How fast does a diver hit the water when jumping off a 30m platform?

[Medgirl314]

Homework Statement

A diver steps off a platform 30m above the water. With what speed does he hit the water?

Homework Equations

v=intial v+at

The Attempt at a Solution

I reached an answer of 2.47 seconds, but I can't see how I got there anymore! Is this answer correct?

Thank you very much!

 

[tiny-tim]

Hi Medgirl314!

You need to learn the standard constant acceleration equations.

In this case, you know v_i and a and s, and you want v_f …

so the equation to use is … ? ​

 

[Medgirl314]

Thanks for the reply!

I think I used several equations, but they faded a bit. Blasted erasable pens. Since it's a fairly new concept, my physics teacher doesn't want me to have them memorized yet. I think I used y=y0+v0+1/2 at^2 and t^2=2y/a .

The answer seems plausible. Did I get it right?

Thanks again, and thanks for the link!

 

[tiny-tim]

(try using the X² and X₂ buttons just

Hi Medgirl314!

(try using the X² and X₂ buttons just above the Reply box )

… my physics teacher doesn't want me to have them memorized yet.

rubbish!

there's only 3 basic equations, memorise them and everything will be much simpler!

I think I used y=y0+v0+1/2 at^2 and t^2=2y/a .

you mean y=y₀+v₀t + 1/2 at²

(and the second one is just another version of the first one, with zero constants)

The answer seems plausible. Did I get it right?

(the question asked for a speed, and your answer is a time )

on this forum, you need to show your calculations, not just the answer

 

[Medgirl314]

Oh! I'm sorry! My answer was actually 24.2 m/s. I gave you a different part that I used to find that answer. I think I used t=√2y/a, and got t=√2.3 m/9.8 m/s^2, giving a time of 2.47 seconds. Then I used v=vo+at , plugging in my numbers to get v=0+9.8 m/s^2(2.42 s), and then got my answer of v=24.2 m/s. Is this better? Thank you!

 

[tiny-tim]

HI Medgirl314!

(never use . for multiplication … use x or * or brackets )

… I used t=√2y/a, and got t=√2.3 m/9.8 m/s^2, giving a time of 2.47 seconds. Then I used v=vo+at , plugging in my numbers to get v=0+9.8 m/s^2(2.42 s), and then got my answer of v=24.2 m/s. Is this better? Thank you!

yes, that's correct (you meant 30 not 3)

but quicker would have been to use v_f² = v_i² + 2as ​

 

[Medgirl314]

Thank you! I must have used a dot for multiplication on my paper and didn't notice that it was 30, not 3.0!