# Homework Help: Finding car speed

1. Oct 31, 2011

### vaironl

1. The problem statement, all variables and given/known data
A race car starting from rest accelerates uniformly at a rate of $4.90m/s^{2}$ . What is the car's speed after it has traveled 200m?

2. Relevant equations

Δv = $\frac{Change in position}{change in time}$

3. The attempt at a solution
v=$\sqrt{980m^2/s^2}$

=31.3m/s

2. Oct 31, 2011

### SammyS

Staff Emeritus

How did you get $v=\sqrt{980m^2/s^2}\,?$

3. Oct 31, 2011

### Staff: Mentor

What kinematic formula are you using? It looks like you've missed out a constant.

4. Nov 1, 2011

### vaironl

Well I'm required to find the speed after 200 meters, but I was never given any time.

So I assume... Which is not a good thing to do, multiplying the acceleration * the distanced travel would give me the speed.

But I really can't remember the actual equation.

5. Nov 1, 2011

### Staff: Mentor

If you can't remember you should check your notes or text book! Your formula is close to being a valid kinematic expression,... but it's missing a constant. Do you have a list of the common kinematic expressions?

6. Nov 1, 2011

### vaironl

I have the text book, and I did have an example sheet but my friend borrowed to check her work. I found this expression which seems to satisfy my problem, v2 = vo2 + 2a(X - Xo)

Therefore V2= 0m/s + 2(4.90m/s^2)(200m)

v2= 9.8m/s^2(200m)
v2= 1960m^2/s^2
v=$\sqrt{1960m^2/s^2}$
v= 44.3m/s

7. Nov 1, 2011

### Staff: Mentor

That looks better