Newton’s Third Law
Newton’s First Law
(Law of Inertia)
∑F = 0
An object will remain at rest or in a
constant state of motion unless acted upon
by net external forces.
Newton’s First Law
If
∑ F = 0 => No Change in Motion
Dynamic Equilibrium
Static Equilibrium
Net Forces cause Acceleration
Newton’s 2nd Law
Fnet = ma
Fnet
a=
m
The acceleration of object is directly
related to the net force acting on it and
inversely proportional to its mass.
Newton’s 2nd Law
Fnet = ma
⎡m⎤
= [ kg ] ⎢ 2 ⎥ = N
⎣s ⎦
Acceleration ~ Net Force
Fnet
a=
m
Acceleration ~ 1/Mass
Fnet
a=
m
(Work Book!)
Frictional Forces
Static Friction
Sliding Friction
Frictional Forces
Static Friction
Fnet
a=
m
Sliding Friction
F− f
a=
m
Question: Frictional Forces
Static Friction
F− f
a=
m
Sliding Friction
What net force does a
sliding crate experience
when you exert a force of
110 N and the sliding
friction between the crate
and the floor is 110 N?
What can you say about
the motion of the crate?
Is it moving?
Is it accelerating?
(Workbook)
Force of Gravity: Weight
F = ma
W = mg
g = 9.8m / s
2
Rock & Feather
A rock and feather fall with the same acceleration due to gravity.
Is the force of gravity acting on them the same?
Rock & Feather
NO! The force of gravity acting on them is their weight!
Rock & Feather
Why does the rock and feather fall with the same acceleration
due to gravity?
Rock & Feather
The acceleration due to gravity is the same for all object regardless of
weight. More massive objects resist their change in motion more than
less massive objects. The ratio F/m always turns out to be g.
Free Fall Acceleration
Air Resistance
Air resistance is proportional to the size and speed of an object.
WHY?
Air Resistance
Air resistance is proportional to the size and speed of an object.
The bigger the object, the more air it has to push aside.
The faster the object, the faster it has to push air aside.
Terminal Velocity
When the air resistance balances the weight (R = W),
the object stops accelerating and it falls with constant velocity
called the Terminal Velocity.
Fnet
a=
m
W −R
a=
m
W −W
=
m
=0
Terminal Velocity
Which person will have the greatest terminal velocity.
Why?
Terminal Velocity
The heavier an object, the greater the terminal velocity.
WHY?
Terminal Velocity
The heavier object requires greater air resistance to balance its
weight and therefore more speed so the heavier object accelerates
longer and has a greater speed and greater terminal velocity.
(Work Book!)
Newton’s First Law
(Law of Inertia)
∑F = 0
An object will remain at rest or in a
constant state of motion unless acted upon
by net external forces.
Newton’s 2nd Law
Fnet = ma
Fnet
a=
m
The acceleration of object is directly
related to the net force acting on it and
inversely proportional to its mass.
Newton’s 3rd Law
Fhand on wall = − Fwall on hand
To every force there is an equal but
opposite reaction force.
Newton’s 3rd Law
Fhand on wall = − Fwall on hand
You can’t TOUCH without being
TOUCHED back!!
An interaction requires a pair of
forces acting on two objects.
Action-Reaction Pairs
If ACTION is A acting on B, then REACTION is B acting on A.
Bug Splat
A bug and bus have a head on collision.
Compared to the FORCE that acts on the bug,
how much force acts on the bus?
More Same Less
Newton’s 3rd Law:
Fbus −bug = − Fbug −bus
Bug Splat
Which undergoes the greater acceleration?
Bug
F
a=
m
Which suffers the
greatest damage?
Bug
Same
Bus
Same
Bus
Action-Reaction Pairs
Action-Reaction
You push a heavy car by hand. The car, in turn,
pushes back with an opposite but equal force on
you. Doesn’t this mean the forces cancel one
another, making acceleration impossible?
Why or Why not?
Action-Reaction pairs act on different objects.
For F = ma, all the forces act on the same object.
Action Reaction Pairs
kick
Gun Pushes Bullet out.
Bullet Pushes back on Gun (& Man)
Rocket Thrust
Rocket Pushes Gas Out.
Gas Pushes Back on Rocket.
Newton’s 3rd Law
Fhand on wall = − Fwall on hand
This is an INTERACTIVE Universe.
Gravity is an Interaction
FEarth on Rock = − FRock on Earth
Gravity is an Interaction
The Earth pulls on you, you pull on the Earth.
You fall to the Earth, the Earth Falls to you.
You accelerate towards the Earth with g = 10m/s2.
With what acceleration is the Earth falling towards you?
FEarth on You = − FYou on Earth
mg = − M E aE
mg
aE =
ME
662 N
−22
2
aE =
= 1.1x10 m / s
24
5.98 x10 kg
Force is not Acceleration
FEarth on You = − FYou on Earth
The forces are equal but the accelerations are not!
Free Fall
Is the acceleration due to gravity zero?
Free Fall
You feel weightless in free fall because there is no floor
pushing up against you.