# Homework Help: Finding the height of a building that a projectile is thrown from using kinematics

1. Jun 15, 2011

### garcia1

1. The problem statement, all variables and given/known data
A 0.21 kg rock is projected from the edge of
the top of a building with an initial velocity of
7.82 m/s at an angle 56 above the horizontal.
Due to gravity, the rock strikes the ground at
a horizontal distance of 10.5 m from the base
of the building.

How tall is the building? Assume the
ground is level and that the side of the build-
ing is vertical. The acceleration of gravity is
9.8 m/s2 .
Answer in units of m.

2. Relevant equations

Kinematics equations. Pythagorean Theorem: A +B = C

3. The attempt at a solution

i tried to think about what I needed, and what equation would help me find it. In this case, the height h, I chose the following equation:

H = Vy - VoY / 2A from the initial kinematics equation of Vy = Voy + 2AH

From here, however, I am unsure how to determine Vy, which is the only variable I would need.

2. Jun 15, 2011

### garcia1

Re: Finding the height of a building that a projectile is thrown from using kinematic

All those variables in the exponent section should have exponents of 2 themselves.

3. Jun 15, 2011

### iRaid

Re: Finding the height of a building that a projectile is thrown from using kinematic

Vy = $vsin \Theta$

4. Jun 15, 2011

### Vim

Re: Finding the height of a building that a projectile is thrown from using kinematic

You'll have to split your initial velocity into x and y components. (Use the Pythagorean theorum, your initial velocity will be the hypotenuse.)

Once you've established what the initial y-velocity is, use your kinematics equations to find the final y-velocity. ( vf = vi + a*t )

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