Hi. It's Mr. Andersen and this is AP Physics essentials video 40. It is on interaction
forces which are forces between different objects. And this whole video revolves around
this one statement, " A force on an object always requires another object." So let's
look at some scenarios and make sure this holds true. Let's say a box is being pushed
across the floor. There is clearly a force being applied to the box. What is the other
object? It is the person pushing the box. Or if we look at the moon as it orbits. Clearly
a force is being applied to the moon. What is the other object? Well that is just the
earth. Or if we look at this charged particle as it accelerates up and to the right, there
is clearly an electric field. There must be another object. There has to be a positive
charge down and to the left or maybe a negative charge up and to the right. So those are pretty
easy. Let's make it a little more difficult. If we see a rocket accelerating through space
there must be another object. But we do not see one. What is it? It is actually the rocket
fuel. And so as the rocket applies the force on the fuel in one direction, the fuel applies
an opposite and equal force in the other direction causing it to accelerate. Or if we look at
an arrow in mid flight. And this in one that has puzzled scientists forever. As it is right
in the middle of its flight what is the other object? Well there is no other object. In
other words this just has constant motion and so there is no force being applied to
it at this point. It just has what we would call inertia. So if you ever have an object
that is having a force applied to it that means there is another object. And just because
an object is at rest does not mean that there are not any other forces acting on it. They
might just be all be balanced. So we could have balanced net forces acting on that object.
But if they are unbalanced those net forces are in one direction, we know this, that there
is going to be an object accelerating in that direction. The net force on an object and
the acceleration will always be in the same exact direction. Now the velocity is different.
You could have an object that is moving with a velocity to the left and it is accelerating
to the right. We will look at that scenario in just a second. Now this idea of motion
has been confusing to scientists for a long time. Even Aristotle thought if you push an
object across the floor then we have to have a constant force being applied on to that
object to keep it moving at a constant rate. But what he did not see is friction. And he
did not understand why an arrow keeps moving through space if there is nothing pushing
it. It simply has inertia. And the scientist that really figured this out was Galileo.
So he took a sphere and he would roll it down like that. Its potential energy becomes kinetic
energy and back to potential energy. And it almost goes as high as it was before. So what
he said was let's imagine we change the shape, what is going to happen now? It still almost
makes it back to where it was. And now let's make it look like this. What is going to happen?
Well it is just going to keep rolling. If we eliminate that friction it is just going
to keep moving. So he discovered this idea of inertia. And so what is an interaction
force? Remember it is a force between one object and another object. And if we have
an astronaut like this who is out in space and his gear breaks down, so he can not maneuver
at all, he is really stuck. He can not apply a force to himself to move anywhere. And so
there is no way that he can accelerate. You can not apply a force to yourself. And a really
cool experiment to do this, in a physics class, is a set up like this where we have a cart.
And on it we have a sail. And then we have a fan that is blowing wind into the sail.
And so this in one object. And so you can not apply a force to yourself, and so this
actually will not move. If we want to get it to move we have to remove one of those
objects. If we remove the sail the wind is going to push against the sail and the cart
is going to move to the right. Or if we leave the sail attached and we move the fan to the
right the whole thing is going to move to the left. We have to have that external object.
Now remember if we have an object at rest, it does not mean that forces are not being
applied to it. It is simply not accelerating. And so what forces are being applied to this
object right here? There is clearly gravity down. But it is not accelerating down so there
must be an opposite and equal force, we call that the normal force that is being exerted
by the table itself. And so there is no net force. There is no net acceleration. Let's
take that same object and slide it across ice with constant velocity. Let me pause the
video here for a second. So what are the forces acting on that object now? We still have gravity
down. We still have a normal force going up. But there is no net force in either direction.
It simply has motion already and it is going to continue with that motion. Now let's say
that astronaut who is out there kind of stuck in space, if we apply a net force to it then
we can get acceleration. And so the way this works with these suits is that you will shoot
nitrogen gas out one side. And as you do that you are apply a net force, in other words
you are applying a force to that gas and it applies an opposite and equal force back on
you. And so our net force is going to be to the right. So what is that astronaut going
to do? It is going to accelerate to the right. And that is Newton's second law. Now let's
take that same astronaut and say that it is just drifting from the right to the left.
And again let me pause the video right here. What happens if we turn on those same jets?
Well we know now that the net force is going to be to the right so the acceleration is
to the right. But watch what happens when I turn on those jets and then I start the
video again. You can see it is now moving to the left. Velocity to the left but the
acceleration is going to be to the right. Remember net force and acceleration are always
in the same direction. So did you learn to analyze scenarios and make claims about the
forces being exerted on different objects and what those other objects are? Remember
if it is just at rest or moving with constant velocity there is no net force. Did you also
learn to challenge this claim that you can exert forces on yourself? And finally, this
is an AP Physics 2 concept, did you learn that if we want to apply a force to a charge
or something that has gravity we have to use something else that has that same kind of
a charge. The earth has mass. It has gravity as well. And so that is interaction forces,
again forces between objects. There always have to be two objects. was
And I hope that helpful.