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 cyclist 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 – Due
Wednesday
Test Corrections – Due Wednesday
Thought Experiment
What happens when you apply a force to
an object on wheels?
The object accelerates!
Thought Experiment
What happens when you apply a force to
something rigid, like a wall?
Ow!
Thought Experiment
What happens if take your hand (open)
and apply a force to the top of the table?
Ow!
Let’s draw a free body diagram of this
situation
Thought Experiment
When you apply a force to the table, what
is the table doing back to you?
Hand
Table
Reaction
The table must be applying a force back!
What is Happening?
Newton discovered that forces tend
to come in pairs.
Newton found that a force is always
part of a mutual action that involves
another force.
These force pairs are referred to as
interactions.
Newton’s
rd
3
Law
This realization spawned Newton’s
3rd Law.
Whenever one object exerts a force on
a second object, the second object
exerts and equal and opposite force on
the first object.
For every action, there is an equal and
opposite reaction.
What is Happening?
For example, let’s look at a hammer hitting a
nail.
A hammer imparts a force on the nail, which in
turn, drives the nail into a piece of wood.
Nail
imparts a force on the
hammer – otherwise the
hammer would go right
through the nail – and
that does not happen.
Hammer
However, the nail also
The Third Law
One force is called the action force.
The other force is called the reaction
force.
Reaction
partners in a single
interaction.
Action
These forces are
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 scooter, 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 scooters.
Draw the FBD
Identify the action force and the
reaction force
More Demonstrations
Imagine if two students are sitting on
a scooter, 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.