Whenever one object exerts a force on a second object the second object exerts a force equal in strength, BUT opposite in direction back on the first

object.

Action/Reaction?Action and Reaction forces act simultaneously. It

is often hard to identify the pair of action and reaction forces. Remember that

Action: Object A exerts a force on object B.Reaction: Object B exerts a force (same strength,

and type) on object A in the opposite direction.Identify the action-reaction pairs in following situations:

Driving a car swimming falling boulder.

Driving

A car’s wheels spin so that they rub the road’s surface creating a friction force pointing to the rear of the car.

The road “reacts” by exerting a friction force of equal magnitude on the car wheels in the forward direction.

Friction Force from the car pushing on the road

Friction Force from the road pushing on the car

Swimming

A fish’s tail moves so that it exerts a contact force on the water, pushing the water backwards.

The water “reacts” by pushing on the fish’s tail with a contact force of equal magnitude, but opposite direction making the fish move forward.

Contact force of the fish on the water

Contact force of the water on the fish

Falling BoulderThe force of gravity from the planet pulls downward on the boulder causing it to fall.

The falling boulder reacts by pulling upward on the Earth (trying to “lift it”) with a gravitational force.

Fg of planet on boulder

Fg of boulder on planet

Visualizing Newton's 3rd lawWhenever something places a force on

another object, that object being pushed exerts a force back onto the thing that is pushing it.

The Box exerts a force of gravity pushing down on the floor

The floor pushes back on the box just as hard

A person pushes a block with a force

The block pushes back on the personjust as hard

The Box exerts a Force of gravity Pushing down on the floor

The floor pushes back on the box just as hard

Smooth floor

The Person exertsa Force of gravity Pushing down on the floor

The floor pushes back on the person just as hard

X

X

X

X

Newton’s 3rd law and accelerationsNewton’s third law states that the forces

between two objects pushing on each other are the same in size, however it does not state anything about acceleration.

The accelerations of the these objects is governed by Newton’s 2nd Law, not the 3rd law.

Smooth floor

A 50kg person pushes on a 20 kg box with a force of 10 Newtons. If both are standing on a smooth (frictionless) surface. What are the accelerations of the two objects?

A person pushes a block with a force of +10 N

The block pushes back on the personjust as hard with -10 N

The Box exerts a Force of gravity Pushing down on the floorFg = (20kg)*(-9.8m/s2)

The floor pushes back on the box just as hardFfloor = +196 N

The Person exertsa Force of gravity Pushing down on the floorFg = (50kg)*(-9.8m/s2)

The floor pushes back on the person just as hardFfloor = +490 N

Fg = -196 NFg = -490 N

Ffloor = +490 N Ffloor = +196 N

+10 N-10 N

Step 1: Draw a Free Body Diagram

Smooth floor

A 50kg person pushes on a 20 kg box with a force of 10 Newtons, both standing on a smooth (frictionless) surface.

Fg = -196 NFg = -490 N

Ffloor = +490 N Ffloor = +196 N

+10 N-10 N

Step 2: Look at each object separately

Smooth floor

A 50kg person pushes on a 20 kg box with a force of 10 Newtons, both standing on a smooth (frictionless) surface.

Fg = -490 N

Ffloor = +490 N

-10 N

Step 2: Look at each object separately

The forces for the Y axis cancel

FnetY = Ffloor + Fg = maY

FnetY = +490 N + (-490 N) = (50 kg)aY

FnetY = 0 N= (50 kg)aY

This leaves only one force along the X axis.

aY = 0 m/s2

FnetX = Fright + FLeft = maX

FnetX = 0N + -(10N) = (50 kg)aX

aX = -(1/5) m/s2

Smooth floor

A 50kg person pushes on a 20 kg box with a force of 10 Newtons, both standing on a smooth (frictionless) surface.

Fg = -196 NFg = -490 N

Ffloor = +490 N Ffloor = +196 N

+10 N-10 N

Step 2: Look at each object separately

Smooth floor

A 50kg person pushes on a 20 kg box with a force of 10 Newtons, both standing on a smooth (frictionless) surface.

Fg = -196 N

Ffloor = +196 N

+10 N

Step 2: Look at each object separately

The forces for the Y axis cancel

FnetY = Ffloor + Fg = maY

FnetY = +196 N + (-196 N) = (20 kg)aY

FnetY = 0 N= (20 kg)aY

This leaves only one force along the X axis.

aY = 0 m/s2

FnetX = Fright + FLeft = maX

FnetX = 10N + (0N) = (20 kg)aX

aX = (+1/2) m/s2

Smooth floor

+10 N-10 N

60 Kg 20 Kg

aperson = -(1/6) m/s2 aBox

= (+1/2) m/s2

Net force on block = - net force on person

Massblock * AccelerationBlock = - Massperson * Accelerationperson

In ShortWhen dealing with two objects pulling or

pushing on each other, and there are no other forces being applied we can always say

Mass1 * Acceleration1 = -Mass2 * Acceleration2

This means that the acceleration of the block is based on the ratio of the masses of both the person and the block itself

Acceleration1= - (Mass2 / Mass1)* Acceleration2