Hi. It's Mr. Andersen and this AP Physics essentials video 9. It is on inertial mass.
Have you ever wondered how astronauts figure out their mass when they are in space? If
they were to stand on a specific scale like we have on earth it would read zero. They
are weightless. And so they use a device like this. Here is Andre Kulpers. He is a Dutch
astronaut on the International Space Station. And he is using a body mass measuring device.
And it is essentially a platform that has a spring in the base. And so you can give
it a push and it will go up and down based on the mass of that platform. And you can
see that it has a pretty high frequency. What you can then do is load it with an astronaut
and then you can give it a push. And you can see that since the astronaut has a greater
mass it has greater inertia. The frequency is going to be much slower. And so since the
spring constant remains the same you could figure out the mass of the astronaut. And
so this is one way to measure mass. We call that inertial mass. But there is also gravitational
mass. And I will talk more specifically about that in the next video. However, when we are
dealing with inertial mass it is essentially interactions between objects and systems.
But what we are looking at is some kind of a net force on an object which causes acceleration.
And if we know the force and we know the acceleration then, in the case of this astronaut, we can
measure their inertial mass. And so let's look at those three things. Net force, inertial
mass and acceleration. How would you study that in a physics lab? Well we could use a
modified Atwood machine like this. And so we have a cart attached to a string and then
this goes over a pulley. And then we have a stop at the end so it just does not keep
flying off the table. And so let's put these things in the right spot. So the inertial
mass we are trying to calculate is the mass of this cart. The net force is going to be
constant. Why is it constant? Because we are attaching it to this pulley. And so the force
of gravity is applying a net force on that cart. And what we will see happening is that
it is going to accelerate over time. You could also just pull this with a scale and make
sure the constant force is maintained throughout that whole pull. Now let's watch what happens
with a small mass of the cart. So just get a sense of what is going on. So you can see
it is accelerating. In other words it is starting slow and then it is speeding up. Now we could
calculate that acceleration. We could use photo gates or a motion sensor. We could video
it with a camera and then do some motion analysis on a computer. We could calculate that acceleration.
So let's see what happens if we increase the mass. Now we are keeping this net force the
same. And so we can see that we are having a slower acceleration. And so again, force
stays the same but it is not accelerating as quickly. Still accelerating. Now let's
load it with a bunch of mass and what happens? Constant force. You can see that we have a
really small acceleration. It is still accelerating, but it is much smaller than it was before.
And so this is just a simple experiment where we are keeping one thing the same. In this
case it is going to be the net force. And then we are varying one variable. We will
call that the mass of the cart. And then we are seeing what happens to the acceleration.
And so in this kind of equation we are keeping net force the same. And what are we doing
to inertial mass? You can see that we are increasing it. And what happens to the acceleration
is it decreases. We increase the inertial mass again. And it decreases again. And so
how could you set up an equation where he have force on one side and then this indirect
relationship between inertial mass and acceleration? Well you are probably familiar with this formula.
It is Newton's second law, force equals mass times acceleration. And so if I solve for
mass like that, what have I just discovered here? This is our inertial mass. So it is
mass not based on how much the earth is pulling down on the object but it is based on the
inertia that that object has. And so we were also measuring inertial mass at the beginning
when we were talking about that astronaut oscillating up and down on a spring. And so
did you learn to design an experiment for collecting data to determine the relationship
between the net force? It is inertial mass and it is acceleration. I hope so. And I hope
that was helpful.