# Homework Help: Guys im hopeless

1. Jan 29, 2006

### nick727kcin

ive been working on my physics hw since lasnt night. i dont get hardly any of this

pplease help me on just one other impossible question

Your school science club has devised a special event for homecoming. You've attached a rocket to the rear of a small car that has been decorated in the blue-and-gold school colors. The rocket provides a constant acceleration for 9.0s. As the rocket shuts off, a parachute opens and slows the car at a rate of 5.0m/s^2. The car passes the judges' box in the center of the grandstand, 990m from the starting line, exactly 12s after you fire the rocket.
What is the car's speed as it passes the judges?

2. Jan 29, 2006

### Staff: Mentor

So you have a constant accel and a constant decel. Try graphing the speed and displacement versus time for several different accel rates, and see what that does for the speed versus distance. Anytime something like this stumps you for more than a few minutes, start graphing some possible trajectories to get a feel for what is going on. Like, you'll see how the different possible accel numbers change the speed later on under decel at the judges' position. Try a few sketches, and see if that helps you to work out the right equations...

3. Jan 29, 2006

### nick727kcin

thanks for the reply, but i have charts for v/t, position/t, and accel/t, but its not really helping much. ill try to list each of the factors i have

tf=3s
a=-5m/s^s
vf=?
vi=0

xo=0
xf=990m

i dont really know how to go about this since it changes its accel

4. Jan 29, 2006

### nick727kcin

ah i think i got it. is it 15m/s?

EDIT: no its not 15. two tries remaining

Last edited: Jan 29, 2006
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