# At what speed must I throw vertically an object.

1. Sep 18, 2015

### astrololo

1. The problem statement, all variables and given/known data
At what speed must I throw vertically an object so that it will lose 25 % of its speed during the first second.

2. Relevant equations
V final = v initial + a * t
3. The attempt at a solution

v final * 0.75 = v initial + -9.8 m/s^2 * 1 second

That's pretty much what I figured out only ...

2. Sep 18, 2015

### Bystander

"Only,..." what?

3. Sep 18, 2015

### astrololo

Is there a problem ?

4. Sep 18, 2015

### Bystander

You haven't answered the original question, what's the original vertical speed?

5. Sep 19, 2015

### astrololo

Well that's the thing I have no idea of how to obtain the answwer...

6. Sep 19, 2015

### Bystander

You're interested in the rate at which the object loses speed, and specifically asked for a 25% loss in one second. What's "vfinal" for 100% loss?

7. Sep 19, 2015

### astrololo

Well it's 0.

8. Sep 19, 2015

### Bystander

Substitute that value, and rearrange. What's "v(t=1s)" relative to "vinitial?"

9. Sep 19, 2015

### astrololo

Vi=9,8 m/s

10. Sep 19, 2015

### Bystander

What's "a?"

11. Sep 19, 2015

### astrololo

-9.8 m/s^2

12. Sep 19, 2015

### Bystander

9.8 is what percentage of 9.8?

13. Sep 19, 2015

### astrololo

100%

14. Sep 19, 2015

### Bystander

In one second, the speed changes by how much?

15. Sep 19, 2015

### astrololo

it goes from 9.8 to 0.

16. Sep 19, 2015

### Bystander

No. It changes by 9.8.

17. Sep 19, 2015

### astrololo

Yes, that's what I said. ok, it changes by 9.8.

18. Sep 19, 2015

### Staff: Mentor

Fix the left hand side of that equation. Then you should be able to solve for vinitial.

19. Sep 19, 2015

### astrololo

I put it that way so that the speed would be 75%... it's not correct?

20. Sep 19, 2015

### Staff: Mentor

No, it's not correct.

This is the formula you are trying to use:
Hint: express Vfinal in terms of Vinitial.