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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