Saturday, May 3, 2014

Kid's Rocket Science: First Law of Motion

 Shifting gears from Astronomy to Physics, today's class was about Newton's First Law of Motion: a body at rest tends to stay at rest, a body in motion tends to stay in motion, unless acted upon by an outside force.

We started out defining force as a push or a pull on something (although there are forces like drag that are a little less straight forward than "push/pull").

And we defined mass again: it's how much "stuff" is in you (as opposed to volume: the amount of space you take up). So, you can see that the scale measures a force (gravity pulling down on a certain amount of mass), while the balance measures mass (this much mass over here is equal to that much mass over there).

The first half of the law is easy to demonstrate: a body at rest (a lab assistant) tends to stay at rest, unless acted upon by an outside force (alarm clock).   I also set up a model rocket and let the kids do a countdown to... nothing happening.  Without the engine to provide an outside force, it just sits there!

But there are more fun ways to show this: we gave each kid a cup, a card (to set on the cup) and a quarter (to set on the card).

When you flicked the card away, the quarter tended to stay in place until acted upon by an outside force (gravity), at which point it fell into the cup.

We also did the quarter on the elbow trick. And I used a string of rubber bands to drag a large (smooth bottomed) rock.  You can measure the force it takes to move the rock by the length of the bands.  The bands stretch a long way to get the rock moving, but then they shorten up as it continues moving.

The fancy term for the first half of Newton's First Law is inertia, while the fancy term for the second half is momentum.  Once you start looking, there are practical examples of both everywhere!

Think about how difficult starting a hula hoop versus keeping it going or balancing a still bicycle, versus a fast moving bicycle.

Let alone biking up a big hill from a standing start!

Perhaps most importantly, these laws tell you exactly why you should wear seat belts.  To demonstrate this, we played a game I'll call Train Wreck.

I had a ll the kids form a train behind me, an off we went.

After we picked up some speed, I stopped quickly.  Train wreck!

There are some other fun ways to use momentum.

 For example, eggs.  Of these 18 eggs, 6 are hard boiled.  I had the kids spin them to determine which ones were raw( how's that for bravery in the name of science?!).

If you spin a raw egg, stop it briefly (with a light touch), and let go, the liquid inside will continue to spin,and the egg will turn.

You want us to what?
A hard boiled egg will take a tiny bit more energy to stop, but it will stay stopped.

 And, of course, there's always spinning around until you get really dizzy.  That's the momentum in the fluid of your semi circular canals, incidentally.

So that's about it.  Am I missing something?
Oh, yes, the rockets!!

This week's rocket is the balloon rocket.

We ran strings between chairs for a track (2 tracks per set of chairs so they could "race.").

We clipped a section of drinking straw  and put it on the string to attach the balloons to the strings.

We blew up the balloons (without tying them off) and gave it a go!  And it totally did not work.

I had only been able to find round balloons instead of long tubulat balloons.  The round balloons can't be oriented enough along the track to make this experiment work as planned.

So they took the balloons outside and raced them in crazy patterns on the lawn, so, no harm done!

My young cousin got the brilliant idea to put a penny in the balloon.  It takes a bit of shaking, but once the penny gets going, it spins along the interior of the balloon for a long time: inertia and momentum!!

At the end, we had a bit of extra time, so we threw Oob a little early birthday party, much to everyone's satisfaction.  Balloons and cupcakes, what's not to like?

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