 In the last series of demonstrations, we tried to reduce the horizontal force as much as we possibly could so that there would be no horizontal force acting on the object. In that particular case, it would continue to move at constant velocity. So what we did was make the friction smaller and smaller. In this case, we're going to look at a car, and cars generally have lots of friction. They have a lot of friction with the road. After all, they need that in order to keep moving. So for this particular car, we're going to arrange for it to move at a constant velocity simply by turning on the motor. Now, why did it move at a constant velocity? Well, according to Newton's first law, the net force acting on it must have been zero. That net force is composed of two parts. There is a force, a friction, that is resisting its motion. It acts like this. But there is also a force pushing forward, and that results from the motor, and that propels the car this way. Those two forces are opposite in direction, but equal in magnitude, and as a result, they add up to zero. So the net force acting on the car is zero. As a result, it will move at constant velocity. To continue our demonstrations on Newton's first law, we're going to look at a little bit different situation. We're going to look at the forces experienced by a passenger in a car. Now, you've all had the experience of being in a car, and the car suddenly speeds up or suddenly slows down. And we tend to describe those experiences in terms of the way that we're feeling. But what we need to do is look at them from the point of view of a passenger on the ground. A passenger on the ground has what we call an inertial point of view. And this is the point of view in which Newton's laws are valid. So Newton's first law to review says that an object continues in a state of constant velocity unless acted upon by net external force. All right, let's apply this to a car. Now, for a car, we're going to use this laboratory cart, and we need to have a windshield on it. And so we've got this plastic pipe plate that's taped to the front of it. And we need a passenger, so we're going to use this troll, and we'll put the passenger right there. Now, first of all, we'll look at the situation where we apply force on the cart to accelerate it. So we're going to push the cart this way to begin with. And we want to see what happens to the passenger. Now, I'm going to go ahead and demonstrate this once, and then we're going to talk about it. Now, you need to have a reference point so that you can see what the troll is doing with respect to that reference point. So I'm going to use my hand here. Again, remember, I'm an observer standing on the ground. Okay, I'm an inertial observer. So here's my hand, and I'm going to push the cart, and I want you to watch where the troll moves with respect to my hand. Watch the feet and watch the head. Here it goes. Let's do that one more time. I hope you notice that the troll was always in front of my hand. The troll did not fall behind my hand. What I'm trying to do is do away with the notion that the troll is somehow thrown backwards or forced backwards. Instead, the troll is being pulled forward. If you notice the feet, the feet moved ahead of the rest of the body, okay? That's because the force of the friction of the cart surface on the feet were pulling it forward. So the force on the troll was that way. Now, as the feet were pulled this way, the head and the shoulders tended to remain where they were. After all, an object tends to remain in a state of rest unless acted on by a net external force. So the head and shoulders tended to stay where they were while a net external force pulled the feet forward, and the result was that the troll's feet were pulled out from under it. Now we're going to look at the other side of the situation, namely what happens when the car comes to a quick stop. We often say that the passenger is thrown forward, but in fact from our point of view as an observer on the ground, that is not happening because we know from Newton's first law that an object continues moving at constant velocity unless acted on by a net external force. In the case of the passenger, there is no force on the passenger pushing it forward. Let's take a look at that. This time what we're going to do is crash the cart into this brick. And so when the brick hits, of course there will be a force acting on it bringing it to a stop. But there is no force acting on the troll, so the troll should continue moving forward and in fact will move forward until it hits the windshield. Let's take a look at that. Now here I want to push this so that the troll doesn't, so the troll stays on its feet for the whole trip. So I'm not going to push quite as hard as I did last time. Okay, let's take a look at that one more time. What happens again is that the cart comes to a stop and turns around because the brick is exerting a force on it to do that, but the troll continues moving in a state of constant velocity at least until it comes into contact with the windshield and then the windshield exerts a force on it to bring it to a stop. Now to show that a little bit more dramatically, I'm going to take the windshield off. And you might guess what's going to happen to the troll this time when the cart hits the troll will simply keep going and actually falls over the front of the cart. Let's see that one more time. So what we've seen in this series of demonstrations is that from our point of view as inertial observers, objects continue to move at constant velocity or remain in their state of rest unless acted on by net external force.