We got an email from the people running SteelyKid's summer camp asking for volunteers to speak at a career day sort of event early next week. I said "Sure, I can do that, and talk about the glamorous life of a physics professor and book author." They said "Great, you'll be talking to several groups, ranging from second-graders down to 3-4 year olds."
That's... not quite the audience I was expecting (the camp runs up through 5th grade or so). I don't think they're going to care all that much about physics research, so instead, I'll probably say "My job is to teach people about science" and then show them some simple physics tricks that don't need many props. I was thinking of showing how light and heavy objects fall at the same rate, maybe a center-of-mass trick (the "stand against a wall and try to pick an object off the floor" game), and possibly the elastic-collision trick where you drop a tennis ball on top of a basketball and it hits the ceiling. The theory being that simple demos involving things and/or people falling on the floor will wow them even if they don't understand the science.
But this seems like a good topic for polling my wise and worldly readers. So, if you were going to talk to a bunch of little kids about physics, and wanted a single really simple demo (i.e., no fancy electronics, no liquid nitrogen or other expensive props) to show them, what would you do?
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I don't know particularly, but I'd look through the UMD lecture demos
http://lecdem.physics.umd.edu/index.php/demos-by-class
for ideas. Some of them are basically equipment-free.
Also: lasers! You could shine a laser pointer on a diffraction grating / CD.
No liquid nitrogen? Why do you hate education?
(I used to volunteer at the Houston Nat Sci museum and do demos. Though they stopped stocking lN2 for us, along with everything else. We--the volunteers--were pretty annoyed at them.)
Is this indoor or outdoors? Can you get some pairs of spectroscopic glasses? If they have any florescent lights around they could look at that, as well as other chemical light sources.
Alternately, some polarizing filters might be interesting, particularly if you can show them how it cuts glare from the ground or what the sky looks like.
Using a beach-ball and a heatgun to show thermal expansion might also be interesting.
Kids this age loooove playing with magnets. Get some magnet toys and show them action at a distance. Ask a volunteer to try to do an impossible thing: vertically balance two stick magnets N on N. Then ask them to do N-S and explain why it works.
Also play with static electricity: get a balloon, rub it against the head of a volunteer, then drip some water from a glass (if outside) or do it on a sink (if inside), and place the balloon near the jet. Amaze them with the deflection of the water jet.
Something that should be reasonably easy to make yourself, if you can't borrow one from the lab demo room: Five identical steel balls, each suspended as a pendulum, such that at rest the balls are all touching. Lift the ball at one end, let it go, and one ball at the other end pops out after the collision. Two balls together at one end, two pop out at the other end. Et cetera. Momentum and energy are conserved!
Or, if you have a chair that can freely rotate about the appropriate axis (and only that axis), take that and two weights (e.g., the sort of sphere used in the shot put), hold one in each hand, stretch your arms out fully to the side, and start spinning. Then bring the weights in next to your body, so that you spin faster.
There are a few others I could come up with that are appropriate for small children but would require fancy equipment and/or liquid nitrogen, so stick with those two.
Tennis ball on basketball. Drop, and the tennis ball pops up on landing. Explain that this is the basic physics behind outer space probes "slingshotting" and gaining speed from planets.
Get a tricycle of some sort. Set the pedal near the audience to the bottom position. Ask what will happen when you pull forward on that bottom pedal. Then do the demo.
Paper helicopters generally go over well, particularly if there is a high place to stand to drop them from. And you can use them to talk about gyroscopic stabilization, torque, and air resistance.
@Eric Lund: It turns out that the Newton's cradle does not exactly work with one sphere in, one sphere out (basically, you've got 5 different velocities available and E and p conservation only provide two constraints) . In fact you always get spalling and eventually the system will be chaotic. This always happens, no matter what people say about damping (the damping does make the system settle down to a point where all the spheres are swinging together in synchrony though).
As an idea, couldn't you give a simpler version of your "what is color?" talk? You could probably get away with a few flashlights and color filters.
Also, with a magnifying glass or two, kids could look at newsprint and see dots of the various colors (probably the subtractive ones plus black) for a nice example of "this is some science you see every time you read the Sunday comics".
