1. Newton’s 3rd Law
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.

2. 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.

3. Driving Friction Force from the road pushing on the car
Friction Force from the car
pushing on the road
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.

4. Swimming Contact force of the water on the fish Contact force of
the fish on the water
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.

5. Falling Boulder The force of gravity from the planet
pulls downward on the boulder
causing it to fall.
Fg of planet on boulder
The falling boulder reacts by pulling
upward on the Earth (trying to “lift it”)
with a gravitational force.
Fg of boulder on planet

6. Visualizing Newton's 3rd law
Whenever something places a force on another object, that object being pushed exerts a force back onto the thing that is pushing it.
The floor pushes back on the box just as hard
The Box exerts a force of gravity pushing down on the floor

7. X X X X The floor pushes back on the person just as hard
back on the box
just as hard X X
The block pushes
back on the person
just as hard
A person pushes a
block with a force X X
Smooth floor
The Person exerts
a Force of gravity
Pushing down on
the floor
The Box exerts a
Force of gravity
Pushing down on the floor

8. Newton’s 3rd law and accelerations
Newton’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.

9. A 50kg person pushes on a 20 kg box with a force of 10 Newtons
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?
Step 1: Draw a Free Body Diagram
The floor pushes
back on the box
just as hard
Ffloor = +196 N
The floor pushes
back on the person
just as hard
Ffloor = +490 N
Ffloor = +490 N
Ffloor = +196 N
The block pushes
back on the person
just as hard with -10 N
A person pushes a
block with a force of +10 N
+10 N
-10 N
Smooth floor
The Person exerts
a Force of gravity
Pushing down on
the floor
Fg = (50kg)*(-9.8m/s2)
The Box exerts a
Force of gravity
Pushing down on the floor
Fg = (20kg)*(-9.8m/s2)
Fg = -490 N
Fg = -196 N

10. A 50kg person pushes on a 20 kg box with a force of 10 Newtons,
both standing on a smooth (frictionless) surface.
Step 2: Look at each object separately
Ffloor = +490 N
Ffloor = +196 N
+10 N
-10 N
Smooth floor
Fg = -490 N
Fg = -196 N

11. A 50kg person pushes on a 20 kg box with a force of 10 Newtons,
both standing on a smooth (frictionless) surface.
Step 2: Look at each object separately
The forces for the Y axis cancel
Ffloor = +490 N
FnetY = Ffloor + Fg = maY
FnetY = +490 N + (-490 N) = (50 kg)aY
FnetY = 0 N= (50 kg)aY
aY = 0 m/s2
-10 N
Smooth floor
This leaves only one force along
the X axis.
Fg = -490 N
FnetX = Fright + FLeft = maX
FnetX = 0N + -(10N) = (50 kg)aX
aX = -(1/5) m/s2

12. A 50kg person pushes on a 20 kg box with a force of 10 Newtons,
both standing on a smooth (frictionless) surface.
Step 2: Look at each object separately
Ffloor = +490 N
Ffloor = +196 N
+10 N
-10 N
Smooth floor
Fg = -490 N
Fg = -196 N

13. A 50kg person pushes on a 20 kg box with a force of 10 Newtons,
both standing on a smooth (frictionless) surface.
Step 2: Look at each object separately
The forces for the Y axis cancel
Ffloor = +196 N
FnetY = Ffloor + Fg = maY
FnetY = +196 N + (-196 N) = (20 kg)aY
FnetY = 0 N= (20 kg)aY
aY = 0 m/s2
+10 N
Smooth floor
This leaves only one force along
the X axis.
Fg = -196 N
FnetX = Fright + FLeft = maX
FnetX = 10N + (0N) = (20 kg)aX
aX = (+1/2) m/s2

14. aperson = -(1/6) m/s2
aBox = (+1/2) m/s2
60 Kg
20 Kg
+10 N
-10 N
Smooth floor
Net force on block = - net force on person
Massblock * AccelerationBlock = - Massperson * Accelerationperson

15. In Short
When 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