Suggestion needed for 3rd grade physics demonstation

[Geek4Life]

If you were a parent volunteer, and you were asked to do a fifteen minute 3rd-grade classroom presentation to demonstrate ONE "law of physics" or to teach one thing about physics or physics related, what would you do as a "fun" example that 3rd graders would enjoy and hopefully learn from and remember.
Thanks in advance!

 

[Drakkith]

I wouldn't worry too much about sticking to a single "law", as the label is mostly pointless. Just do something fun. Perhaps something with magnets?

 

[Bobbywhy]

I like using rubber bands and folded paper wads to make slingshots with our fingers.
Here we demonstrate elasticity, energy storage and release, acceleration, trajectories, and more. It's also fun.

Cheers,
Bobbywhy

 

[Thundagere]

As a third grader, I personally found the idea that heavier things don't fall faster fascinating. Mind you, I was a complete nerd, so others might not find it as interesting. Maybe make a simple motor with a coil or wire, or make a parallel plate capacitor and get some tiny sparks in the dark?

 

[Lsos]

Wire in a lighbulb to a generator to show that the generator gets harder to turn when there's a load on it. This will save ALOT of future man-hours which would otherwise be wasted on perpetual-motion "ideas".

 

[Geek4Life]

I like using rubber bands and folded paper wads to make slingshots with our fingers.
Here we demonstrate elasticity, energy storage and release, acceleration, trajectories, and more. It's also fun.

Cheers,
Bobbywhy

Thanks for all the great ideas! I especially like the finger slingshot idea, though I'd have to clear it with the teacher. Or it might be safer to make a little catapult and show the effect of different angles, and adding more rubber bands. Perhaps a competition between two groups. But the time is limited to fifteen minutes. Hmmmm. Good ideas, thanks!

 

[ModusPwnd]

My thought is to crush a soda can with atmospheric pressure by boiling some water in it then cooling it upside down in an ice bath.

Then maybe show them this;

 

[Geek4Life]

My thought is to crush a soda can with atmospheric pressure by boiling some water in it then cooling it upside down in an ice bath.

Then maybe show them this;

Thanks for that video. It's awesome. I like the idea of the coke can, and I think some of the kids will relate because some of them go to a hiking spot that is about 4,000 ft above us, and if you close any empty plastic water bottle up there and bring it back down, it will partially collapse.
I can just see the kids faces when they see what happens to that tank in the video. Good suggestion.