Q:

In science we are building rockets to see who will have the one to stay in the air the longest. I was wondering if i paint the bottle will the weight be heavier than if i put tinfoil or the weight wouldnt matter?

- Aryanna (age 14)

Canada

- Aryanna (age 14)

Canada

A:

Weight definitely matters. But it's worth thinking about why it matters—after all, you probably know that two objects dropped from the same height will hit the ground at the same time, even if they have different weights (as Galileo famously observed). Two projectiles launched at the same speed will also stay in the air for the same amount of time even if one is heavier. However, a heavier rocket won't be able to speed up as much as lighter one with the same amount of fuel, so the lighter rocket will go faster, higher and will stay in the air longer.

I'm not sure whether paint or tin foil would be heavier. The density of aluminum (which is what "tin" foil is actually made of) is about 2.7 grams per cubic centimeter, and typical foil is about 0.0016 centimeters thick. 2.7 g/cm^{3 }× 0.0016 cm = 0.00432 g/cm^{2}, so you'll add about 4.3 milligrams of weight for each square centimeter of foil you use.

The weight of the paint is a little harder to estimate. It will depend on how thick your layer of paint is, and different kinds of paint probably have different densities. The weight might also change as the paint dries. If you have a precise scale, you could measure the weight of something (maybe a piece of paper of a certain size) before and after you paint it to find out how much weight it adds.

Weight is important, but you should also consider how paint and foil might affect air resistance. Good luck with your rocket!

Rebecca Holmes

PS: Did you know that the paint on a Boeing 747 airliner weighs ?

*(published on 11/18/2014)*