Rocket Science: Laws of Motion 2018

1st Law: A body at rest stays at rest, a body in motion stays in motion unless acted on by another force.

Looking at inertia with the quarter/card/jar trick.

 Looking at mass with scales and balances (and jelly beans!).

We also looked at momentum with raw and hard boiled eggs.

And we made balloon rockets.

Full  1st Law class here.

 2nd Law of Motion: F=ma, force equals mass times acceleration.

It takes twice as much force to move twice as much mass, or half as much force to move half as much mass.

We looked at this by dropping a heavy marble and a light foil ball into flour.  They hit at the same time, but the heavy ball made a big crater, and the light one barely sunk in.

We did a few other demonstrations including a race of a heavier and lighter car down a ramp.

And we made puff rockets!

Full 2nd law class here.

3rd Law: Every reaction has an equal and opposite reaction.

Gotta love a Newton's Cradle!

We also did the thing where the kids stand in a wagon and push on me.

And the billiard ball thing with angles.

And Alka-Seltzer rockets!

Which worked this time since I had bought new film canisters.  It was still pretty random: some popped up 15 feet in the air or more, but many only popped up a couple of feet.  We got the best results with half a tablet and just enough water to cover it.

Full 3rd Law class here.

 Lots of science fun!


Next week, we'll build and paint our model rockets, and the following Sunday we'll launch them at NASA Goddard!

(Mostly) Wordless Wednesday: The Big Launch

This was the big finale of our Astronomy/Rocket science class, launching at NASA Goddard!

Out of 16 kids, I believe everyone had at least one successful launch, and most had two launches.  Super fun on a beautiful day!

Kids' Rocket Science: Third Law of Motion

 The third law is the one everyone remembers: for every action there is an equal and opposite reaction.

The favorite demonstration of this is the Newton's Cradle.  Perfectly equal and opposite reactions!


Having done this lesson a number of times, here is my advice: yes, you need a Newton's Cradle; no, you can't make one yourself; yes, you will need a new one each time because the kids will play with it until they tangle the lines irretrievably.  The good news is that it's less than $10 counting shipping. Do not take it out of the box until just before class time.

An interesting  part of the equal and opposite law is what I think of as the "billiards corollary" because this is how professional pool sharks actually make their shots.  When a ball strikes another object, it bounces off with equal force (minus whatever was absorbed by the object) in the exact opposite direction (at the opposite angle).  Lacking a pool table, we showed  this by rolling golf balls against the wall at different angles.

It's easy to see how this law works when things are moving, but most every day encounters with it are fairly static: when you sit down on a chair, you are pushing down on the chair with your entire weight, say, 100 lbs. The chair is pushing back 100 pounds. One kid asked what happened if the chair didn't push back, the answer is that if the chair can't push back enough, it breaks and I fall on the floor, which hopefully can still push back enough!

My favorite way of looking at this law is by having two kids (of roughly equal mass) stand on skateboards and push each other.  Lacking skateboards, I had them take turns standing in our wagon and pushing against me. Of course, as hard as they pushed, they moved away!

That led to a conversation about recoil: cannons, guns, baseball bats, and so forth.

 Then it was time to make the Alka-Seltzer rockets.  When I was setting up the class, I ran a few test flights to make sure that my Alka-Seltzer tablets were still good.

The good news was that the tablets were fine.



The bad news was that some of my old film canisters (which I had successfully used for this the last four times I ran this class) were no longer holding pressure. 

 Worse news: during the actual class, all the canisters, even the ones that had worked for the tests failed.  The farthest a rocket got was 6 inches off the ground! Usually we can get 20 feet in the air!

What a disappointment.

 

So I decided to do some tea rockets instead! 


Not a perfect demonstration of the third law, but much more satisfying!


 And I could use the left over tea to serve the class tea and strawberries!

Kids' Rocket Science: Second Law of Motion

 The Second Law is probably the hardest to explain to young kids: F=ma.  Force is equal to mass times acceleration.  I like to put it as: "The heavier it is, the harder it is to push.", and "The harder you push the farther it goes."

