Newton’s First Law
August 29, 2007
Principles of Physical Science

Newton’s first law of motion states that any object that is moving will continue to move until a force acts upon it. It also says that any object that is at rest will remain at rest until some force causes it to move. This is because all objects possess inertia or the unwillingness to change.

For example, if I was in outer space and threw a tennis ball, the ball would continue traveling indefinitely. If the ball were to meet a force, its travel could be slowed, halted or accelerated. It could collide with a piece of debris, which might cause it to slow down. It could be pulled into the gravitational field of a planet, which might cause it to go faster. It might be caught by an alien space ship, and thus come to rest. All of these – the debris, the gravitational field and the aliens – are forces that could alter the motion of my tennis ball. However, if no forces interfered with the ball’s motion, it would continue to soar through space until the end of time.

There are many simple experiments that demonstrate Newton’s first law. These are two of them:

1. For this experiment you will need a cup, an index card and a penny. Place the index card on top of the cup and the penny on the card. Now, flick the card straight out with your finger. Your finger provides a force that causes the index card to move horizontally. The penny will not move with the card; it will fall into the cup. The penny does not move horizontally because, as Newton’s first law says, objects will not move unless a force acts on them. The force of your finger acted on the index card only. The penny fell into the cup because once the index card – which was supporting the penny – moved, gravity acted as a force on the penny and pulled it downward.

2. This experiment requires a golf ball and a cup. Put the ball in the cup, and then lay the cup on its side. Push the cup forward then stop suddenly. The cup will stop but the ball will continue moving and roll out of the cup. This happens because the bottom of the cup supported the ball and forced it to move with the cup, but when you stop the cup, there is no force acting on the ball so it remains in motion.

Seatbelts in cars also demonstrate Newton’s first law of motion. If a person is in a car, with no seatbelt on, traveling fifty miles per hour and the car stops suddenly – perhaps because it hits a tree (the force terminating the cars motion) – the person will continue to travel fifty miles per hour through the windshield. This happens because a force (the tree) acted upon the car, but not the person. If we reenact the scenario, this time with the person wearing a seatbelt, when the car hits the tree and stops, the person will also stop. This is because the seatbelt essentially makes the person part of the car. Any force that acts on the car will in turn act on the person. This is how seatbelts can save lives.