# Help! I need somebody

1. Oct 8, 2006

### Ahmedstein

Ok guys, I really need your help on this one. A free faling object from an unknown height. How can I get an equation to calculate the time that takes that object to reach the ground with only using "g" (free fall acceleration) and no other constant.

Pleease, if you love The Beatles, you'd help me. I tried everything and couldn't do it.

2. Oct 8, 2006

...not just anybody?

Well, write down the equation of displacement and the equation of velocity of the object. Use a condition in every equation, and you can retrieve the time. (Hint 1: what is the velocity when it hits the ground? ; Hint 2: what is the displacement at the same time? )

3. Oct 8, 2006

### arildno

Set up Newton's 2.law with "m" signifying the (unknown) mass.
See what you can do with that m!

4. Oct 8, 2006

### Ahmedstein

@radou:The displacement when it hits the ground is the height from which the object was dropped, which I cannot use. and the velocity at that point is soo unknown.

@arildno: Mass does not matter in free fall

So the conclusion is: initial velocity is ZERO, accerleration is (-g) and t=?? .. maan, that's the most difficult problem, I've ever dealt with

5. Oct 8, 2006

### arildno

Set up Newton's 2.law! In symbols please!

6. Oct 8, 2006

F=m a

7. Oct 8, 2006

### Ahmedstein

dude, I'm not supposed to add new variables that are not mentioned in the problem.

8. Oct 8, 2006

### arildno

Quite so!
And, what is F in this case?

9. Oct 8, 2006

### arildno

It doesn't say that explicitly, does it?

10. Oct 8, 2006

### Ahmedstein

the force of gravity?!

11. Oct 8, 2006

### arildno

Which in symbols can be written as??

12. Oct 8, 2006

### Ahmedstein

actually, it does. It is a computed homework system and if I use variables that are not in the problem it says "the answer doesn't not depend on that variable"

for example I tried that:
h is height or displacement
h= -0.5gt(squared)
t= square root of (2h/g)

I know it's a square root of a negative number, but I couldn't think of anything else. and it said "the correct answer doesn't depend on h" ... actually I am supposed to calculate the height. I just didn't tell you the whole problem.

13. Oct 8, 2006

### Ahmedstein

Ok, here it is. An object was dropped from the top of a cliff and the dropper heard the sound after (t) time. ok, and it's given in the question that speed of light is V, and free fall accerelation is a, and I should find the height of the cliff, so I figured out that I should use h= V * t, but I should subtract the time that took the object to reach the ground from the given (t), as the given t should equal (time that takes the object to hit the ground + time that takes sound to reach the top of the buiding to be heard) am I right?

14. Oct 8, 2006

Yes, sounds right.

15. Oct 8, 2006

### Ahmedstein

so it is h= v (t- ???) :(

16. Oct 8, 2006

Let's slow down. The equation for the y-displacement (which is the only direction of displacement, btw) is y(t) = y0 - 1/2*g*t^2, where y0 is the height of the cliff, you may call it h, if you want, doesn't really matter. So, you know the time, let's call it t1. So, we can write y(t1) = y0 - 1/2*g*t1^2. Now, what does y(t1) equal?

17. Oct 8, 2006

### Ahmedstein

shouldn't it be y(t) = V0 - 0.5 g t^2 , where V0 is the initial velocity, which in this case is equal to zero

18. Oct 8, 2006

### Ahmedstein

:uhh:

19. Oct 8, 2006