checked at security for a suspected bomb I thought it might be the best idea to leave any spherical-shaped items at home when heading to Turkey for a three week diving holiday. But after being convinced by a friend I packed my dog’s chewed up tennis ball, justifying it only by curiosity to see if it floated in the sea.


Would the tennis ball float or sink in the sea?

The tennis ball was packed in my suitcase (I wasn’t having my hand luggage checked at security again no matter what) on arrival it was packed directly into my beach bag. A few days after our arrival, after having done the first dive of the day (Flying Fish Reef) I decided not to do the second dive of the morning, instead, holding only the old tennis ball  I cannon balled into the sea, forgetting mask, snorkels and fins. Upon impact of my jump on the water I let go of the ball and I felt a moment of dread, my only thoughts being:  ‘if the ball sank now…’

When I swam to the surface however I laughed when I found the half chewed lime green ball next to my head, the tennis ball had beaten me to the surface! After my mini experiment the tennis ball was played with constantly on the dive boat, whether this be a game of catch between various people on our dive boat, or even with divers from other boats, thrown from one boat back to the other.

However I did feel rather stupid after having done my mini experiment, of course a tennis ball would float in the sea for two reasons;

  1. A tennis ball’s shell is made of thin layer of rubber. Rubber is less dense than water and thus floats.
  2. A tennis ball is hollow and filled with air. Air, as all divers know, is less dense than water helping the ball to float.

    My dog eaten tennis ball and my well-travelled cricket ball both float

Overall the tennis ball floats because the ball is less dense than water this means it is positively buoyant, and therefore floats on the sea. If I had taken my cricket ball with me, the boat would have more than a few dents in its frame right now, and the cricket ball made primarily  of cork and leather are both, also, less dense than water so would be positively buoyant and float, my releasing the ball when I jumped would have also meant that the cricket ball would float to the surface, however the salt water would have damaged the leather coat of the ball.

Something I found really interesting was that if you were to puncture the tennis ball you would be able to take the ball on a dive and attempt to play catch with it underwater. This also makes sense. When puncturing the ball you allow the air in the tennis ball to escape. When putting the ball in the sea you let the ball fill with water. This makes the ball less positively buoyant. Resulting in the possibility of playing catch/tennis underwater.

One of the diving instructors on the boat had offered to puncture my tennis ball, but on realisation that the puncture would prevent it from ever bouncing again I declined, although it would have been really interesting to play tennis underwater, above the water a deflated tennis ball is not really very useful.

After having conducted my mini-experiment I will be taking my tennis ball with me on holiday more often, rather than buying a beachball, which after a short period of time will definitely get a puncture and become useless, even when my tennis ball eventually gets a puncture I will be able to take it on a dive and attempt to play tennis with it.  For me tennis balls will always come before a beach ball, although I will not be carrying it in my hand luggage anytime soon.

5 responses to “Physics and a Tennis Ball

  1. Pingback: Fuzzy Balls « Blair Pettrey·

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