Solving Projectile Motion: A Building 70.0 m High

[ScienceGirl90]

Homework Statement

A ball is thrown horizontally from the top of a building 70.0 m high. The ball strikes the ground 90.0 m from the base of the building (Neglecting air resistance). How fast and at what angle (relative to horizontal) does the ball strike the ground?

Homework Equations

I'm not sure!

The Attempt at a Solution

My teacher briefly explained projectile motion but I don't understand it at all! Please help me.

 

[JaredJames]

You should use the SUVAT equations.

First, calculate the time taking to fall the 70m from the top of the building using s= u*t + 0.5*a*(t^2).

Where s = distance, u = initial speed, v = final speed, a = acceleration and t = time.

(Can you see why they're called SUVAT equations?)

You know u = 0m/s, a = 9.8m/s^2, t = ? and s = 70m. Plug them in and rearrange to get the fall time t.

Then, using v = u + at with the values from above, you can get vertical impact speed v.

Because fall time vertically = flight time horizontally, you now know how long it took to travel the 90m horizontally. Because you are neglecting air resistance, for the horizontal flight u = v and a = 0.

So using s = u*t + 0.5*a*(t^2), where a = 0, s = 90 and t = answer from first section, you can plug in your values and get u = v.

You now have both vertical impact speed and horizontal impact speed. Combine the two using basic trigonometry and you get your impact speed and angle.

Jared