Newton’s Laws

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Newton’s LawsThe Father of Force

Isaac Newton(1642 – 1727)

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What is a Force?

Force can be defined as a push or a pull…

or anything else that has the ability to change motion.or anything else that has the ability to change motion.

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What is Force?

Force = mass x accelerationNewton (N) (kg) (m/s2 )

NN Is even easier!Is even easier!

Easier than saying Easier than saying kilogramkilogram m/sec m/sec22

F = ma

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To Understand Force…

You must understand the difference between

AND

2 Kg Mass

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What is MassMass?

1 KgMass

2 KgMass

1.5 KgMass

The amount of “matter” in an object

MassMass is measured in kilograms.kilograms.

6 How is Mass Different Than Weight?

1.0 Kg1.0 KgMassMass

9.8 N9.8 N

1.5 Kg1.5 KgMassMass

14.8 N14.8 N

2.0 Kg2.0 KgMassMass

19.7 N19.7 N

Weight is measurement of FORCE.Weight is measurement of FORCE.

DO NOT USE kilogramskilograms (kg) (kg) as a measurement for weightweight (force of (force of gravity acting on a gravity acting on a massmass). ).

(1 kg x 9.86 m/s(1 kg x 9.86 m/s22))F=maF=ma

(1.5 kg x 9.8 6m/s(1.5 kg x 9.8 6m/s22)) (2 kg x 9.86 m/s(2 kg x 9.86 m/s22))

Acceleration Acceleration due to gravity due to gravity

(9.8 m/s(9.8 m/s22))

Acceleration due Acceleration due to gravity to gravity (9.8 m/s(9.8 m/s22))

Acceleration Acceleration due to gravitydue to gravity

(9.8 m/s(9.8 m/s22))

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http://www.exploratorium.edu/ronh/weight/

100 KgMass is220 lbs

On the sunOn the sun 220 lbs.220 lbs. wouldwould weigh 5956 lbs. weigh 5956 lbs.

On MarsOn Mars 220 lbs.220 lbs. wouldwould weigh 82.9 lbs. weigh 82.9 lbs.

On the moonOn the moon 220 lbs.220 lbs. would would weigh 35 lbs. weigh 35 lbs.

Being pulled by different gravity? Being pulled by different gravity? Your weight may be different.Your weight may be different.

However, your mass never changes.However, your mass never changes.

What’s “Your Weight On Other Worlds?”

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Let’s look at Newton's three laws of motion...

These laws explain why objects move (or don't move).

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An object will remain at rest unless acted upon by an “unbalanced” force. An object in motion will

continue with constant speed and direction, unless acted on by an unbalanced force.

This law shows how force, mass and acceleration are related as shown in the equation below:

Force = mass x acceleration

For every action force there is an equal and opposite reaction force.

10 Newton’s First Law: Balanced Forces

Stay at rest: a book on a table

Stay in motion: a bowling ball tossed

in spaceInertiaInertia

An object will maintain a constant state of motion. (Speed and direction remain unchanged)

This means an object at rest tends to stay at rest and

an object in motion continues in the same direction with the same speed.

It will go on and on forever.

This is known as

IP&C

Table pushes up on book

Gravity pulls down on book

Balanced Forces

11 Newton’s First Law: Unbalanced Forces

The book below slides and then stops because of a resistant force called friction. Friction is force that opposes motion.

In space there is no resistance to cause friction, so a bowling ball would stay in motion ...unless another object got in the way…

It kind of reminds me of a few movies

I’ve seen lately...

An unbalanced force is a force that changes the motion.

IP&C

Force of friction stops the book

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Of course “frictional” force happens on earth...

but, if you weren’t wearing a safety belt

you would continue your motion…

Do not let this happen to you; buckle up!

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FF = = m maa FF = = mmaa

Force and acceleration are related.

Newton’s Second Law: Force = Mass X Acceleration

120 kg Lineman120 kg Lineman

Scenario # 1Look at the situation above. If the acceleration 3 m/s2 from the snap was equal between these players,

then who would have the advantage of force?

Lineman = 3 m/s2 x 120 kg is 360 N

Center = 3 m/s2 x 100 kg is 300 N

The lineman has the advantage.

100 kg Center100 kg Center

Scenario # 2 Now, let’s say the center has a

1 m/s2 advantage over his opponent at acceleration of 4 m/s2 from the snap. Now who has the

advantage of force?

Lineman = 3 m/s2 x 120 kg is 360 N

Center = 4 m/s2 x 100 kg is 400 N

Now, the center has the advantage.

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Solving Force Problems

Step 1 Read the problem.Draw a picture.

Step 2 Write down what you know, What are you trying to find?

