# Find the time of its fall

1. Feb 2, 2007

### whaler

I have been having some problems with this question:

An object falls from a height h from rest. If it travels a fraction of the total height of 0.5665 in the last 1.00 s, find the time of its fall.

I started by saying h=(0.5)(g)(T^2)
Then I said that 1-.5665h=(0.5)(g)(T-1)^2

2. Feb 2, 2007

### Kurdt

Staff Emeritus
You have the correct equation. All you need to do is set T=1 and you know that the distance travelled is 0.5665h. Then you can solve for the height put it back in the original equation and solve for total time.

3. Feb 2, 2007

### whaler

so that it states

h=(.5)(9.81)(1)?????

4. Feb 2, 2007

### Kurdt

Staff Emeritus
you would have $$0.5665 h =\frac{1}{2}g$$

Now rearrange for h.

When you find h put it back into the original equation and solve for t.

5. Feb 2, 2007

### whaler

i got an h=8.66
Then i put that in and got a value
t=1.33s

That is not the right answer though.....where am i going wrong???

6. Feb 2, 2007

### Kurdt

Staff Emeritus
No worries its my fault. Give me a minute (doing many things at once) and I'll correct my mistake.

7. Feb 2, 2007

### whaler

know the feeling....not a problem

8. Feb 2, 2007

### Kurdt

Staff Emeritus
Ok so the equation that describes the motion is:

$$s=h-\frac{1}{2}gt^2$$

Now the question tells you that in the final second the particle falls $s=0.5665h$ therefore,

$$0.5665h = h-v_t-\frac{1}{2}g$$

where vt is the velocity at t-1 seconds. You can work out this velocity and should get a quadratic in h.

9. Feb 2, 2007

### whaler

so how would I find the vt though?
would i solve the first equation for h and then plug that into the second?

10. Feb 2, 2007

### Kurdt

Staff Emeritus
What about other kinematic equations? Do you know any more?

11. Feb 2, 2007

### whaler

my prof does not teach? have not be given any others yet

12. Feb 2, 2007

### Kurdt

Staff Emeritus
13. Feb 2, 2007

### whaler

Thank You For All The Help!!!!!