- Good morning, today we are going to define
and show examples of inertia and inertial mass.
- Hey guys.
- [Billy] Hey Bo. - [Bobby] Hi Bo.
- [Mr. P.] Bo, could you please define inertia?
- Inertia is the tendency of an object
to resist a change in state of motion.
At least that's what the book said, but I don't get it.
What's a state of motion?
- A state of motion just basically means velocity.
- Oh, so a change in state of motion would be
a change in velocity.
- Right, and change in velocity over change in time
is acceleration so inertia is a resistance to acceleration.
- Right, inertia is the tendency of an object to resist
a change in state of motion.
A change in state of motion is acceleration,
therefore inertia is the tendency of an object
to resist acceleration.
For example, the steel sphere on your left is at rest
and its inertia will cause it to tend to stay at rest.
The steel sphere on your right has a constant velocity
and its inertia will mean that it will tend to maintain
this constant velocity, and the constant velocity means
a constant speed and a constant direction.
Now, let's define mass, and before you tell me that
mass is the--
- [Billy] The amount of matter in an object.
- [Mr. P.] Before you tell me that mass is the amount
of matter in an object, I want to define mass more precisely
as a measure of inertia.
Again, the amount of matter in an object
is not a very clear definition for mass.
Mass as a measurement of an object's inertia is
a precise definition for mass.
Now, Bobby, could you please define mass
using the definition of inertia?
- Uh, mass is a measure of inertia,
and inertia is the tendency of an object
to resist acceleration, so mass is a measure of an object's
resistance to acceleration.
- That means that the more mass an object has,
the more it will resist acceleration.
Let's go to our Flipping Physics correspondent
Marcia in Mexico for an example of this concept.
- Thanks, Mr. P.
You see, to my left I have a desk,
and to my right, I have a chair.
Literally, the desk has more mass than the chair
and therefore has more resistance to change in motion.
Both the chair and the desk are currently in a state of rest
and now I will accelerate each one.
The chair is easy to accelerate because it has little mass.
Pushing the desk, however, is more difficult
to change the state of motion because it has a larger mass.
Thanks, this is Marcia Vilchis reporting
from Monterrey, Mexico, back to you, Mr. P.
- Thank you for your help, Marcia.
Now we need to add one word to our definition of mass.
Technically, the mass we have defined here
is called inertial mass.
There is also something called gravitational mass,
which we'll define at a later point, but I do want to
point out right now that gravitational mass
and inertial mass appear to be identical.
When we measure the inertial mass of an object,
when we measure its resistance to acceleration,
we actually get the same result as we do
when we measure its gravitational mass.
Therefore, because gravitational mass and inertial mass
are experimentally identical, unless there's a reason
to differentiate between the two,
I will simply refer to each as mass.
Thank you again to Marcia for your help with today's lesson.
And thank you for learning with me today.
I enjoyed learning with you.