# Homework Help: Acceleration problem (HELP! ()

1. Sep 30, 2011

### princesssuckz

acceleration problem (HELP!:(()

1. The problem statement, all variables and given/known data
A vehicle accelerates to 14 m/s after being stopped at a red light. The vehicle covers 26 m to reach its final velocity. How fast was it accelerating?

2. Sep 30, 2011

### vela

Staff Emeritus
Re: acceleration problem (HELP!:(()

What equations do you have?

3. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

umm.. acceleration is equal to displacement over time squared.

4. Sep 30, 2011

### vela

Staff Emeritus
Re: acceleration problem (HELP!:(()

I'm afraid you're going to have to do better than that. Look in your textbook for equations for constant acceleration. Those are the ones you want to choose from.

5. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

well the only equation here related to acceleration is a=v-vo/delta t

6. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

Welcome to PF, your highness princesssuckz!

Try:
$$v_f^2 = v_i^2 + 2 a \Delta x$$
Can you make sense of this formula?

7. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

no not really. we haven't discussed this in class yet. the teacher sort of just threw it at us.

8. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

$$x=x_o + v_o t + {1 \over 2} a t^2$$

9. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

i don't get it. why is there an x?

10. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

Because the problem statement says: "The vehicle covers 26 m".
This is "x".

11. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

oh now i get it. :rofl: i feel really stupid.

12. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

Uhh?

So you solved the problem?

13. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

sorry. i tried but no. could you please walk me through it. :(

14. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

So which equations do you have now?

15. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

just the ones you gave. i tried the first one. but i don't think it's right.
v2f= 14m/s2+2(9.8m/s) (26 m)

16. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

Ah, well, let me explain:

$v_i$ is the initial speed. What is it?
$v_f$ is the final speed. What is it?
$a$ is the acceleration you have to calculate - it is not gravity!
$\Delta x$ is the change in position - you have that right.

17. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

$v_i$= 14 m/s
i don't know the $v_f$.

18. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

Initially the vehicle is in front of a red light.
At this time the vehicle has its initial speed.

After accelerating over a distance of 26 m, the vehicle achieves its final speed.

Try again?

19. Sep 30, 2011

### princesssuckz

Re: acceleration problem (HELP!:(()

hmm..so the $v_i$= 0
and the $v_f$= 14m/s

right?

20. Sep 30, 2011

### I like Serena

Re: acceleration problem (HELP!:(()

Right!