Newton’s Three Laws of Motion

Physics

We’ll discuss the following in

this lesson:

Newton’s First Law

Newton’s Second Law

Newton’s Third Law

Inertia

• The INERTIA of an object is its resistance to CHANGE in motion.

• In other words, inertia is sluggishness.

Inertia• Galileo is given credit for first coming up with the concept of inertia.

• Galileo identified inertia by performing experiments with inclined planes.

Click HERE to learn a little bit more about Galileo and

inertia.

Mass

• MASS is the measure of inertia or the measure of how much matter an object contains.

• Quite often, the words “mass” and “inertia” are used interchangeably.

Abbreviation for Mass

Mass is abbreviated m.

m

Mass is a Scalar Quantity

Mass is a scalar quantity. In other words, when

expressing mass, you only need to include a

magnitude and a unit (NO DIRECTION IS NECESSARY).

SI Unit for Mass The SI unit for mass is the kilogram (abbreviated kg).

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Mass

• An object’s mass is independent of its location.

• In other words, an object’s mass does not change if it is moved.

Your mass is the same on the moon as it is on the

earth.

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Newton’s First Law of Motion

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The sum of

The

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ide

forc

es a

cting

on

an

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ct

Equals

Zero

When this happens, an object will NOT

accelerate.

Newton’s First Law of Motion

Newton’s First Law of Motion says that if zero net (total) outside force acts on an object (or system of objects), that object will not accelerate.

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Net means “the sum of.”

Newton’s First Law of Motion

In other words, if zero net outside force acts on an object, it will do one of two things:

1. it will stay at rest if it is already at rest, or

2. it will keep moving in a straight line at a constant speed if it is already moving.

“Moving in a straight line at a constant speed” means the same thing as “moving

at a constant velocity.”

Newton’s First Law of Motion

• Newton’s First Law holds even when forces act on an object.

• The forces acting on the object just must ADD UP TO ZERO.

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left does not open, click HERE.

Newton’s First Law of Motion

Newton’s First Law of Motion is sometimes called the Law of Inertia because it is essentially a restatement of inertia.

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Newton’s Second Law of Motion amF

Newton’s Second Law of Motion says that the

net external force acting on an object (or

system of objects) is equal to the mass of the object multiplied by the acceleration of

the object.

Watch the video below

to find out what the

symbols above mean.

Newton’s Second Law of Motion amF

Newton’s Second Law of Motion

Newton’s Second Law of Motion says that a larger net force results in a larger acceleration and that a smaller net force results in a smaller acceleration.

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Newton’s Second Law of Motion

• Newton’s Second Law of Motion also says that if an object has a larger mass, it will have a smaller acceleration for a given net force.

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HERE.

Newton’s Second Law of Motion

Remember: Newton’s Second Law deals with NET FORCE, not just force.

Click HERE to use open a web page. Once the web page

opens, click the “Run Now!” button to use a force

simulation.

Weight and Newton’s Second Law

For right now, we will define weight as the force due to gravity acting on an object.

Weight and Newton’s Second Law

• Weight is equal to mass x acceleration due to gravity.

• Weight = mg (direction is downward).

Remember: g ≈ 10 m/s/s

Weight and Newton’s Second Law

• An object’s weight depends on its location.

• The astronauts in the video to the right do not weigh the same on the moon as they do on the earth because the moon does not exert the same gravitational force on them as the earth does.

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open.

Free-Fall Motion

An object is in free fall when the only force exerted on it is the force due to gravity.

Remember: an object is not really in free fall if

wind resistance is acting on it.

Free-Fall Motion• An object is in free fall when the only force exerted on it is the force due to gravity.

• So, if we draw a force diagram for an object in free fall, the diagram will only have one force vector (for the force due to gravity).

• Let’s draw a force diagram for a basketball in free fall.

Non-Free-Fall Motion

• When an object falls through the atmosphere, at least two forces act on that object:1. gravity and2. wind resistance.

• In this case, the object is not in free fall, and the object will NOT have an acceleration of g.

Terminal Velocity/Speed

Interaction

• If I exert a force on the wall, the wall exerts a force back on me.

• This is always the case. Any time Object A exerts a force on Object B, Object B exerts a force back on Object A.

• This pair of forces makes up ONE INTERACTION.

Action/Reaction Pair of Forces

• If Object A exerts a force on Object B, we call that an ACTION force.

• When Object B exerts a force back on Object A, we call that a REACTION force.

• Remember: an action force + a reaction force makes a SINGLE INTERACTION.

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Newton’s Third Law of Motion

Newton’s Third Law of Motion says the following:

“For every action, there is an equal and opposite reaction.”

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Newton’s Third Law of Motion

Newton’s Third Law says two things:1. If Object A exerts

a force on Object B, Object B exerts a force back on Object A, and

2. The two objects exert forces on one another that are equal in size and opposite in direction.

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Examples of Newton’s Third Law of Motion in Action

• A bird’s wings push down on the air. The air pushes up on the bird’s wings. In other words, air keeps a bird aloft.

• You pull down on a ladder. The ladder pushes you up. In other words, the ladder propels you upward.

• A car’s wheels push backward on the road. The road pushes forward on the car’s tires. In other words, the road propels the car forward.

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at the right does not

open, click

HERE.

Newton’s Second and Third Laws for Objects of Different Masses

• If I hit a grasshopper with my PT Cruiser, which exerts the greater force on the other? Click on the correct choice below.

The car exerts the greater force

on the grasshopper.

The grasshopper exerts the greater force on the car.

The two do not exert forces on

each other.

The two exert forces with equal

magnitudes on each other.

Newton’s Second and Third Laws for Objects of Different Masses

• Although the PT Cruiser and the grasshopper exert equal sized forces on one another, they do not sustain the same amount of damage upon impact.

• According to Newton’s Second Law, because the grasshopper has a tiny mass, it will experience a huge acceleration compared to that of the PT Cruiser.

• It is the huge acceleration of the grasshopper that causes it to sustain such damage.

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open, click

HERE.