Solving quadratic equations using formula

[Hyzon]

1. An Acapulco diver dives into the sea from a height of 35 m. His height h metres t seconds after leaving the cliff is given by h=-4.9t^2+t+35. How long is it until he reaches the water? How long does it take him to fall from 35 m to 25 m?
2. (-b +/-(sqrt)b^2-4ac)/2b
3. I don't have any work done on this problem because I don't even know where to start. In previous questions I was merely using the quadratic formula to solve for the roots, but in this question I have to solve for a variable. The textbook doesn't mention this type of problem anywhere, any help on getting started would be wonderful.

 

[cepheid]

Welcome to PF Hyzon!

There is nothing new here. You're given the equation for the height, h, as a function of time, and you need to solve it in order to figure out the elapsed time at two different heights.

The first height is h = 0, since the question asks you to figure out how long it takes him to hit the water, and the height is measured from the water's surface. Plugging in h = 0 gives you:

-4.9t² + t + 35 = 0

The second height for which you have to solve the equation is h = 25 m, which gives you:

-4.9t² + t + 35 = 25

These are both quadratic equations. You can solve them both using the quadratic formula.

EDIT: Since these are quadratic equations, you will always get two answers for t. However, one of them will probably be unphysical (unless it just corresponds to reaching that height on the way up, before he reaches max height and starts falling again).

 

[Hyzon]

Ah, yeah I knew I was looking at it wrong. Thanks a lot for the help!