Vectors and Trigonometry Homework Help

[Lui4]

Homework Statement

A stone is thrown straight up with a speed of 15.0m/s
(a) How fast will it be moving when its altitude is 8.0m above the point from which it was thrown? How much time elapses while the stone is reaching that height?
(b) Is there one answer or are there two answers? Why?

Relevant Equations

D=vot+(1/2) (a) (t)^2

Hey, I am new to this community and I am in need of help with this physics problem. I have used the formula above and the answer I get is 1.43s. The correct two answers are 0.68s and 2.4s. For the Vf the answer is 8.3 m/s.

 

[haruspex]

Homework Statement:: A stone is thrown straight up with a speed of 15.0m/s
(a) How fast will it be moving when its altitude is 8.0m above the point from which it was thrown? How much time elapses while the stone is reaching that height?
(b) Is there one answer or are there two answers? Why?
Relevant Equations:: D=vot+(1/2) (a) (t)^2

Hey, I am new to this community and I am in need of help with this physics problem. I have used the formula above and the answer I get is 1.43s. The correct two answers are 0.68s and 2.4s. For the Vf the answer is 8.3 m/s.

If you would like someone to point out where you went wrong it would be helpful if you were to post your working.

 

[Lui4]

Hey, thanks for the quick reply. Hope this helps understand the problem better!

 

[haruspex]

You don't seem to know how to solve quadratic equations.

 

[Lui4]

Oh, okay.

 

[Delta2]

A guide on how to solve a quadratic equation Quadratic equation - Wikipedia

If you are bored to read all that (lol) here is the core of that article:
A quadratic equation $ax^2+bx+c=0$ has (in the most general case) two roots:
$$x_1=\frac{-b+\sqrt{b^2-4ac}}{2a}$$ and $$x_2=\frac{-b-\sqrt{b^2-4ac}}{2a}$$

 

[Lui4]

Thank you for your help, I’ll keep this in mind when looking at the question!