# Stopping Speed

1. Jun 25, 2010

A driver in a car traveling at a speed of 77 km/h sees a cat 107 m away on the road. How long will it take for the car to accelerate uniformly to stop in exactly 103m? Answer in units of s.

I really have no idea where to begin with this problem.

2. Jun 25, 2010

### Redbelly98

Staff Emeritus
Any problem dealing with uniform acceleration starts with the equations for uniformly accelerated motion. Surely these are in your class notes or textbook.

Also, writing out the known quantities (vinitial=___, etc.) would help with this and most other physics problems.

3. Jun 27, 2010

I know that I need to convert 77 km/h to 21. 38888889 m/s which is the initial Velocity.

I was thinking of finding T with the equation d=V(initial)*T. So i could therefore put it in the question in: d=V(initial)*T + 1/2at^2 and solve for a.
However, I am unsure if I use 103m or 107 m in the equations.

Am I doing this correct?

4. Jun 27, 2010

### Bartek

Hint: cat is negligible . This is just a trap. To confuse you what distance to use. And make problem more dramatic.

regards

Last edited: Jun 27, 2010
5. Jun 27, 2010

### Redbelly98

Staff Emeritus
That is almost the right equation, except that you need v_average, not v_initial. For v_average you may take the average of the initial and final velocities.
You could do that, but look again at what the question is asking you to find. (It is not asking for the acceleration.)
The question asks how long it takes for the car to stop in 103 m.