# Homework Help: Touble with acceleration/velocity problem

1. May 31, 2005

### Hwarang

trouble with acceleration/velocity problem

The land speed record of 13.9 m/s (31 mi/h) for birds is held by the Australian emu. An emu running due south in a straight line at this speed slows down to a speed of 12.0 m/s in 3.3 s.

Assuming that the acceleration remains the same, what is the bird's velocity after an additional 3.7 s has elapsed?

i know that you have to use...

A = ((Vfinal - Vinitial)/(Tfinal - Tinitial))

So we're solving for Vfinal.. For Vinitial, it would be 13.9m/s? And A would be -12m/s (neg. because slowing down). And for time.. would it be 7 sec-0 sec.?

thanks

Last edited: May 31, 2005
2. May 31, 2005

### whozum

You want to find the acceleration from the information given in thefirst two sentnces. Then from there use that acceleration to find the velocity at 7s. 13.9 would be hte initial velocity.

3. May 31, 2005

### Hwarang

thx

thx! that helped a lot.
i got -4.0 m/s as the velocity... but i was wondering, can velocity be negative? is it because the emu was slowing down?

4. May 31, 2005

### whozum

Was your average acceleration 0.58m/s^2? Thats what I got, then

$$\Delta v = a t$$

$$\Delta v = 0.58 \times 7 = 4.06 m/s$$

Recall this is the CHANGE in velocity. You started at 13.9m/s, and accelerating at that rate for 7 seconds you will lose 4.06 m/s. Whatis your final velocity?

Velocity CAN be negative, but in this case it shouldnt be.

5. May 31, 2005

### Hwarang

sorry.. i'm a bit confused.

if Vinitial =13.9m/s.. and the change in velocity = 4m/s...
by plugging in everything
-0.5758m/s^2 = ((Vfinal - 13.9)/(7sec))
Vfinal = 9.86 m/s

6. May 31, 2005

### omairt2001

yep thts rite

y dont u just use

v = u + at

where u=13.9 t=7 and a=-0.576

7. May 31, 2005

### Hwarang

thx everyone!

8. May 31, 2005

### whozum

$$V_f - V_i = at$$ where [itex] v_0 \ and \ v_i [/tex] are analogous.