Newton's 3rd Law: Popping Canisters

 This activity can be done as one activity in a series of stations on Newton's 3rd law, or it could be done as a demonstration if performed on an overhead projector* (does anyone still have those in their classrooms?!?!)

You'll need 2 film canisters (another relic), Alka-Seltzer, water and a pan with a line drawn down the center. 

I took the set-up picture with the transparent canisters, but later switched to the black**

Fill both canisters about half full with water.  You'll want to have the same amount in each canister.  

Cap one of the canisters and lay it on its side so the cap is against the line in the pan. 

For the next portion, you'll need to work quickly....
Add about 1/4 - 1/2 of an Alka-Seltzer tablet to the second canister and cap it.  Then lay that canister so its cap is against the line in the pan. (The two caps should abut one another). 

When the Alka-Seltzer creates enough gas to fill the canister, it will pop the top off.  At the same time it will push the second canister.  Equal force will be applied to each canister, but in opposite directions.  After the explosion, the two canisters will end up in mirrored positions.  

*If you want to do it on an overhead projector, draw a line down the center of a transparency using a permanent marker.  And use a minimal amount of water.  

**This is definitely a demonstration to play around with before you plan to do it in front of your students!  I've done it successfully several times in the past without problems, yet when I went to photograph it, I ran into problem after problem.  The first canisters I grabbed to use leaked so that enough pressure never built up to pop the top off.  Then I used too large a piece of Alka-Seltzer and sent the canister flying out of the pan and off the table (fortunately it didn't go through the brand-new dining room window!).  It's a great little activity, just give yourself a chance to practice it in advance! 

Action/Reaction: Spring Scale Demonstration

For this demonstration, you'll need 2 student volunteers and 2 identical spring scales. 

Hand each student a spring scale.  They'll hold the loop end with one finger.  Hook the opposite ends of the spring scales together.

Instruct one of the students to pull his/her spring scale with 10 N of force (or whatever number is appropriate for the spring scales you are using; something in the middle of the scale) and the other student to pull with 0 N of force (or 5 N or any number as long as it's different from the first student). 

Then let them try to do it.

When they can't get it to work, I usually step in and try to "help".  I have the one student pull the spring scale so it reads 10N.  Once that one's set, I tell the other student to then pull his spring scale to the predetermined number. As they're doing this, they'll notice that both spring scales are always at the same number, no matter what they do.






It's that whole "For every action there is an equal and opposite reaction" thing. If a student pulls on one scale with 10N of force, the other scale pulls with 10N of force (equal force), but in the opposite direction. 

It's a really simple demonstration, but it really exemplifies the "equal" part of Newton's third law. 

Action/Reaction: Car on Marbles

Here's a little activity/demonstration that shows that when a car drives, it pushes on the ground and the ground pushes on it. 

Place a handful of marbles on a cookie sheet. 

Place a piece of cardboard (box board or corrugated, either works) on top of the marbles.

Then you need self-propelled toy car or truck.  Wind up the vehicle and hold the wheels (so it doesn't go yet).  Place the car on the cardboard and let go.

The car will move forward and the cardboard will move backward. 

The ground (cardboard) pushes on the car, and pushes it forward.  The car pushes on the ground (cardboard), and pushes it backward. 

In the real world, we aren't able to see the car pushing on the ground, because the ground is part of the Earth and is SO much more massive than the car that it doesn't move. 

This activity is good for demonstrating the usually invisible forces at work when a vehicle moves.  You could use it as a demonstration or as one station in a series of 3rd law activities.

Action/Reaction: Hero's Engine

First, a little story...
Back when I first graduated and was looking for a teaching job, I needed to prepare a demo lesson on the 3rd Law of Motion. Being the dilligent (and hopeful) interviewee, I scoured the internet for ideas to bring to the lesson. One of the things I found was to make a Hero's Engine using a soda can. I had never heard of such a thing before, but gave it a shot. Turns out it worked pretty well. Fast forward a few months, when I had a teaching job at a well-supported school with a science lab filled with goodies and gadgets of all sorts... one of those nifty gadgets was a fancy, purchased-from-a-catalog Hero's Engine. Unfortunately, you needed to heat it (with a flame) for it to work and even then it really didn't work!

So, today, I provide you with some guidance for making your own Hero's engine - it's generally safe, it costs next-to-nothing, and it WORKS!

Get an empty, clean soda can out of the recycling. You'll be using a nail to punch holes around the bottom of the can. Before doing any hole-punching, you'll want to plan approximately where you'd like the holes to appear, as you'd like them as evenly spaced as possible.

To make the holes: use a hammer to pound the nail into the can. Once the nail is poking through the can, push it (the nail) to one side, so you create an angled hole (see picture below). Remove the nail and make additional holes. Make sure you angle all of the holes in the same direction.

Lift the tab on the top of the can, so it's sticking straight up. Tie a length of string to the tab.

To use: You'll want to work over a large sink/bin or outside and you'll want to work quickly. Fill the can with water and hold onto the string. The water will come out of the holes in one direction and push/spin the can in the opposite direction. Can be used as a demonstration or as one station in a series of activities related to the 3rd Law of Motion.