The fact is, it's more specific than that.  If push something exactly twice as hard, it goes exactly twice as far.  If something is exactly half the mass of the first thing, it will go exactly twice as far, all other things being equal.

 We looked at this several ways.  I used a very light ball, and a sand filled ball that happened to be a similar size, and tried tossing them. This is good because it confirms their everyday understanding that, yeah, you have to use more force to throw a heavier ball. Naturally, any boys in the class will try to throw the heavy ball as far as possible, but, in my view, that's okay- they can feel the extra effort it takes to heave the extra mass.

Then we tried dropping the two balls in the sand box.  This was actually a better experiment because you could easily see the dents left behind by the two balls (the heavier ball left a deeper dent because it fell with more force, while the acceleration of gravity was constant)

We also rolled real golf balls and foam golf balls (you could use ping pong balls) down an incline and saw that the more massive balls rolled farther. Again, this is somewhat more useful than throwing because the gravitational acceleration is constant when you use gravity.

It also explains why, in the Pinewood Derby, you are not allowed to have your car weigh more (have more mass) than a certain standard.  It would be an unfair advantage because the heavier cars would always win.

Now, I did say these rules applied with all other things being equal... Such circumstances are pretty rare on Earth.  A baseball hit with twice as much force won't actually go twice as far.  Why not?  One reason is that it will have gravity pulling on it for a longer  time since it will be in the air a longer time. Another reason is that it has to travel through more air, and therefore more air resistance.

Ah, air resistance.  That's pretty important when you're talking about rockets.  How far can you throw a piece of paper?

Try throwing a plain flat sheet of paper as hard as you can.  It's pretty hilarious, actually.  One of the kids immediately suggested that we wad it up into a ball.  That went much farther, even though it was the same mass as the flat sheet.  Then I folded a sheet into a paper airplane, and that went clear across the room and pegged Zorg.  Sorry, Zorg!

 We made Puff Rockets from this printable pattern.  They are designed to have a very low mass, as well as minimal air resistance, and they go remarkably far on a puff of air (provided by you!).

You can also experiment with flying them with and without the fin assembly.  It really lets the kids discover for themselves why rockets have fins.

 Super fun!  Thank you Mxyl for the pictures!

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?

Kids' Astronomy and Rocket Science: the Big Picture

 Okay, technically, it's not rocket science.  It's model rocket science.

And it's super fun!

We started our younger kids science class last Thursday.  I have 11 students aged 6 to 12 (including Oob, Choclo, and Leena) and 5 older lab assistants (including Zorg, Klenda and Mxyl, our staff photographer).  All photos will be from from Mxyl.




Here's how our class is set up:
1 Motion and Distance
2 Earth and Other Planets
3 The Moon
4 The Sun and Other Stars
(Easter Break)
5 Rockets: Newton's First Law
6 Rockets: Newton's Second Law
7 Rockets: Newton's Third Law
8 Visit Observatory (at night!)
9 Build Rockets
9.5 Optional extra build class for older kids and fancy paint jobs
10 Launch Rockets at NASA Goddard

I'm drawing a lot from Janice Van Cleave's Astronomy for Every Kid, and from the rocket program developed by my cousin, Ed (whose granddaughter is taking the class!).   I'll post all the materials and sources as I go - most of it's free, either printable on line or stuff you probably have.  The exception is the model rockets themselves.  I'm getting them in a class pack from AC Supply - way the cheapest and easiest way to do it.

Cheapest because it breaks down to $15-$20 a complete rocket (including 2 engines per rocket) counting the shipping.

 Easiest because I'm getting rockets with pre-molded fin assemblies.  If you are doing this with single digit kids, this is the way to go.  Older kids can get their own fins on straight, but with younger kids, it's an exercise in frustration..

I've now done the rocket program three or four times and I keep adding on and fiddling with it.  I blogged about the rockets last time in 2011.  This time around, each class is an hour.