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Newton's First Law: Motion of a Passenger in a Car

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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 a constant velocity. So what we did was make the friction smaller and smaller. In this case we are going to look at a car, since cars generally have a lot 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 are going to arrange for it to move at a constant velocity simply by turning on the motor.

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 force is composed of parts, a friction that is resisting that motion and it acts like this. But there is also a force that is pushing forward and that results from the motor that propels the car this way. Those two forces are opposite directions but the magnitude result adds up to zero. So the net force acting on the force is zero, as a result, it will move at a constant velocity.

To continue our demonstration on Newton's First Law we're going to look at a different situation. We are going to look at the forces experienced by a passenger in a car. Now you've all had the experience being in a car and the car can suddenly speeds up or suddenly slows down and we tend to describe those experiences in terms of the way we are feeling, but what we need to do is look at it 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 where Newton's first law is valid.

So Newton's first law states that an objects stays at a constant velocity unless acted upon by a net external force. Let's apply this to a car; now for a car we are going to use this laboratory car and we need to have a windshield on it and so we have this plastic plate and we need a passenger so we are going to use this troll, and we will put the passenger right there. Now first of all we will look at the situation where we apply a force on the car to accelerate it, so we are going to push the cart this way to begin with and we want to see what will happen to the passenger. Now I'm going to go ahead and demonstrate this once and then we are going to talk about it.

Now you need to have a reference point so you can see what the troll is doing with respect to that reference point. So I am going to use my hand here and remember I am an observer standing on the ground, 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. I want you to watch the feet and watch the head. Here it goes. Let's do that one more time. I hope you noticed 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 being pulled backwards. Instead, the troll is being moved forward. If you noticed the feet, the feet moved ahead of the rest of the body. That's because the force of the friction of the car surface on the feet was pulling it forward, so the force on the troll was going that way. When the feet were pulled this way, the head and shoulders tend 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 heads and shoulders tend to stay where they were while a net external force pulled the feet forward, and the result was the troll's feet was pulled from under the troll.

Now we are going to look at the other situation, namely what happens when the car comes to a quick stop and we often say that the passenger is thrown forward, but from our point of view on the ground, that is not happening because we know from Newton's first law that an object will keep moving at a 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.

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