Newton’s Laws of Motion

A journey into inertia, net force, and

other topics….

Force is central to understanding Newton’s Laws

A Force can be thought of as a

push, like a quarterback throwing a football.

Force is central to understanding Newton’s Laws

A Force can also be

thought of as a pull, like a boat towing a skier.

Another idea central to Newton’s Laws is MASS.

Anything which is made of matter, or atoms, has

mass. Anything you can touch or

see has mass, including

students at Central!!

Along with the idea of

mass is Inertia.

More mass means more

Inertia.

Less mass means less Inertia.

Inertia is the tendency of an object to stay at rest or

move at a constant velocity

• What does that sound like?

Newton’s 1st Law

• Objects tend to either remain at rest or move at constant velocity unless acted on by a Net Force.

• What is a Net Force?

When an object is at rest the forces are all equal or in

equilibrium.

Gravity pulls down on the

rock

But the rock is pushing up

with equal force!

When forces are equal there is NO Net Force!

What if the wind was pushing on the rock with greater force

than the rock could push back?

There would be a Net Force in the direction of the wind!

The rock would fall over in the direction of the Net

Force!

What is the Net Force in this picture?

Contact of the bat on the baseball!!

What other forces are in action in this picture?

Air Resistance

Gravity

Initially the force of the ball coming off of the bat is

stronger!Air

Resistance

Gravity

It is the Net Force!!

Acceleration can also be thought of as a signal that a

force has been applied!!Air

Resistance

Gravity

Even changing directions

requires force!!

Newton’s 2nd Law

Fnet =ma

Net Force

mass

acceleration

Net Force has direction. We call this a vector !

We have already talked about velocity, acceleration, and momentum, which have

direction and are also vectors !

Force is measured in Newtons (N)

1 N =1 kg ∙ ms 2

Free-Body Diagrams

Engine of truck exerting a force forward

Free-Body Diagrams

Why will the truck not go on forever at constant

velocity?

Free-Body Diagrams

Air Resistance, a form of friction, pushes against the

truck.

Free-Body Diagrams

There is also friction between the road and the

tires.

Free-Body Diagrams

Gravity pulls

down on the

sewing machine

Free-Body Diagrams

The desk pushes up

on the sewing

machine

The desk is exerting what is called Normal Force!

Free-Body Diagrams

Is there a Net Force

in this case?

No! The forces are equal and the desk is not moving!

Let’s Look at Types of Forces

• Gravity or Weight• Normal force• Friction• Tension• Thrust

Gravity or Weight

• Can be as simple as an object sitting on a table.

• Formula for Weight is W=mg.

Thrust

• Forces that move

objects from

rockets to cars to people.

Normal Force

• The force exerted by the surface the object is on.

• It is always perpendicular or at 90°.

Normal Force at an Angle

FN

Fg

FNet

FNet is the sum of the vectors. More on that later….

Friction

FN

Fg

FNet

Friction is the resistance of two surfaces sliding past one another

Ff

Tension

• Tension is force being applied to a rope or string.

• It applies when an object is hanging by a string or being pulled by a rope.

FT

Tension

FN

Fg

FNet

When someone is pulling a box up hill by a rope we now have thrust

going up the ramp.

Ff

FT

FThrust

Tension

FN

Fg

FNet

Notice when someone is pulling a box up hill by a rope Net Force and

Friction reverse directions.

Ff

FT

FThrust

Newton’s Third Law

• Whenever one body exerts a

force on another body, the second body exerts an

oppositely directed force of equal magnitude on the first body.

Newton’s Third Law

You probably learned this as

“For every action there is an equal but

opposite reaction.”

The End