# New Problem I dont get!

1. Aug 28, 2005

### thschica

A sports car is advertised to be able to stop in a distance of 55 m from a speed of 85 km/h.
(a) What is its acceleration in m/s2?

(b) How many g's is this (g = 9.80 m/s2)?

I am confused as to how you would find the acceleration.Iwould be able to put it in to g's but i dont understand how to get the acceleration!

Last edited: Aug 28, 2005
2. Aug 28, 2005

### VietDao29

Do you know the equation:
$$v_f ^ 2 = v_i ^ 2 + 2ad$$?
It's used to solve for a when v_i, v_f, and d is known.
Viet Dao,

3. Aug 28, 2005

### thschica

would v_i =23.6 m/s,v_f=55m,and d=0

4. Aug 28, 2005

### Nylex

No. Where do you get that 23.6 ms^-1 from? v_f is also a speed, so it isn't a distance. Also, why did you put d = 0?

5. Aug 28, 2005

### thschica

I have no idea what to do in this problem.I divided 85km/h to get it into m/s.

6. Aug 28, 2005

### iNCREDiBLE

$$v_f = 0$$ m/s
$$v_i = 23.611$$ m/s
$$d = 55$$ m

7. Aug 28, 2005

### Dr.Brain

Use the basic kinematics equation
$v^2 - u^2 = 2as$

For your second part , divide your answer by g , that will give you the reqd. number.

Donot forget to convert km/hr to m/sec.

Regards
BJ

8. Aug 28, 2005

### thschica

Thank you!I understand what I did!Is the answer a negative number?

9. Aug 28, 2005

### iNCREDiBLE

Yes, indeed. The velocity has decreased down to zero which means that the acceleration must be negative. A negative acceleration is also called deceleration/retardation.