What Are Some Examples of the Laws of Motion?

Newton's three laws of motion were first published in his famous book "Principia" in 1687. They are the basis of his law of gravitation, which appears in the same work. The laws incorporate and codify significant observations by Galileo preceding him.

Newton's Laws of Motion: The Law of Inertia

Newton's first law, the law of inertia, states that objects in motion tend to stay in motion, and objects at rest tend to stay at rest. This law was noted by Galileo before Newton stated it, but was such an important foundation for his work in "Principia" that it merited repeating as a law.

A trivial example is to leave an object motionless on a table and see that it does not move. A meaningful demonstration, however, requires more work to remove forces that prevent the property from being observed, especially friction. Video clips of floating objects in spacecrafts show the objects travel in straight lines without deceleration.

The Law of Intertia: Addressing a Misconception

Some introductory science students believe that once an object hits another object it stops immediately. This stems from not understanding that objects in motion tend to stay in motion, and therefore take time to slow down. A demonstration to correct this misconception can be performed with tennis balls. From some low height, drop a tennis ball onto carbon paper, to mark a sheet of paper underneath with the area of contact between the ball and paper. Then take the ball in hand and throw it down onto the paper much harder than the ball landed before. The area of the second mark will be much larger. Because of inertia, the throw kept the ball going further than the dropped ball traveled. Though the front of the ball stopped upon contact in both cases, the rest of the ball traveled further toward the floor in the latter case, because of the ball's tendency to maintain its original motion of a high rate of speed.

Newton's Laws of Motion: The Law of Acceleration

Newton's second law, the law of acceleration, states that the acceleration of an object equals the force applied to it divided by its own mass. A cannon firing a lighter ball of the same size (by using a lighter material) would be accelerated to a higher velocity out of the cannon's mouth. The heavier a box is, the harder you have to push it to get it up to the same speed.

An example of a variation in mass can be demonstrated with a turntable. If a record is spinning freely on a turntable, and a weight is dropped onto the record (with some adhesive so that it sticks), the record suddenly slows down much more than if allowed to continue slowing down just from friction.

The second law also incorporates Galileo's observation that objects of unequal masses fall at the same rate. The acceleration of both is the same, but the gravitational force increases with mass. That it is the force that varies with mass explains how the acceleration can stay the same.

Newton's Laws of Motion: The Law of Interaction

Newton's third law, the law of interaction, states that for each action, there is an equal and opposite reaction. One example of this is a rocket in space. The exhaust out the back is pushed back with the same force as the rocket is pushed forward. A child can replicate the lesson sitting in a chair, by kicking his or her legs forward, to send the chair moving backward.

Another example can be performed at an ice rink. The skaters start facing each other, in contact and at rest. They then push each other away. The force they feel from pushing on each other is the same, according to the third law. Therefore, if they are of similar mass, they will travel away from each at a similar speed. If one is larger, that skater will travel backward slower than the other skater.

Newton's Law of Gravitation

Newton's law of gravitation is an application of his second law of motion. Gravitational phenomena are therefore an example of the second law. Such phenomena include both planetary motion and the motion of falling objects. An example is the Moon. It is falling toward the Earth, but its lateral velocity is high enough to always miss the Earth as it falls toward it.

Newton showed the relation between planetary motion and falling terrestrial objects with an illustration in "Principia." Firing a cannon off a mountain produces a parabolic arc. But if the cannon ball is fired hard enough, it attains orbit. Thus the equivalence of objects in orbit and those falling to Earth is demonstrated.

References

• Feynman Lectures on Physics, Vol. 1; Richard Feynman; 1963