Free Fall/Constant Acceleration Problem

[cheerspens]

Homework Statement

An object is launched upwards with a velocity of 25 m/s. At what time (or times) will the object be located twenty meters above the ground?

Homework Equations

The Attempt at a Solution

I started off with the following variable list:
Xo=0 Vo=25
X=0 V=0
t=? a=-9.8
Which seems very off to me.
When trying to solve I got 2.55 seconds. But is there another time where it reaches 20m?
How do you solve this?
Thanks!

$$x=V_0 t-\frac{1}{2}gt^2$$


will help you. Remember it is going up, reaching maximum height and falling back down to the ground. So at two times it will be 20m up in the air.

Would my answers be 0.994sec and 1.557sec?

 

[Delphi51]

I got one of those answers but not the other!
Maybe you should show your work, in particular the quadratic equation so we can see what your a, b and c numbers are for entry into the quadratic formula.

 

[kerimek]

Yes, those would be the two times when the object is 20m above the ground. The first time is when the object is going up and the second time is when it is falling back down. This is because the acceleration due to gravity is constant throughout the motion, so the time it takes for the object to go up and come back down will be the same. Keep in mind that these times may be slightly off due to rounding errors or if you are using an approximation for the value of acceleration due to gravity.