mr.p: Good morning, today we are going to learn about
Newton's Third Law of Motion.
Bo: Hey guys.
Billy: Hey Bo. Bobby: Hi Bo.
♫ Flipping Physics ♫
mr.p: Newton's Third Law is an equation,
for every force one two,
there exists an equal but opposite force two one,
where both forces are vectors.
Newton's Third Law,
for every force one two,
there exists an equal but opposite force two one,
where both forces are vectors.
Every force is the result of an interaction
between two objects,
in this equation the objects are object one,
and object two.
So F's of one two,
is the force from object one on object two.
And F's of two one,
is the force from object two on object one.
Let's take a look at our first example.
As the ball collides with my head,
there are two equal and opposite forces.
The force the ball applies on my head,
or force one two,
and the force my head applies on the ball,
or force two one.
Those two forces are equal in magnitude,
and opposite in direction.
These two forces are called a Newton's Third Law Force Pair,
and it is important to realize the two forces
act on two different objects,
the ball, and my head.
(laughs) That was good, I like that one.
The force the ball applies on my head is equal and opposite
to the force my head applies on the ball,
a Newton's Third Law Force Pair.
Now, you may have heard for every action
there is an equal and opposite reaction.
However, I'm not really a fan of this version
of Newton's Third Law, for two reasons,
both of which are illustrated well in this home video
of ours from 2007, when Geneve was four and Ryan was two.
Geneve: I want a turn now.
[Mr.P] Why don't you guys do it together?
No, you wanna take a turn.
Geneve: No, I wanna take a turn by myself.
[Mr.P] Okay.
Geneve: Ryan, I want a turn by myself. (calls out)
[Mr.P] Geneve, calm down.
(Geneve calls out) Mr.p: Geneve, calm down.
Geneve: Ryan is wrecking my turn.
Ryan, you're wrecking my turn.
[Mr.P] Geneve, you can have turn next honey.
(crying)
Yes, you can.
Geneve: But I was waiting in line.
[Mr.P] You know what, you were waiting in line,
and you know what, I'm sorry,
Ryan probably shouldn't have gone.
mr.p: The action here is Ryan taking Geneve's turn,
and the reaction is Geneve getting upset.
For every action there is an equal and opposite reaction,
right?
Billy: But, that has nothing to do with physics.
mr.p: Exactly, the term's action and reaction here, are unclear.
Newton's Third Law has to do with forces, not actions.
In addition to that,
the two forces act simultaneously,
and the terms action and then reaction imply
that the two forces are not simultaneous.
Also, Newton's Third Law Force Pairs
are sometimes referred to as action/reaction pairs.
Bobby, remind me what is a force?
Bobby: A force is the ability to cause a change
in state of motion of an object.
Bo: Or you could just say the ability
to accelerate an object.
mr.p: Let's do another example,
a hammer striking a nail.
Billy, what is the Newton's Third Law Force Pair
in this example?
Billy: That would be the force the hammer applies to the nail
is equal and opposite to the force
the nail applies to the hammer.
[mr.p] Bo, what evidence do we have
that the hammer applies a force to the nail?
[Bo] The nail starts out at rest,
and when the hammer strikes the nail,
the nail accelerates,
so the hammer applied a force to the nail.
[mr.p] Correct, and Bobby, what evidence do we have
that the nail applies a force on the hammer?
[Bobby] The hammer is moving at the beginning,
and when it strikes the nail, the hammer stops,
so the hammer accelerates because the nail applies
an equal but opposite force on the hammer.
mr.p: Let's do another example.
(laughs) Let's go to Kevin Zhang,
our Flipping Physics correspondent in China,
to discuss this example.
Kevin: I'm here on the Great Wall of China,
to prove that Newton's Third Law still applies,
even with something as vast as the Great Wall.
Note that we know the Wall applied a force on me,
because I experienced an acceleration to slow me down.
I also applied an equal but opposite force on the Wall,
however its mass is too large for me
to change its state of motion.
This is exactly what happened with your wall
back at home, Mr. P.
So there you have it,
Newton's Third Law, on the Great Wall.
This is Kevin Zhang, from Dandong, China,
back to you, Mr. P.
mr.p: Thanks Kevin, it looks like Newton's Third Law
works on the other side of the planet, which is good.
[mr.p] Thank you very much for learning with me today,
I enjoyed learning with you.
mr.p: Good morning, today we are going to learn
about Newton's Third Law of Motion.
Geneve: Sorry, it keeps hitting the ceiling.
mr.p: Good morning, today we are going to learn
about Newton's Third Law of Motion.
(grunts)
(laughs) That was perfect,
we're going to do it several times.