Projectile Motion Equation: Calculating Height from Initial Velocity and Time

[KevinFan]

Homework Statement

A ball is tossed from an upper-story window of a building. The ball is given an initial velocity of 7.90m/s at an angle of 21.9deg below the horizontal. It strikes the ground 2.95s later.
Calculate the height from which the ball was thrown.

Homework Equations

The Attempt at a Solution

y=y0+vy0t- 1/2gt^2
vy0=sin21.9 deg*7.9m/s=2.9466m/s
0=y0+2.9466m/s(2.95s)-(1/2)(9.8m/s^2)(2.95s)^2
y0=33.95m
could anybody tell me where have done wrong?

 

[Bystander]

Check your signs.

 

[haruspex]

Homework Statement

A ball is tossed from an upper-story window of a building. The ball is given an initial velocity of 7.90m/s at an angle of 21.9deg below the horizontal. It strikes the ground 2.95s later.
Calculate the height from which the ball was thrown.

Homework Equations

The Attempt at a Solution

y=y0+vy0t- 1/2gt^2
vy0=sin21.9 deg*7.9m/s=2.9466m/s
0=y0+2.9466m/s(2.95s)-(1/2)(9.8m/s^2)(2.95s)^2
y0=33.95m
could anybody tell me where have done wrong?

You need to be consistent about how you assign the positive directions. If you want positive to be up, what is the sign on y₀? If you want positive to be down, what is the sign on the acceleration?

 

[KevinFan]

Check your signs.

Thank you:)