# The Naked Scientists Forum

### Author Topic: What sorts of waves will propel a ping-pong ball best?  (Read 2130 times)

#### CZARCAR

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##### What sorts of waves will propel a ping-pong ball best?
« on: 27/11/2010 15:25:07 »
a plunger resides at the center of a circular pool & plunges with the same power. Plunger can vibrate fast or plunge deeper & slower so to create different waves?
a ping pong ball is placed in the water
question is = will the ppball travel faster to the edge of the pool & by which method?
« Last Edit: 06/12/2010 08:37:16 by chris »

#### SteveFish

• Guest
##### Re: What sorts of waves will propel a ping-pong ball best?
« Reply #1 on: 27/11/2010 17:18:13 »
Molecular movement in a traveling wave are perpendicular to the direction of the wave. This means that water molecules go up and down as the wave moves across the surface. So water motion itself would not move the ball. I can think of two mechanisms that would move the ball in the direction the wave is moving.

1) As the ball is raised up on the slanted surface of the wave front it will tend to slide downhill a little in the direction of the wave. This means that it takes a little longer for the wave to pass under the ball and there is more time for it to slide down. On the backside of the wave the ball would tend to slide in the direction opposite wave motion but it wouldn't make up for the movement on the front side of the wave because movement backward reduces the time it is on the rear wave face.

2) The moving wave fronts might act as paddles that could create air movement near the water surface in the direction of the waves. This could blow the ball across the surface a little in the direction of the waves.

I don't know how wave period and amplitude would affect these mechanisms.

#### tbarron

• Jr. Member
• Posts: 22
##### Re: What sorts of waves will propel a ping-pong ball best?
« Reply #2 on: 27/11/2010 23:14:16 »
Molecular movement in a traveling wave are perpendicular to the direction of the wave. This means that water molecules go up and down as the wave moves across the surface. So water motion itself would not move the ball. I can think of two mechanisms that would move the ball in the direction the wave is moving.

1) As the ball is raised up on the slanted surface of the wave front it will tend to slide downhill a little in the direction of the wave. This means that it takes a little longer for the wave to pass under the ball and there is more time for it to slide down. On the backside of the wave the ball would tend to slide in the direction opposite wave motion but it wouldn't make up for the movement on the front side of the wave because movement backward reduces the time it is on the rear wave face.

2) The moving wave fronts might act as paddles that could create air movement near the water surface in the direction of the waves. This could blow the ball across the surface a little in the direction of the waves.

I don't know how wave period and amplitude would affect these mechanisms.

At a guess, increased amplitude would emphasize the differential in 1), resulting in increased lateral movement of the ball. I doubt altering the period of the waves would have much effect. I would think increased amplitude would increase any effect due to 2) as well, again with period not making much difference. That's my guess.

#### maffsolo

• Sr. Member
• Posts: 280
##### Re: What sorts of waves will propel a ping-pong ball best?
« Reply #3 on: 28/11/2010 00:18:13 »
If the diameter of the plunger face were half the diameter of the pool the pool water depth is a quarter size of the pools diameter, a quarter wave of a single upward stroke of the plunger. may produce a quarter wave, permitting the PPball to travel to the pools edge in one full swoop.
Sort of a single surf wave ride
« Last Edit: 28/11/2010 00:26:09 by maffsolo »

#### yor_on

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##### Re: What sorts of waves will propel a ping-pong ball best?
« Reply #4 on: 04/12/2010 07:28:51 »
A wind? :)

I think that ping-pong ball would stay still, more or less, the waves water molecules would have a slight friction and suction on it in the direction of their general motion but a wind would do the trick.

The reason being that a ping-pong ball have a very little surface touching the water, and is very light. The deeper submerged into the water the more 'force' I would expect the waves direction to have on it though? Up to a limit of course :)

Maffsolo, could you expand on your example. It was interesting.
===
As for what kind of waves that would propel it I guess fast 'light' waves would do it best, as the faster they come in time, and considering the ping-pong balls light mass, :) they should move it most efficiently.
« Last Edit: 04/12/2010 15:26:40 by yor_on »

#### The Naked Scientists Forum

##### Re: What sorts of waves will propel a ping-pong ball best?
« Reply #4 on: 04/12/2010 07:28:51 »