# Projectile motion equations

## 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.

## 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???

## Answers and Replies

Bystander
Science Advisor
Homework Helper
Gold Member
Check your signs.

• KevinFan
haruspex
Science Advisor
Homework Helper
Gold Member
2020 Award

## 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.

## 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:)