If a car you're riding in happens to be involved in an accident, wearing a seat belt can save your life. How does this work?
Newton's first law declares that objects at rest tend to remain at rest, and objects in motion tend to remain in motion. This means that moving objects will continue to move in a straight line unless a force is applied to change that direction.
When you're traveling in a car, your body is moving at the same speed and in the same direction as the car. If the car stops suddenly, your body, because of inertia will keep moving at the same speed and direction as the car was moving. Seat belts reduce the force of inertia by restraining you when the car slows or stops suddenly.
Explain Newton's first law of motion with this experiment.

Materials:

• a balloon
  
• a 9 cm square of styrofoam
  
• scissors *ask for help from an adult
  
• an empty thread spool
  
• school glue
  
• masking tape
  

Procedure:

Draw two crossing diagonal lines from the corners of the square of styrofoam. The point at which the lines meet is the exact centre of the styrofoam. Ask an adult to help you punch a small hole with the scissors in the centre of the styrofoam. The hole should be about 2 mm wide. Tape the spool directly over the hole int the styrofoam. Place the tape over the ridge that sticks out around the bottom of the spool. Check carefully to be sure of a good seal all the way around the ridge. Inflate the balloon. Pinch the top of the balloon's neck so no air excapes. While you hold the balloon, ask someone to stretch the bottom part of the neck over the spool. Place the air car on a smooth floor or counter. Let go of the neck of the balloon and give the car a small push. If necessary, pull the corners of the styrofoam up slightly.

What happened?

What happens to the air car? How far does it move? Does it change direction?
Normally the force of friction will slow down an object, so it's hard to see that inertia will keep it in motion once it's moving. With your air car you were able to see an object move in a stright line with very little friction acting upon it. Air escaping from the balloon formed a thin, low-friction layer between the surfaces of the air car and the floor. Because there was almost no friction to act upon the car, you were able to see inertia in action. The car moved a long way in the direction in which you pushed it.

More?

When you swing downward on a seesaw, you feel a bump when your end hits the ground. When your partner hits the ground, you bounce upward. Based on what you know about inertia, why do think this happens?

Reproduced with permission from ©1989 Educational Insights, How Things Work, #7.