# Homework Help: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 km?

1. Sep 25, 2010

A person goes skydiving. The plane is one kilometer up and they decide to fall a height of 1.2 kilometeres before opening their chutes. How fast are they going at that point?

Isn't it 9.8 m/s^2 because gravity has to be dragging them down at that rate?

Thank you!

2. Sep 25, 2010

### rock.freak667

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

They will be accelerating at that rate, but that will not be how fast they are going. To get how fast, you need to find the velocity at that point.

Conservation of energy will solve this quickly

3. Sep 25, 2010

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

So velocity is distance / time. I have the distance, but don't have the time. How am I supposed to solve for velocity then?

4. Sep 26, 2010

### Phrak

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Um. Why do they open their parachutes after falling 1.2km when they are only 1.0km high?

5. Sep 26, 2010

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

I meant 1/2. That's what the title says. I made a mistake in the description.

6. Sep 26, 2010

### Phrak

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

99 percent of terminal velocity, with prone attitude, for a typical body type takes about 550 metres. 90% of terminal velocity happens after falling about 375 metres.

7. Sep 26, 2010

### rock.freak667

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Consider the energy before and the energy after.

Conservation of energy: KE+PE = constant.

As for the why the parachute is opened, I am not sure about the height they have to open their chutes at, but if they open it without reaching terminal velocity, then bad things happen.

8. Sep 26, 2010

### Phrak

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Why are you ignoring air friction, rock.freak. Skydivers don't fall in a vacuum.

9. Sep 26, 2010

### Pengwuino

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

This is probably a few weeks into an intro physics course, chill out. You gotta walk before you can run.

10. Sep 26, 2010

### rock.freak667

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

For two reasons, one what Pengwuino said and two, had air resistance been included, more information would need to be given I believe.

11. Sep 26, 2010

### OThePestO

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

I believe it is like this:

Vi=0m/s a=9.8m/s^2 d=500m Vf=?

So we use this formula:

Once you plug in the numbers and calculate, it should come out to 99 m/s

However, I just started learning Physics yesterday, so do not take my word for it.

12. Sep 26, 2010

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Thanks OThePestO.

But really, this is a Physics forum. How is it that nobody knows the answer??? This is a basic physics question, it doesn't involve extremely complicated calculation yet.

13. Sep 26, 2010

### fizzynoob

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Its a simple kinematic problem: don't listen to the others about conservation of energy and drag, because you have not learned that yet.

y = (vi)t - (1/2)gt^2

since vi = 0; your jumping out of a plane you get

y = -(1/2)gt^2

set y to 500m (1/2 km) and solve for the time. Once you have time you can plug it in a kinematic equation to find the velocity.

Velocity is the derivative of position (assuming your class is calculus based)

v = dy/dt = -(gt)

plug n solve

14. Sep 26, 2010

### OThePestO

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Do you have an answer to reference with? If no one else provides a better solution, I would just go with mine, assuming this is for marks. What are the current formulas you are working with, as I assume you would be required to show how you came to use the formula I provided if you had not yet learned it.

15. Sep 26, 2010

### DaveC426913

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Also, there was a template you were given in which you were supposed to supply the question. You deleted it.

16. Sep 27, 2010

### Phrak

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Yeah, but I can't agree. If professors will say what they mean and mean what they say...

Or maybe you're wrong and this is all about terminal velocity for the bright students. Let me consult my Ouji board...

Last edited: Sep 27, 2010
17. Sep 27, 2010

### rock.freak667

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Not enough information is given in the question to utilize finding the terminal velocity.

18. Sep 27, 2010

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

The template was terrible. It was much easier to just write my question. You're not allowed to give answers on this forum? Is that a joke? If I've been working hard on a problem, and all of the help isn't helping, it would help much more to give me the answer so I would have something to work towards.

19. Sep 27, 2010

### rock.freak667

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Thus far, I see that three replies were given that would attempt to aid you, but since they all have differing things which depend on what you've learned, it is hard to say.

They were
Conservation of Energy
Using kinematics
Terminal velocity

Which one have you learned in your class?

Also, you didn't really post what you attempted, so we don't really know along what lines you tried which is why everyone has a differing input.

20. Sep 27, 2010

### fizzynoob

Re: A person goes skydiving from one kilometer up. How fast are they falling at 1/2 k

Look at my post, follow the steps and you will arrive at the correct answers.