# Background

Newton’s Third Law states that for every action force there is an equal and opposite reaction force.  In other words when I walk and I push on the floor with 650 N of force (~145 lbs) the floor pushes back on me with the same 650 N of force.  The concept that all forces come in pairs is the reason why you move forward as your foot pushes backward.  It is also a concept that is difficult for beginning physics students to internalize.  A fast way to see the misconception in action is to ask students to explain how a rocket flies. I use a water rocket since my students are usually very familiar with those.

Why does a water rocket fly?  The most common answer I get is that the water pushes on the ground and the ground pushes on the water.  This pair of forces is the action|reaction pair that somehow causes the rocket to move, even though the rocket is not in the equation at all.  My students consistently miss the interaction between the water and the rocket. The rocket applies pressure to the water, and the water applies pressure to the rocket.  When the water is released the resulting pressure from the rocket becomes a downward action force, and the pressure from the water becomes an upward reaction force. It is the upward force which causes the rocket to fly up.

# Demonstration

What you will need:

• A water cooler bottle (sorry you’ll have to forgo your deposit)
• A stopper that fits in the bottle (Mine is a #9)
• A drill with a ¼  inch drill bit (6.5 mm)
• String
• Ceiling hooks or a large paper clip that can hold the string to the ceiling.
• Long stem lighter
• Propellant – I use 99% isopropyl alcohol in a spray bottle, but any potato gun propellant will work.*
• Hair dryer if you want to fire it multiple times in a single class.

Safety Glasses  should be worn – unless you are clumsy then safety glasses and a hockey mask is a good idea.

## Set Up

1. Drill a hole in the bottom edge of the water bottle
2. Hang the bottle horizontally from the ceiling using the string and ceiling hooks.

## The Demo

1. Spray 2-3 sprays of your propellant into the bottle and shake it to vaporize more propellant.  (If you are using an aerosolized propellant you don’t need to shake)
2. Stand back and to the side.
3. Light the long stemmed lighter and place it to the hole you drilled. If you have the right proportion of propellant to air the propellant will ignite and the expanding gas will go one way as the bottle goes the other.  The force is usually not all that much** – this original bottle set up was modified from a hand held confetti thrower. Put a paper cup filled with confetti in the bottle opening and it is a party.
4. To reset the bottle I use a hair dryer with cool air to speed up the process of reoxygenation.

## How I use this

When I use this demonstration in class I use this paper.***  The students answer the “What do you already know” as a bell ringer (a question or assignment that students begin when they arrive in class), and then I perform the demo.   I normally do it a few times.  The first time without any warning, I like to wake my class up.  The second time for the people who didn’t realize they had to actually look at the demonstration and the third time I put the stopper in the bottle.   If you look at the attached handout you’ll see that I use the paper as think time.  Students answer on their own and maybe share with a partner and then I use their answers in a class discussion.  I like to give individuals this time to process the information on their own since I have found that most people think at different rates.    The third time I do the demo I put a stopper in the bottle, which increases the force created.  I then use this as a jumping off point to talk about momentum and impulse and how they are related to force.  The exercises are homework/classwork.  Please note that some of the questions in this activity are not mine.  In the past I was not always diligent with citing from where I took questions.  No copyright infringement was intended, and if the questions are yours please let me know.

*I do not like to use hair spray because I do not want to even indirectly encourage my students to make flame throwers out of it.  I was once in an Emergency Room next to a person who was there because the hair spray bottle exploded.  He was not a happy camper.

**The amount of force is related to the oxygen|propellant mix and the temperature of the apparatus.  On a warm day I once left the bottle with some liquid propellant in it, and then I went to lunch.  In the class after lunch when I performed the demonstration the resulting boom was very loud and my students swore the bottle touched the ceiling.  After that time I started to leave the bottle upside down over a sink in between classes to let the excess propellant drain out.