- Good morning, today we are going to learn about
Newton's First Law of Motion.
♫ Flipping Physics ♫
- Sir Isaac Newton was a great scientist,
whose first of three laws of motion we are going
to learn about today.
Newton's First Law of Motion is...
An object at rest will remain at rest,
and an object in motion will maintain a constant velocity
unless acted upon by a net external force.
There are clearly two cases here,
an object is either at rest, or in motion.
Let's start with an object at rest.
In order to do so, let's go to our
Flipping Physics correspondent in Chicago, Illinois.
Mrs. Zeller, I believe you have something to tell us
about an object at rest.
- Yes, Mr. P., I do, I'm standing next to a very large rock,
and as you can see, it is at rest.
And, it's still at rest,
therefore we can conclude, according to Newton's First Law,
that there is no net external force acting on the rock.
This is Mrs. Zeller, reporting from Chicago, Illinois.
- Great, thank you, Mrs. Zeller.
Now, Billy, I need to know what it means
that there is no net external force acting on the rock.
- It means there are no forces acting on the rock.
- Actually no, it means there are no net external forces
acting on the rock.
- Oh, right, but what does that mean?
- Net means the sum of, or what you get
when you add all the forces together,
but what does external mean?
- External simply means that the force is caused
by an interaction with an object other than itself.
For example, the rock next to Mrs. Zeller
has a normal force acting on it caused
by a push upward from the ground,
and a force of gravity acting on the rock,
which is caused by an interaction with the planet
pulling down on the rock.
Actually, Mrs. Zeller, could you please explain more
about what it means that there is no net external force
acting on the rock?
- Oh sure, Mr. P.
You can see that the rock is remaining at rest,
and this does not mean that there are
no forces acting on the rock.
There is a normal force acting on the rock,
and the force of gravity.
What the net external force being equal to zero means,
is that when you add up all the forces together,
they add up to zero.
- Thank you again, Mrs. Zeller.
Next, let's talk about the other case,
an object in motion will maintain a constant velocity
unless acted upon by a net external force.
Let me show you.
You can see this steel sphere is in motion
and will continue to move at a constant velocity,
until it experiences a net external force.
Again, it has forces acting on it right now,
the normal force,
and the force of gravity,
however, those two forces are equal in magnitude
and opposite in direction,
so they cancel one another out,
because their sum is zero.
So, the sphere will maintain a constant velocity,
until it experiences a net external force.
For example, it could run into a wood block.
The wood block caused a net external force on the sphere
and therefore changed the velocity of the sphere.
- [Bo] Hey, that block was an object at rest,
until the ball caused a net external force on it
and changed its state of motion.
- [Mr. P.] Very nice Bo, the block was certainly
an object at rest until the sphere caused
a net external force on it, and accelerated it.
Now, let's get back to the moving sphere.
An object in motion will maintain a constant velocity
unless acted upon by a net external force.
Bo, please tell me which two variables remain constant
when an object has a constant velocity.
- The speed and direction of the object.
- Because velocity is a vector,
and has both magnitude and direction.
- Yup, speed and direction.
- So notice you could also say,
an object in motion will maintain a constant speed
and a constant direction,
unless acted upon by a net external force.
This means the same thing
as maintaining a constant velocity.
Newton's First Law is often called his Law of Inertia.
Remind me, Bobby, what is inertia?
- Inertia is the tendency of an object
to resist a change in state of motion.
Right, so staying at rest or staying at a constant velocity
is all about inertia or resisting changes
in states of motion, that makes sense.
- Bo, please say Newton's First Law for me, one last time.
An object at rest will stay at rest,
and an object in motion will stay in motion
unless acted upon by a force.
- Thank you Bo, you just made the two most common mistakes
students make when stating Newton's First Law.
Students often forget that an object in motion
will maintain not just motion,
but rather a constant velocity,
and it's unless acted upon by not just a force,
but rather, a net external force.
Bo, one more time please.
- An object at rest will stay at rest,
and an object in motion will stay at a constant velocity
unless acted on by a net external force.
- Very nice Bo.
Thank you very much for learning with me today,
I enjoyed learning with you.
(birds chirping)
- Yup, it's still at rest.