Do Now – 5 Minutes
Topic – Newton’s 3rd Law - Introduction
A 60 kg cyclist starts riding a 10 kg bike. The
cyclist accelerates from rest to 7 m/s over the
span of 2.5 seconds.
Calculate the force the cycling is applying to
the bike to accelerate at this rate.
Determine the distance the
cyclist traveled over the 2.5
seconds spent accelerating.
Homework
Newton’s 2nd Law Worksheet
(1 Day Late)
Test Corrections – Due Today
Recap
What did we learn about yesterday?
What did Newton’s 3rd Law say?
Give an example of an interaction.
What is the action?
What is the reaction?
Concept Check
To clarify the whole action/reaction thing,
let’s look at a theoretical situation.
For effect, let’s assume it’s a big person hitting
into a smaller person.
Concept Check
Who gets hit harder?
Who gets knocked back further?
Why?
The smaller person is going to
accelerate more because they have less
mass.
Concept Check
Let’s take a look at jumping for a second.
What are you doing when you jump?
How would you explain that using
Newton’s 3rd Law?
What is the action?
What is the reaction?
Concept Check
What is pulling you back down?
How would you explain that using
Newton’s 3rd Law?
What is the action?
What is the reaction?
Concept Check
Let’s draw the FBD of a student
slapping their hand against a desk.
Identify the action force and the
reaction force.
Action force: hand slapping desk
Reaction force: desk slapping hand
Concept Check / Demo
Identify the action force and the reaction
force if a student pushes off of a desk or
wall while sitting on a scooter.
Draw the FBD
Predict what will happen to the student
on the scooter.
Action force: Student pushes wall
Reaction force: Wall pushes back
More Demonstrations
Predict what happens if two students,
each on a skateboard, push off from
each other.
Draw the FBD
Identify the action force and the
reaction force
More Demonstrations
Predict what happens if only one
student pushes off of another student
on their skateboards.
Draw the FBD
Identify the action force and the
reaction force
More Demonstrations
Imagine if two students are sitting on
a skateboard, facing each other, and
hold a rope between them.
Draw the FBD
Predict what happens if one and only
one of the students pulls on the rope.
Problems with Math
How does Newton’s 2nd Law relate to
Newton’s 3rd law?
How can they be combined?
The forces are equal and opposite on
different objects.
The resulting acceleration may not be
the same, however.
How is that possible?
Seeing this in action
We don’t just see Newton’s 3rd law when
we’re slapping tables – it’s literally
everywhere!
Explain why cannons (used for firing
cannonballs) are designed this way:
Predict what would happen if the beam
behind the cannon didn’t exist.
Seeing this in action
Here is a video of a World War 2 Soviet
Artillery Cannon.
Describe what happens to the cannon-
barrel when a shell is fired. Explain the
action force and the reaction force.
Action Force: Cannon exhibits a force on
artillery shell
Reaction Force: Artillery shell exhibits a
force on the cannon
Computational Problems
A 10 kg cannonball is fired with an
acceleration of 45 m/s2.
What is the force the cannon
fires the cannonball with? 450 N
What force does the cannonball exert on
the cannon? 450 N
The cannon that fires the cannonball has
a weight of 10,000 N.
What is the acceleration of the cannon
when the cannonball is fired? 0.45 m/s2