Newton’s Three Laws

Forces and Motion

What Is a Force?

• Any influence that may change the motion of an object

• For example: a push, a pull, an attraction, a repulsion

The pull of gravity

Newton’s Laws of Motion

1. Law of Inertia

2. F = ma

3. Action/reaction

Law of Inertia• All objects resist change.

– The more mass an object has, the more it resists

• An object in motion will remain in motion unless acted upon by force

• An object at rest will remain at rest unless acted upon by a force

Change!

Move!

I don’t want to! Change!

Move!

I don’t want to either, but I

am more willing to

change than that big guy.

An object in motion

• Car and Driver start in motion• Car stops by force from wall• No force on unbelted driver• Driver remains in motion due to

inertia

Because of Inertia. . .

• If there is no force the object does not change what it is doing

Nothing is evenTrying to make

Me move, I’ll just Keep sitting here!

But what if the forces acting are equal?

If the forces are equal, the motion does not change!

• The equal and opposite forces cancel out!

• The object continues to do what it was doing before the balanced forces.

5N

Moving

6m/s

Still moving6m/s

5N

An UNBALANCED net forcecauses acceleration

• The 5N force is unbalanced so the object must accelerate (go faster and faster)

5N

Moving

6m/s

Accelerating!More than 6m/s

Newton’s Second Law

forcemass acceleration

F = ma

times

The force here is the NET force which means the total vector force acting on the

object.

Net Force• All of the forces added together

Net F = 0 no forces

5 N Net F = 5 N

5N Net F = 0 (5N – 5N=0N)5N

2N5N Net F = 3 N (5N – 2N = 3N)

If Net Force Is Zero• Acceleration is zero• Object does not accelerate

(but can still move)

If Acceleration Is Zero•The net force must be zero

•The forces have to be balanced

•Forward force equals backward force, etc.

Because F=ma0 = ma

F = m x 0

Therefore a = 0

Therefore F = 0

F = ma

• Big F gives big a

• Small F gives small a

• Big a comes from big F

• Small a comes from small F

Big A

Small a

Small Acceleration

BIG

small

Solve• A mass of 6kg accelerates at

3m/s/s

• What force was applied?

• Use F=ma

F = (6kg) x (3m/s/s)

F = 18kg m/s/s

F = 18N

Solve This!

• If the force is 10N and the mass is 5kg

• What is the acceleration?

• Use F = ma10N = 5kg x a

10N = a

______ _________

5kg 5kg

Divide both sides by 5 and cancel

5kg a = 2m/s2

                                                                                

Be careful this one’s harder!

Newton’s Third Law

• For every action there is an equal and opposite reaction

                                                                                I push on this wall, the wall

pushes back on me!

Forces Always Happen in Pairs

The force of the guy stepping on the ground

The force of the ground pushing

on the guy

Apple 1 pulls on apple 2

Apple 2 pulls on apple 1

Action/reaction Applies to Earth’s Gravity Too

Force of Earth pulling down on the apple

Force of apple pulling up on Earth

Is that possible?

The apple pulls on the Earth?!

I am confused!

For every action there MUST be a reaction; the Earth being pulled by the apple is the only possibility!

If the force from the Earth moves the apple, why doesn’t the Earth move?

• The force on the apple is large compared to the small mass of an apple.

• Therefore, the apple accelerates with a big acceleration

•The same force on the hugely massive Earth does not cause it to accelerate very much•Therefore, the acceleration of the Earth is not noticeable (but it is there!)

I’m so little I have large

acceleration!

I’m too huge to move much.

So what are Newton’s Laws?

First Law tells us:

Second Law tells us:

Third Law tells us:

An object in motion remains in motion an object at rest remains at rest unless acted upon by a force!

Also known as:The law of inertia

F = ma

For every action there is an equal and opposite reaction

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