Newton’s Third Law

Newton’s First Law(Law of Inertia)

An object will remain at rest or in a constant state of motion unless acted upon

by net external forces.

0F =∑

Newton’s First Law 0 => No Change in MotionIf F =∑

Dynamic Equilibrium Static Equilibrium

Net Forces cause Acceleration

Newton’s 2nd Law

The acceleration of object is directly related to the net force acting on it and

inversely proportional to its mass.

netF ma=

netFam

=

Newton’s 2nd Law

[ ] 2

mkg Ns⎡ ⎤= =⎢ ⎥⎣ ⎦

netF ma=

Acceleration ~ Net Force

netFam

=

Acceleration ~ 1/Mass

netFam

=

(Work Book!)

Frictional ForcesStatic Friction

Sliding Friction

Frictional ForcesStatic Friction

Sliding Friction

netFam

=

F fam−

=

Question: Frictional ForcesStatic Friction

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?

F fam−

=

(Workbook)

Force of Gravity: Weight

F ma=

W mg=29.8 /g m s=

Rock & FeatherA rock and feather fall with the same acceleration due to gravity.

Is the force of gravity acting on them the same?

Rock & FeatherNO! The force of gravity acting on them is their weight!

Rock & FeatherWhy does the rock and feather fall with the same acceleration

due to gravity?

Rock & FeatherThe 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 ResistanceAir resistance is proportional to the size and speed of an object.

WHY?

Air ResistanceAir 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 VelocityWhen the air resistance balances the weight (R = W),

the object stops accelerating and it falls with constant velocity called the Terminal Velocity.

W Ram−

=

W Wm−

=

0=

netFam

=

Terminal VelocityWhich person will have the greatest terminal velocity.

Why?

Terminal VelocityThe heavier an object, the greater the terminal velocity.

WHY?

Terminal VelocityThe 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)

An object will remain at rest or in a constant state of motion unless acted upon

by net external forces.

0F =∑

Newton’s 2nd Law

The acceleration of object is directly related to the net force acting on it and

inversely proportional to its mass.

netF ma=

netFam

=

Newton’s 3rd Law

To every force there is an equal but opposite reaction force.

hand on wall wall on handF F= −

Newton’s 3rd Law

You can’t TOUCH without being TOUCHED back!!

hand on wall wall on handF F= −

An interaction requires a pair of forces acting on two objects.

Action-Reaction PairsIf ACTION is A acting on B, then REACTION is B acting on A.

Bug SplatA 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

bus bug bug busF F− −= −Newton’s 3rd Law:

Bug SplatWhich undergoes the greater acceleration?

Bug Same BusFam

=

Which suffers thegreatest damage?

Bug Same Bus

Action-Reaction Pairs

Action-ReactionYou 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

Gun Pushes Bullet out.Bullet Pushes back on Gun (& Man)

kick

Rocket Thrust

Rocket Pushes Gas Out.Gas Pushes Back on Rocket.

Newton’s 3rd Law

This is an INTERACTIVE Universe.

hand on wall wall on handF F= −

Gravity is an InteractionEarth on Rock Rock on EarthF F= −

Gravity is an InteractionThe 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?

Earth on You You on EarthF F= −

E Emg M a= −

EE

mgaM

=

22 224

662 1.1 10 /5.98 10E

Na x m sx kg

−= =

Force is not Acceleration

The forces are equal but the accelerations are not!

Earth on You You on EarthF F= −

Free FallIs the acceleration due to gravity zero?

Free FallYou feel weightless in free fall because there is no floor

pushing up against you.