Discus: SAT/ACT Tests and Test Preparation: October 2003 Archive: Physics help please
 By Bard (Bard) on Monday, October 06, 2003 - 09:05 pm: Edit

A rocket rises vertically, from rest with an acceleration of 3.2 m/s until it runs out of fuel at an altitude of 1200 m. After this point, its acceleration is that of gravity, downward. (a) what is the velocity when it runs out of fuel.(b) How long does it take to reach this point. (c) what maximum altitude does the rocket reach.(d) how much time does it take to reach maximum altitude. (e) With what velocity does it reach the earth. How long (total) is it in the air

Thank You for you help

 By Neo (Neo) on Monday, October 06, 2003 - 09:42 pm: Edit

It's velocity(a) when it runs out of fuel is approx. 87.64m/s, presuming it continued to accelerate until the very moment it ran out of fuel. Use the vf^2 formula to find vf. Use vf in the vf=vi+at eq. (vi is 0, launch velocity) to find the time (b). The time is 27.39 seconds.

So so far, (a)=87.64m/s, and (b)=27.39sec.

Next, use the vf^2=vi^2+2ad formula again, but use (a) as Vi and 0 as Vf. This will give you (c), after you add this answer to 1200.

(c)=1591.5m.

Next, use vf=vi+at to find time, and add that to (b). You should get something around 36.3 seconds. That's (d). Then, use the drop-equation (d=4.9t^2) to find the time it takes to fall, and plug that into vf=vi+at. You should get around 153.5m/s. That's (e). To find (f), add all the times together. 36.3+15.65(the drop-time), is 51.95 sec.

You're welcome :^) Physics rocks...

 By Bard (Bard) on Monday, October 06, 2003 - 09:44 pm: Edit

thanx a lot ne0. I solved the first two. Just needed help on the last ones.

Thank you very much

 By Neo (Neo) on Monday, October 06, 2003 - 10:09 pm: Edit

No problem :^)

Always glad to help on rocket problems...now I'm off to eat something.