Wednesday, May 21, 2014

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!


Queen of Carrots said...

What are we all going to do without film canisters? Nothing else is quite the same.

Wendy said...

That is the truth! The dozen I have now, I begged off of a film processing store ten years ago. When's the last time you saw a film processing place?

We are going to have to BUY them from some one who makes them for this purpose. It's like buying bluing for crystal gardens: the original point was that it was something everyone was sure to have around the house!