NEWTON'S LAWS OF MOTION

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The Physics Classroom » Physics Tutorial » Newton's Laws » First Law
Newton"s Laws - Lesson 1 - Newton"s First Law of Motion
Newton"s First Law
First Law

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In a previous chapter of study, the variety of ways by which motion can be described (words, graphs, diagrams, numbers, etc.) was discussed. In this unit (Newton's Laws of Motion), the ways in which motion can be explained will be discussed. Isaac Newton (a 17th century scientist) put forth a variety of laws that explain why objects move (or don't move) as they do. 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 lớn as the law of inertia.

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Newton's first law of motion is often stated as

An object at rest stays at rest & an object in motion stays in motion with the same speed & in the same direction unless acted upon by an unbalanced force.

Two Clauses & a Condition

There are two clauses or parts to lớn this statement - one that predicts the behavior of stationary objects và the other that predicts the behavior of moving objects. The two parts are summarized in the following diagram.

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The behavior of all objects can be described by saying that objects tend lớn "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 lớn "keep on doing what they're doing."

There is an important condition that must be met in order for the first law lớn be applicable khổng lồ any given motion. The condition is described by the phrase "... Unless acted upon by an unbalanced force." As the long as the forces are not unbalanced - that is, as long as the forces are balanced - the first law of motion applies. This concept of a balanced versus & unbalanced force will be discussed in more detail later in Lesson 1.


Suppose that you filled a baking dish to lớn the rim with water and walked around an oval track making an attempt khổng lồ complete a lap in the least amount of time. The water would have a tendency khổng lồ spill from the container during specific locations on the track. In general the water spilled when:

the container was at rest & you attempted to lớn move itthe container was in motion and you attempted to stop itthe container was moving in one direction and you attempted khổng lồ change its direction.

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The water spills whenever the state of motion of the container is changed. The water resisted this change in its own state of motion. The water tended khổng lồ "keep on doing what it was doing." The container was moved from rest khổng lồ a high tốc độ at the starting line; the water remained at rest & spilled onto the table. The container was stopped near the finish line; the water kept moving and spilled over container's leading edge. The container was forced to lớn move in a different direction lớn make it around a curve; the water kept moving in the same direction và spilled over its edge. The behavior of the water during the lap around the track can be explained by Newton's first law of motion.

Everyday Applications of Newton's First Law

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 lớn the rim while starting a oto from rest or while bringing a oto to rest from a state of motion? Coffee "keeps on doing what it is doing." When you accelerate a oto from rest, the road provides an unbalanced force on the spinning wheels khổng lồ push the car forward; yet the coffee (that was at rest) wants to stay at rest. While the oto accelerates forward, the coffee remains in the same position; subsequently, the car accelerates out from under the coffee và the coffee spills in your lap. On the other hand, when braking from a state of motion the coffee continues forward with the same speed và in the same direction, ultimately hitting the windshield or the dash. Coffee in motion stays in motion.

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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 lớn change the car's state of motion, yet there is no unbalanced force khổng lồ change your own state of motion. Thus, you continue in motion, sliding along the seat in forward motion. A person in motion stays in motion with the same speed và in the same direction ... Unless acted upon by the unbalanced force of a seat belt. Yes! Seat belts are used to lớn provide safety for passengers whose motion is governed by Newton's laws. The seat belt provides the unbalanced force that brings you from a state of motion khổng lồ a state of rest. Perhaps you could speculate what would occur when no seat belt is used.


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There are many more applications of Newton's first law of motion. Several applications are listed below. Perhaps you could think about the law of inertia & provide explanations for each application.

Blood rushes from your head khổng lồ your feet while quickly stopping when riding on a descending elevator.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 it with a hammer. (CAUTION: vị not attempt this at home!)To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down & thrusted downward at high speeds và 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 or rock or other object that abruptly halts the motion of the skateboard.

Try This At Home

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Acquire a metal coat hanger for which you have permission to lớn destroy. Pull the coat hanger apart. Using duct tape, attach two tennis balls lớn opposite ends of the coat hanger as shown in the diagram at the right. Bend the hanger so that there is a flat part that balances on the head of a person. The ends of the hanger with the tennis balls should hang low (below the balancing point). Place the hanger on your head & balance it. Then quickly spin in a circle. What vị the tennis balls do?

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