Step 4 Plug-in the numbers.Solve.

Step 3 Set up the formula.

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“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

Consider the problem…

F =m =a =

Formula Plug-in Answer

Units, units, units!

Step 1 Read the problem. Draw a picture.

1400-kgcar

2 m/s2

How much force?

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F =m =a =

Formula Plug-in Answer

Units, units, units!

1400 kg2 m/s2

Step 2 Write down what you know, What are you trying to find?

1400-kgcar

2 m/s2

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

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Formula Plug-in Answer

Units, units, units!

Step 3 Set up the formula.Solve.

F =m =a =

1400 kg2 m/s2

m x a

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

F = m x a

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Formula Plug-in Answer

Units, units, units!

Step 4 Plug-in the numbers.Solve.

2800kg x m/s2

F =m =a =

1400 kg2 m/s2

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

F = ma 1400 kg x 2 m/s2 2800 kg x m/s2

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Be sure to do the problems.

• Helpful Hints:a = F / m

F = m x a

m = F / a

20The unit for force:

1 kg x m/s2 = 1 newton or 1N

…I thought so.

Which one would you rather write for your answer?

2800 N

21 Newton’s Third Law: Action Forces = Reaction Forces

For every action there is an equal and opposite reaction.

Look at the picture below:

What is the action?

What is the reaction?

As the gases push downward out of the rocket,

The rocket is pushed upward by gases.

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An object will remain at rest unless acted upon by an “unbalanced” force. An object in motion will

continue with constant speed and direction, unless acted on by an unbalanced force.

This law shows how force, mass and acceleration are related as shown in the equation below:

Force = mass x acceleration

For every action there is an equal and opposite reaction.

23 Forces and Geological Processes…

Most of the time the Earth’s massive plates Most of the time the Earth’s massive plates push on each and move very slowly.push on each and move very slowly.

But, sometimes a massmassive plate exerts a greater amount of forceforce that accelerates another plate.

To learn more about earthquakes: http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeIntro.html

Results in an Results in an earthquakeearthquake..

Gravitational Force Gravitational Force and waterfallsand waterfalls

Results in Results in erosion erosion

24 The Story of Newton’s AppleNewton sat under an apple tree, and an apple fell on his head.

That falling apple gave him a revelation

GRAVITY.GRAVITY.But DON’T think of gravity as falling…

It is the pull (force) that an object has on another object.

and led him to describe a force called

25 Gravity is the force that ALL objects in the UNIVERSE exert between each other...

Newton said that gravitational force depends on 2 things2 things:

The DISTANCEDISTANCE of the objects The MASSMASS of the objects

Less gravitational force compared to...

more gravitational force here.

The blue objectsblue objects have more gravitational force between them than the Green objectsGreen objects. This is because the Blue Blue objectsobjects have more MASSMASS.

More FMore F gg

Less F

Less F gg

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The acceleration due to gravity is known as “g.”

When calculating FORCEFORCE due to gravity…

F=ma should be rewritten as

Fg = m x g

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Why is the apple pulled toward the Earth Why is the apple pulled toward the Earth rather than the Earth pulled toward the apple?rather than the Earth pulled toward the apple?

The apple and the Earth The apple and the Earth exert the same gravitational force.exert the same gravitational force.

FFgg = F = Fgg

Why doesn’t the Why doesn’t the EarthEarth move towards the apple? move towards the apple?

28 Acceleration of AppleAcceleration of Apple Is greater thanIs greater than

Acceleration of the EarthAcceleration of the EarthThe apple moves faster toward the Earth The apple moves faster toward the Earth than the Earth moves toward the apple.than the Earth moves toward the apple.

Gravitational force (FGravitational force (Fgg) being equally exerted… ) being equally exerted…

mmaa = = mmaa

TheThe appleapple hashas very little (very little (mm) mass) mass

compared to the compared to the Earth (Earth (mm).).

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The moon is a satellite too..

The pull of gravity keeps satellites orbiting around

the Earth.

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The Earth is round The Earth is round and the satellite and the satellite

continues to “just continues to “just miss” entering into miss” entering into

the Earth’s the Earth’s atmosphere.atmosphere.

Satellites are sent away and placed in the exact spot where the Earth’s gravity pulls them into a “continuous free fall” or orbit... The moon does

the same thing.

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31 Does the moon’s gravitational pull affect the Does the moon’s gravitational pull affect the Earth? Earth? Yes, the moon’s

gravity is large enough to actually pull water out of the ocean from space…

Well, not quite that much...just enough to create motion of the

tides.

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Newton is even credited with inventing

CALCULUS.Needless to say, Newton was Needless to say, Newton was very important in shaping our very important in shaping our

understanding of science.understanding of science.