Science Buddies: "Ask an Expert"

I'm after a bit of help. We've been set an 'egg race' challenge, with a few constraints. We simply have to get an egg as far as possible from the start point, measured horizontally. It can be ground based or launched skywards, however if we do send it airborne the horizontal measurement is taken from the touchdown point rather than the eventual resting place. We can do whatever we want, with points for distance, egg condition, and style, but the device can only be powered by the energy stored in a large elastic band OR a 1.5V AAA battery.

I'm sure the logical route would be to think about designing some sort of buggy considering we are allowed to go ground based, but that just isn't as much fun! So with this in mind my initial thought was to use the battery to drive a motor that would, via gearing, tension our own set of elastic bands to form a catapult for a balsa wood glider. I don't know whether there would be enough power in a battery to do this to great effect however, and the gearing isn't particularly efficient.

I'm sure there are other alternatives - cannon launch via power transfer maybe? - but I have absolutely no idea whether the availability of a battery is a bit of a red herring, with a large elastic band containing far more energy! Any advice would be welcome, but ultimately I'm not too fussed if we obliterate the egg as long as we do it in style!

Thanks

Hello stevie_j_b,

It seems like you are in for some fun

I think many, many considerations bear on your choice. I'm sure that I can't think of all of them, so here are some random thoughts.

1. The battery can deliver its energy only over an extended period of time, whereas the elastic could be rigged to deliver it rapidly (e.g. a catapult) or slowly (e.g. turning a regulated mechanical device). It would be very difficult to design an airplane that could lift an egg while powered only by a AAA-driven propeller.
2. Could you use a blimp (He filled vehicle) powered by a AAA-driven propeller? It might travel a very long distance.
3. Since you can choose an elastic band of any size, it can contain more energy than a fixed-size battery. This means it always will win the distance competition if well-engineered.
4. Your mention of using a battery to tension an elastic band seems unnecessarily complicated since you could do it by hand, so I think I understand that you're more interested in style than performance.
5. The airborne route surely exposes the egg to more risk than via ground.

Hi,

Thanks for the reply!

I should probably elaborate a bit on point 3 and 4. The egg, battery, and elastic band are all provided on the day, and all we know about the latter is that it is a 'large' elastic band. It'll be a typical office one, that's for sure! The only propulsion rule is that the egg must be transported using the energy of the battery or the elastic band.

Now, with this as a basis (and considering I really want to get this thing airborne!), I'm trying to find ways of yeilding the energy from the most capable device. That has got to be the battery, right? Now logically, I'm not going to be able to get this 'glider' off the ground using a battery-driven prop, so I need to find a way to harness the battery power into a more suitable launch method. This is where my mention of a larger piece of elastic comes in. According to the rules I should be able to design a nice, powerful catapult to launch the glider on the basis that I use solely the energy of the battery to tension the elastic. This breaks no rules, as I'm not physically or otherwise imparting any additional energy into the 'system'!

This therefore brings me back to my original question: If a catapult is the best way of getting an egg airborne, would I be better to use the provided elastic band and physically tension it (similar to the catapult in a childrens glider), or use the energy of the battery to tension a much larger catapult? Is there even enough power in a AAA battery to do the latter? Yes, this could be very messy

stevie_j_b,

Okay. Since you don't know the kind of elastic band, you can't know whether it would provide more energy than the battery or not.

The amount of charge (current x time) in an alkaline AAA cell is typically given as 1200 mAh. The corresponding energy would be 1.2 Ah x 1.5 v. = 1.8 Watt-hours = 6480 J.

Let's guess at the elastic band energy. Suppose it stretches 6" (15.24 cm) and has a force that increases linearly from zero to 3 lb. (13.34 N) at full extension. That's an average force of 1.5 lb (6.67 N). The energy used to extend it would be 6.67 N x 0.1524 m = 1.02J.

Please check my calculations and unit conversions. I pulled the numbers for the elastic band out of thin air. If you find a big office rubber band and measure its force, that would be a much better estimate.

You will lose a lot of the battery energy in whatever mechanical rig you use, but it seems you can get much more out of the battery.

Be sure that the rules allow your catapult design.

You're going to need a BIG elastic band. Surgical rubber tubing is often used for such a purpose. The glider will need to be large enough to carry the egg and withstand the sudden force of the launch.

Good luck and be careful. Once the elastic band is pulled tight, it could do some damage.

WW