# Mechanics Notes Riding in a Car

BHS -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page

## Riding in a Car

The common experience of riding in a car provides some excellent examples of Newton's First Law in action.

## Accelerating from Rest - "G Forces"

### Situation:

Your car is at rest. Suddenly you push down on the "accelerator" and the car accelerates forward. You feel that you are pushed back into your seat.

### Common Explanation:

"G Forces," caused by the acceleration, push you back into your seat.

### Problems with the Common Explanation:

First of all, there is nothing pushing you back in your seat! (Acceleration is a concept, not an object, and only objects can push you. See Thoughts on Force... for more discussion.)

### Correct Explanation:

You were at rest, so you remain at rest. The car accelerates forward, and you stay where you are.

## Decelerating - Being "Thrown Forward"

### Situation:

You are moving at constant velocity (constant speed in a straight line). Suddenly you apply the brakes. You feel "thrown forward" into the dashboard!

### Common Explanation:

You are thrown forward.

What throws you?

### Correct Explanation:

You are moving with a constant velocity. You keep that velocity (while the car slows down) until something stops you.

## Turning - "Centrifugal Force"

### Situation:

You go around a curve (at constant speed). You feel pulled toward the outside of the curve.

### Common Explanation:

The "centrifugal force" pulls you to the outside of the curve.

### Problems with the Common Explanation:

: What exerts this "centrifugal force"?

### Correct Explanation:

You were moving with a constant velocity (straight line at constant speed). The car turned, and you didn't (since Newton's First Law says your velocity stays the same unless an unbalanced force acts on you.)

## Summary

Notice that explanations involving Newton's First Law are simpler and more logical than explanations involving strange, mysterious forces that seem to have nothing exerting them.

See also "Thoughts on Force..." and the lab activity Newton's First Law in a Car.

BHS -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page

last update November 10, 2007 by JL Stanbrough