Newton’s Third Law
The Push Back (Demo)
Demo: Dynamics Carts with Plunger, with and without mass.
The Push Back
Whenever one body exerts a force on a second body, the
second body pushes back with an equal and opposite force
on the first body.
“For every action there is an equal and opposite
reaction.”
Reaction Force
The reaction force is equal in magnitude and opposite in
direction.
FBDs
Wall pushes
on man
Man
Man pushes
on wall
Wall
Always in Pairs
• "action...reaction" means that forces
always occur in pairs. Single, isolated
forces never happen.
• Either force in an interaction can be the
"action" force or the "reaction" force.
• "equal" means:
• Both forces are exactly the same size.
• Both forces exist at exactly the same time.
• "opposite" means that the two forces
always act in opposite directions –
exactly 180o apart.
IMPORTANT
When two objects are involved, the action and reaction forces
act on different objects.
For example,
Blocks X and Y are attached to each other by a light rope and can
slide along a horizontal, frictionless surface. Block X has a mass
of 10 kg and block Y a mass of 5.0 kg. An applied force of 36 N
[right] acts on block X.
XY Together FBD
X Alone FBD
Y Alone FBD
Consider the following situations:
Consider the following:
Now Predict
Action
force
Reaction
force
WHY?
Blow up balloon and
release
Balloon
molecules
push on air
Air molecules push
on balloon
Air molecules in rm have greater
mass than air molecules in balloon
– therefore balloon moves
Student steps off
skateboard
Person on
skateboard
Skateboard off
person
Person has more mass therefore
skateboard has greater
acceleration
(x2) 2 students on
skateboards- push
each other – equal
mass, unequal mass
One student
Other student
The student with the greater mass
will move less
2 students stand on
skateboards –
student throws ball
to other student
Student on
ball
Ball on student
Ball goes further, less mass but
both move
Scenario
Predicted
Result
Observed
Result
Other Scenarios
•
•
•
•
•
•
•
Firefighters holding a hose
Recoil of a gun
Canon
Fan cart
Rocket
Propellers on a plane
Jet engine in space: jet engine on air particles, particles on jet
engine
• Astronaut tethered to space station – tether breaks – drift
indefinitely but you are holding onto a wrench – throw away
and it will propel you back
Right or Wrong?
• If an object is moving, there must be some force making it
Wrong! It could be moving without accelerating.
move.
• If v = 0, then
a and F
net
must be zero.
Wrong! Think of a projectile shot straight up at
its peak.
• An object must move in the direction of the net force.
Wrong! It must accelerate that way
but not necessarily move that way.
Right or Wrong?
• Heavy objects must fall faster than light ones.
Wrong! The rate is the same in a
vacuum.
• When a big object collides with a little one, the big one hits the little
one harder than the little one hits the big one.
Wrong! The 3rd Law says they hit it
each other with the same force.
• If an object accelerates, its speed must change.
Wrong! It could be turning at
constant speed.
Example 1
Blocks X and Y are attached to each other by a light rope and can
slide along a horizontal, frictionless surface. Block X has a mass
of 10 kg and block Y a mass of 5.0 kg. An applied force of 36 N
[right] acts on block X.
XY Together FBD
X Alone FBD
Y Alone FBD
(a) Calculate the action-reaction forces the blocks exert on each
other.
(b) Suppose the magnitudes of the force of friction on blocks X
and Y are 8.0 N and 4.0 N respectively. Calculate the actionreaction forces the blocks exert on each other.
Example 2
A worker applied a small 30-N horizontal force to box A which
also pushes onto box B in front. Box A is 25kg and box B is 15kg.
They sit on a smooth surface. The two-box system accelerates
forward at 0.75m/s2. What size of forces do the boxes exert on
each other? How do their directions compare?
Given:
Required:
Example 2 Continued
Consider box A alone: Newton’s second law applies. The mass times
the acceleration of A must equal the applied force plus the force of
box B pushing back on A.
mA a  Fapp  FBA
FBA  mA a  Fapp
FBA  25kg(0.75m /s2 )[ for]  30N[ for]
 18.75N[ for]  30N[ for]
 11.25N[ forward]
 11.25N[backward]

Consider box B alone: Newton’s second law still applies; however, the
only force acting on box B is from box A.
This demonstrates that the forces acting from box to box are equal in
magnitude and opposite in direction, as we would expect from
Newton’s third law. If the forces were not equal (unbalanced), on box
would be accelerating away from the other box.
Practice
Complete Practice Problems posted on our class blog.