# Description using Newton's laws

1. Oct 27, 2004

### TheShapeOfTime

Let's use the scenerio of a car going from uniform motion, to a turn to the left. I'm trying to describe this in relation to Newton's laws in hopes to get a greater understanding. What I know is that objects in the car are affected by Newton's first law, which states (among other things) that objects in motion wish to stay in motion. Thus, the objects move towards the outside of the turn. As for the second law, what causes the unbalanced force? Does the third law come into play here?

2. Oct 27, 2004

### kuenmao

You forgot that the car itself isn't an inertial reference frame. This is an accelerating reference frame we're talking about. To compensate for the frame change, we have to add a force on every object inside the frame, pointing in the opposite direction to the movement of the reference frame.
If you judge from an outsider's point of view, the object is just moving in a straight line.

You don't need the third law to explain this.

3. Oct 27, 2004

### Staff: Mentor

Right. Unless a force is exerted on an object, it will keep moving in a straight line. For objects not firmly attached to the car, that's just what they do. (From inside the car it looks like they are being pulled towards the outside of the turn.)
What do you think? What's the only thing (except for air) in contact with the car?

4. Oct 28, 2004

### TheShapeOfTime

From what I understand, the third law is applies only when a force is applied to an object. In this example, the car pushes down on the road, and the road pushes back with equal force. But, the third law has nothing to do with the turning (unbalanced forces).

5. Oct 28, 2004

### Staff: Mentor

Right. If not for the road exerting a sideways force on the car, the car could not turn. For an unbanked road, that force is a friction force. Take away the friction--try making a sharp turn on a patch of oil or a sheet of ice--and you can't make the turn.

Of course, whenever a force is exerted, Newton's 3rd law comes into play. If the road pushes the car, the car pushes the road.