Aim: What are Newton’s 1st and 3rd Laws of Motion?

Do Now:

What is the weight of the box? m = 20

kg

Fg = mg

Fg = (20 kg)(9.8 m/s2)

Fg = 196 N

So why isn’t the box accelerating downward?

No acceleration means no unbalanced forces

In other words: Forces are balanced

FNet = 0

The table is pushing up with an equal force

This is called the Normal Force (FN)

Fg = 196 N

FN = 196 N

Normal ForceNormal Force►Represented by FRepresented by FNN

►Supporting force from the Supporting force from the surface the object is onsurface the object is on

►Perpendicular and away Perpendicular and away from the surfacefrom the surface

► If object is at rest in that If object is at rest in that direction, Fdirection, FNN is equal to is equal to the objects weightthe objects weight

Fg

FN

Newton’s 1st Law of Motion

Law of Inertia

If all the forces acting on an object are balanced (FNet = 0):

a. an object at rest remains at rest

Penny in beaker Demo

b. an object in motion will remain in motion with a constant velocity

Inertia – the ability of an object to resist a change in motion

Objects at rest want to remain that way

Objects in motion want to remain that way

Inertia is dependent only on mass(the more mass, the more

inertia)

A boy is pulling a 30 kg sled with a force of 100 N at a constant velocity

Draw a free – body diagram to illustrate this

Fg = 294 N

FN = 294 N

F = 100 N

Are we missing any forces?

FF = 100 N

Force of friction (Ff) – opposes the direction of motion

Forces must be balanced

Object is at rest. What are the values of Forces A and B?

5 N

20 N

Force A =

Force B =

20 N

5 N

Newton’s 3rd Law – Law of Action – Reaction

For every action there exists an equal but opposite reaction

Demo – Balloon

What is the action? What is the reaction?

Action – Jumper pushes on floor

Reaction – floor pushes on jumper

Action – Foot pushes backward on floor

Reaction – Floor pushes forward on foot

Action – Boat pushes person forward

Reaction – Person pushes boat backwards

Action – Rocket pushes the air down

Reaction – Air pushes the rocket up