11.3 Newton’s
rd
3
Law
I CAN:
-I CAN APPLY NEWTON’S LAWS USING MULTIPLE FORCES ON A SINGLE
OBJECT.
When you kick a ball…
•Force exerted on ball by foot= action force
•Force exerted on foot by ball= reaction force
Newton’s Third Law
For every action force, there is an equal and
opposite reaction force.
All forces act in pairs
Ex: Action force is sitting in a chair. What is
the reaction force and what is it equal to?
Swimming?
Force pairs do not act on the same
object
If the if reaction is equal but opposite,
wouldn’t the net force be zero?
Swimmer acts on water, water acts on
swimmer. (occurring at the same time)
Equal forces do not always have equal
effects
• It is easy to see the effect
of the action force, but
what is the reaction
force?
• Why cant we see it?
• -Newton’s Second Law
Momentum
Momentum: a quantity defined as the product of mass and velocity of an object.
Momentum=mass x velocity
P= mv
 More mass, more momentum
 More velocity, more momentum
Momentum must also have direction.
What is the difference between inertia
and momentum?
Mass is a fundamental measure of inertia; it measures the resistance of the body to changes in
its motion. Thus, inertia is resistance to motion changes. Whereas, momentum is mass in
motion, and, is defined as the mass times the velocity.
Math Practice
Calculate the momentum of a 6.00 kg bowling ball moving at 10 m/s down the alley toward the
pins.
Step 1: list the given and unknown values.
◦ m=6.00 kg
◦ V=10 m/s
◦ p= unknown
Step 2: Plug them in
◦ p=6.00kg x 10.0 m/s
◦ p=60.0kg x m/s down the alley
More Practice
A 75kg speed skater moving forward at 16m/s
A 135kg ostrich running north at 16.2 m/s
A 5.0 kg baby on a train moving eastward at 72m/s
A seated 48.5kg passenger on a train that is stopped.
Calculate the velocity of a 0.8kg kitten with a momentum of 5kg x m/s forward.
Force is related to change in momentum
MOVING THE GLOVE BACK DURING THE CATCH
INCREASES THE TIME OF THE MOMENTUM’S
CHANGE AND DECREASES THE IMPACT FORCE.
How do you catch a baseball if you are not
wearing a glove?
Law of conservation of momentum
The total amount of momentum in an isolated
system is conserved.
Billiard balls on pg. 364 or Newtonian Balls
example.
Any time two or more objects interact they
may exchange momentum, but the total
momentum stays the same.
Action and Reaction pairs are
everywhere.
11.3 Review
1) State Newton’s third law of motion, and give an example that shows how the law works.
2) List three examples of action-reaction force pairs that we did not mention in this section.
3) Define momentum and explain what the law of conservation of momentum means.
4) Explain why, when a soccer ball is kicked, the action and reaction forces don’t cancel each
other.
a)
b)
c)
The force of a player’s foot on the ball is greater than the force of the call on the player’s foot
They act on two different objects.
The reaction force happens after the action force.