# Light propelled toy boat?

## Main Question or Discussion Point

[SOLVED] Light propelled toy boat?

Hi,

I was wondering if it would be possible to to produce an LED propelled model boat - no practical application, just a toy.

I think it would be possible to make a boat using something on the order of 3 high-power LEDs, with a small lithium based battery, with a total weight of the order of 100 grams. The LEDs can be pretty well focussed, so nearly all the light would go out in one direction, to about 10 degrees of arc. The boat would be something like 100mm long by 50mm wide. The battery wouldn't last long, but nevermind.

This is how far I got...

Total power consumption of LEDs = ~10 watts.
Light output = ~700 lumens (3x Cree XR-E "Q5" grade).

From: www dot lumitex dot com slash spec_1.html

I think you get about 1 watt out of that in light (1 lumen = 1.5* 10^-3 watts) i.e. around 10% efficiency - you'll have to dump the other 9 watts of heat into the water.

My somewhat rusty physics tells me that pushing 1 watt worth of photons out the back of the boat should result in an equal and opposite forward force on the boat? Is this the case? If so, would the force be enough to actually push the boat forward?

Assuming no friction, I tried to calculate using (ke=1/2mv^2) the velocity after once second (i.e. 1 joule of k.e), and got a speed of 4.4 meters per second, which didn't seem to make much sense to me...

What am I doing wrong?

Cheers,

Tim.

Related Other Physics Topics News on Phys.org
Another opportunity for a xkcd link

http://blag.xkcd.com/2008/02/15/the-laser-elevator/" [Broken]

look for the squirrel

Last edited by a moderator:
A question of momentum....

Ahh, that makes sense.

Strangely enough, I did have the idea of mounting the LEDs at the centre of a mirror, and then mounting a fixed mirror on the side of the tank, with the vague feeling that it'd get more energy out....

Oh well, could have been worse - I could have actually gone to the trouble of building one.

I'll stick to building the LEDs into bike lights instead.... diyled.org.

Tim.