Ask the kids if they think one of them could lift you up (or perhaps a more familiar adult camp worker). Get one of them (perhaps the one they think is strongest) to try - unsuccessfully. Then produce a 2x4 and fulcrum and have another (perhaps the smallest kid) succeed using it as a lever.
These kids are probably just learning to read, teaching them about energy levels and molecular hybridization is probably gonna fail. I think we can agree on that. I say bring a drink. Like lemonade. Plug the end of the straw. No mass in, no mass out. They'll at least understand it's what they do.
My kids at that sort of age loved experimenting with things that floated or sank. Do you get to do a hands-on demonstration? A tank of water and a lot of different objects to drop in?
You can drop a tennis ball on a basketball and hit the ceiling?
For that audience, at that age who often feel small or ineffectual, I think the power of simple machines is a home run. Levers and pulleys properly prepared demonstrate what the smallest of children can do with applied physics- and a perfect segue into how learning (physics and mathematics too, of course) amplifies every individual's effect on a great big universe.
@Wesley: Yep--if if you drop much larger object just below a smaller one, the smaller one bounces three times higher than the drop height. You can also stack more objects (each much smaller than the one below) to get a 3x gain in height term.
There is a demo where a person stands on a rotating table and is given a spinning bicycle wheel. They tilt the wheel and spin due to the gyroscopic effect. Main problem is likely to find the right size wheel for the kids.
Spinning gyroscope st the end of a string, hung at 90 degrees. If you've never seen what it does when released, no way you'd guess it.
Not sure what is illustrates, though, other than "things sometimes behave in counterintuitive ways."
Water bottle rocket. Next question.
Some of these suggestions are interesting, but dull. You need to grab their attention. I suggest a flour explosion. Its perfectly safe, but exciting. Take a coffee (or similar) can with a plastic lid. Run a tubing in thru the bottom. Pile flour over the end of the tubing. Put a lighted candle in the can, close the lid, and blow in the tubing. POOF, the lid goes flying.
Two easy demos come to mind that kids tend to really enjoy: (1) Get a ping pong ball to hover over a hair dryer to showcase Bernoulli principles. If you want to do dramatic, however, you could use a leaf blower and a child's play ball (both of which I'm sure you have); (2) get a flask from your college's chem lab with an opening of diameter only slightly larger than the diameter of the semi-major axis of a hard boiled egg. Put a lit match into the flask, then place the hard boiled egg on the opening. The egg will be pushed into the flask with a satisfying "thump." (I'll let you figure out why.) If you want to be even more dramatic, get the egg out of the flask by turning it upside down, let the egg settle on the inside of the flask opening, blow very hard into it, and watch the egg fly out!
There's the old speaker/perspex tube demo of soundwaves that is beloved of science activity centres: might be too complex to set up though.
With water, you can demonstrate the power of a vacuum by filling a glass tumbler (or something larger, but transparent) in a big bowl of water and then lifting the upended tumbler out to show the water inside 'rising out' of the water. If you have plastic ones you can drill holes in various parts of the tumbler and ask the kids what will happen next.
If you have a big pool and a big bucket you could get the kids to test their strength in trying to get such a water-filled bucket out of the pool.
You could also try the 'collapsing can' experiment if you have access to a stove, though if you have no helpers to corral the youngest kids it might be best to skip it:
http://scifun.chem.wisc.edu/homeexpts/COLLAPSE.html
Air vortex gun.
https://www.youtube.com/watch?v=fxZu_Y_m8i4
or the many modern self-built versions on Youtube.
I'll second magnets, but not just the examples mentioned above. For example, you can have them do some experiments to see what a magnet will pick up (paper, chalk, a penny, a "nickel" (25% Ni), actual nickel, a paper clip, anything else in the room). In addition, show action THROUGH things in addition to the usual action at a distance. I'm always surprised by how many college students have never seen a magnet move a paper clip on the other side of a sheet of cardboard. I suspect that experience with refrigerator magnets has them thinking the force ends when it gets to something.