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Introduction
The Knowledge Bears enjoy racing toy cars, but we hate it when the batteries run out. You can make a rocket car that moves without any batteries or electricity. Your rocket car will run on physics! To learn more about how this works, see the explanation and links below. 

Collect the materials and follow the instructions to make your very own rocket car.  Teach your friends and have a race!
      Safety Information
Experiments are dangerous, but scientists are always safe!

→Always have an adult help you collect the materials and conduct the experiment.

 

 

  
Materials
→ styrafoam meat tray
→ ruler
→ marker/pen
→ drawing compass (or circular stencil)
→ scissors
→ small party balloon
→ Flexi-straw
→ cellophane tape
→ 4 pins
Instructions
1) On the flat surface of the styrafoam meat tray, draw a 7.5 cm x 18 cm rectangle and four circles with a diameter of 7.5 cm each. Cut out all five shapes.  
2) Stretch out the balloon by inflating it a few times. Then, fit the opening of the balloon over the  flexi-straw at the end that is closest to the bend. Tape the balloon in place. Seal the opening of the balloon with tape so that air can only get into it through the straw.  
3) Tape the straw to the styrafoam rectangle. The straw should run lengthwise along the middle of the rectangle.  
4) Using the pins, attach the four styrafoam circles to the sides of the rectangle to make the wheels. Push one pin through the center of the circle and into the side of the rectangle. Leave room between the circle and the rectangle so that the wheels can roll.  
5) Blow through the straw to inflate the balloon, then pinch the opening of the straw to trap the air.  Place your rocket car on a smooth surface, and remove your fingers from the straw.

3 . . . 2 . . . 1 . . . Blast Off!

 

  
6) Now that you have seen how this works, you can design your own rocket car.  Experiment with the size of the car, wheels, and balloon. Different measurements will produce different results. Try changing the materials; lightweight materials, like styrafoam, are easier to move. As an experiment, you could test different cars, measure how far and how fast they travel, and record your results in a table. You can even decorate your car and amaze your friends.   
Explanation
  Are you sitting down? If so, your body is pushing down on the chair. Did you know the chair is pushing you back? This "pushing" is called a force. Your body applies a downward force on the chair, and the chair applies an upward force on you.
  Here's another example: imagine you are a fish with a fin on each side of your body. You push your fins backward, applying force to the water. The water pushes on your fins in the opposite direction, moving you forward.
  Newton's Third Law of Motion says:
"For every action there is an equal and opposite reaction."
  When you push down on the chair, that is an action. When the chair pushes you up, that is a reaction. When the fish's fins push backwards on the water, that is an action. When the water pushes forward, that is a reaction. 
  Sir Isaac Newton figured out that actions and reactions are always equal. This means that an action and a reaction will have the same force. 
  He also said that the action and reaction are opposite. This means that if an action pushes in one direction, the reaction pushes in the opposite direction. You push the chair down, it pushes you up. The fish pushes water backward, the water pushes the fish forward. Wheels on a car spin backward against the road; the road pushes the wheels forward, and the whole car moves forward. 
  So let's take a look at how Newton's Third Law of Motion makes the rocket car work.  When the air is released from the balloon, it applies a backward force on the rest of the air around the car. The car is then pushed forward with equal force. The wheels help the car move more easily along the floor.
Links
A Century of Physics
Explore the history of physics, including Sir Isaac Newton and his laws of motion
The Physics Classroom
Learn more about Newton's three laws
The Physics Behind Airbags and Crumple Zones
A student's project that applies Newton's Laws to car safety features
The Physics of Hockey
A student's project that applies Newton's Laws to hockey
Home Alone Physics
A student's project that applies Newton's Laws to the movie Home Alone
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