Test Your Knowledge #3

Could You Walk Across a Pool Filled with Ping-Pong Balls?
That was the odd but intriguing question posed on the Web site PhysicsForums.com. To read the entire message posted by the inquisitive Matt and the answer that makes the most sense to WaterShapes EXTRA!, see below.

Here is the question posed by “Alaskan Matt” on the Web site PhysicsForums.com:

I feel a little out of place here, as I have no physics background whatsoever, but I was hoping you folks here might be able to help settle a debate. My brother and I like to have “what if” discussions, and one we had quite awhile ago was whether a person could walk across a swimming pool filled with Ping-Pong balls (EDIT: No water in the pool, only ping pong balls).

I said that it would not work, because the balls, being so light and having a smooth sphere shape, would cause a standard person to sink into the balls, and one would have to “wade through” them to get out of the pool.

My brother, however, insists that since the balls would be supported by the sides and bottom of the pool, they would support a person who could easily walk across.

If this is too unscientific of a question, feel free to ignore or delete it, but this debate has been going on for quite a while and I was hoping that someone more experienced in weight/friction/physics issues could explain what would happen better than myself.

Thanks for your time,

Matt.

The Correct Answer?

The following response – by “Borg” – made the most sense to WaterShapes EXTRA!:

Yes, you would sink. When you walk, your weight is spread over the area of your feet. Assuming that your foot covers 10 balls and you weigh 150 pounds, each ball is roughly exerting 15 pounds of force in some direction when you take your first step. Since the other balls can’t go down, this would result in them rising up around your foot and you would sink. In order to walk on Ping-Pong balls, you would have to spread your weight over a larger area by using snowshoes or something like that.

However, we should point out that since the person walking on the Ping-Pong balls presumably has two feet, they would be standing on 20 balls (not 10). Does that mean each ball would be exerting only about 7.5 pounds of force – and if so, does it make a difference? Are there any watershaping physicists out there who can clear this up?