- #1
flyingpig
- 2,579
- 1
Homework Statement
A student throws a set of keys vertically upward to her sorority sister, who is in a window 2.00 m above. The second student catches the keys 2.30 s later.
(a) What was the velocity of the keys just before they were caught?
I did y(t) = 2 - 4.9t² + v₀t
2 = 0 - 4.9(2.3)² + v₀(2.3)
v₀ = 12.1m/s
Now I have two formulas, v² = v₀² + 2ad and v = v₀ + at, but they give me different answers.
v² = v₀² + 2ad
v² = (12.1)² + 2(-9.8)(2)
v² = 107.21
v = 10.4m/s
v = v₀ + at
v = 12.1 - 9.8(2.3)
v = -10.4m/s
I understand the square root will give us a negative as well, but the question is, HOW DO WE KNOW which to take? I hadn't use the v = v₀ + at, I wouldn't know whether to take positive or negative.