# Newton's Law Question

1. Jan 28, 2007

### robbondo

1. The problem statement, all variables and given/known data
An advertisement claims that a particular automobile can "stop on a dime." What net force would actually be necessary to stop an automobile of mass m traveling initially at a speed of v in a distance equal to the diameter of a dime, which is d?

2. Relevant equations
F=ma

3. The attempt at a solution
Well I know that to stop the car you need to have a force equal to the force of the car at that point. And I know that if there is a distance and change in acceleration there must be some way to relate it to force. I'm just about completely lost though. I'm assuming there must be some simple way to solve this using Newtons laws. I think I just need a little direction.

2. Jan 28, 2007

### P3X-018

Assuming constant acceleration, you can use the kinematic equation

$$a = \frac{v_2^2-v_1^2}{2d}$$

Where $v_2-v_1$ is the change in speed over the distance $d$, $a$ is the acceleration you seek.

Last edited: Jan 28, 2007
3. Jan 28, 2007

### SGT

Use conservation of energy. The kinetic energy of the car must be canceled by the work done by the force.