# Homework Help: Acceleration problem (HELP!)

1. Sep 29, 2008

### jcumby

A jet is diving vertically downward at 1200 km/h. If the pilot can withstand a maxiumum acceleration of 5g before losing consciousness, at what height must the plane start a quarter turn to pull out of the dive? Assume the speed remains constant.

I really don't know where to start here....All I've done is convert 1200 km/h to m/s and draw the jet's ideal flight path

2. Sep 29, 2008

### tiny-tim

Welcome to PF!

Hi jcumby! Welcome to PF!

Obviously, the jet has to complete the 90º turn before it reaches the ground!

So essentially the question is asking, if you go in a circle at 1200 km/h, what is the radius at which the maximum acceleration is 5g (or is it 4g or 6g ).

3. Sep 29, 2008

### minifhncc

The speed remains constant? So acceleration = 0? Sounds like a dodgy question again...? Maybe someone else can help you.

4. Sep 29, 2008

### tiny-tim

Hi minifhncc!

The speed is constant, but the velocity isn't.

5. Sep 29, 2008

### minifhncc

Well judging by the question, I'd imagine it would've had asked something along the lines of how far it can accelerate down at 9.8ms-2 and then when or after how much travelling to turn to get the correct force/acceleration for 5g, but anyhow...

6. Sep 29, 2008

### tiny-tim

Nooo … that would be free-falling … the question is about powered flight, with constant speed.

7. Sep 29, 2008

### minifhncc

Yes, but the question is asking at what height the plane needs to turn so that acceleration is under 5g. It doesn't ask what height it needs to turn to avoid hitting the ground... You see now?

EDIT: I can see where you're getting at, but I still think it's a dodgy question.

8. Sep 29, 2008

### tiny-tim

At constant speed, the acceleration depends only on the radius, not the height!

So why else would the question ask for the height?

Not hitting the ground is very important!

9. Sep 29, 2008

### minifhncc

Yeah I suck at word based stuff, Maths is my main 'area of expertise' =P

10. Jan 30, 2012

### metaender

Re: Welcome to PF!

An excellent explanation. I was doing the same as OP and completely forgot about circular motion.

Another helpful tip for this question- a = (v2)/r or r = (v2)/a

Don't forget to keep your units uniform.