Lesson 1: Newton's First Law of Motion

Newton's First Law

Unit 1 of the Physics Classroom discussed a variety of ways by which motion can be described – words, graphs, diagrams, numbers, etc. This unit, Newton's Laws of Motion, will discuss the ways in which motion can be explained. Isaac Newton (a 17th century scientist) put forth three laws which explain why objects move (or don't move) as they do and these three laws have become known as Newton's three laws of motion. The focus of Lesson 1 is Newton's first law of motion – sometimes referred to as the "law of inertia."

Newton's first law of motion is often stated as:

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

There are two parts to this statement – one which predicts the behavior of stationary objects and the other which predicts the behavior of moving objects. These two parts are summarized in the following diagram.
 

The behavior of all objects can be described by saying that objects tend to "keep on doing what they're doing" (unless acted upon by an unbalanced force). If at rest, they will continue in this same state of rest. If in motion with an eastward velocity of 5 m/s, they will continue in this same state of motion (5 m/s, East). If in motion with a leftward velocity of 2 m/s, they will continue in this same state of motion (2 m/s, left). The state of motion of an object is maintained as long as the object is not acted upon by an unbalanced force. All objects resist changes in their state of motion – they tend to "keep on doing what they're doing."

There is a Pass the Water exercise that demonstrates this principle. If students participate in a relay race carrying a plastic container of water around a race track, the water will have a tendency to spill from the container at specific locations on the track. In general the water will spill when:

• the container is at rest and you attempt to move it
• the container is in motion and you attempt to stop it
• the container is moving in one direction and you attempt to change its direction.

The behavior of the water during the relay race can be explained by Newton's first law of motion. The water spills whenever the state of motion of the container changes. The water resists this change in its own state of motion and tends to "keep on doing what it is doing." If the container is moved from rest to a high speed at the starting line; the water remains at rest and spills onto the table. When the container stops near the finish line; the water keeps moving and spills over the container's leading edge. If the container is forced to move in a different direction to make it around a curve; the water keeps moving in the original direction and spills over its edge.

There are many applications of Newton's first law of motion. Consider some of your experiences in an automobile. Have you ever observed the behavior of coffee in a coffee cup filled to the rim while starting a car from rest or while bringing a car to rest from a state of motion? Coffee tends to "keep on doing what it is doing." When you accelerate a car from rest, the road provides an unbalanced force on the spinning wheels to push the car forward; yet the coffee (which is at rest) wants to stay at rest. While the car accelerates forward, the coffee remains in the same position; subsequently, the car accelerates out from under the coffee and the coffee spills in your lap. On the other hand, when braking from a state of motion the coffee continues to move forward with the same speed and in the same direction, ultimately hitting the windshield or the dashboard. Coffee in motion tends to stay in motion.

Have you ever experienced inertia (resisting changes in your state of motion) in an automobile while it is braking to a stop? The force of the road on the locked wheels provides the unbalanced force to change the car's state of motion, yet there is no unbalanced force to change your own state of motion. Thus, you continue in motion, sliding forward along the seat. A person in motion tends to stay in motion with the same speed and in the same direction ... unless acted upon by the unbalanced force of a seat belt. Yes, seat belts are used to provide safety for passengers whose motion is governed by Newton's laws. The seat belt provides the unbalanced force which brings you from a state of motion to a state of rest. Perhaps you could speculate what would occur when no seat belt is used.

There are many more applications of Newton's first law of motion. Several applications are listed below – try to provide explanations for each application.

• blood rushes from your head to your feet when riding on a descending elevator which suddenly stops.
• the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
• a brick is painlessly broken over the hand of a physics teacher by slamming the brick with a hammer. (CAUTION: Do not attempt this at home!)
• to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.
• headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
• while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard.

Can you think of a few more examples which further illustrate applications of Newton's first law?