A ball is thrown horizontally from the roof of a building 52 m tall and lands 50 m from the base. What was the ball's initial speed?

Question

A ball is thrown horizontally from the roof of a building 52 m tall and lands 50 m from the base. What was the ball's initial speed?

Answer 1

Horizontal motion is constant velocity_, so get from v=(x/t)=50m/t

But time in air is determined by vertical constant acceleration due to gravity.  Initial vertical velocity component v_y0= 0,

since ball was thrown horizontally.

_{Get t from solving} y=v₀t+(1/2)at²  for t=sqr[2y/a]=3.3s

Then v_0x=(x/t)=50/3.3=15.2m/s

 

 

Answer 2

Sounds like a projectile motion question. To answer these, you need to set up a system of two kinematic equations -- one for the horizontal direction and one for the vertical direction. One of these two equations will contain the unknown quantity you're looking for and a second unknown quantity; you can use the other equation to solve to this second unknown quantity.

 

I invite you to reread the problem and think critically about which of the following quantities you know: Δx, Δy, v_ix, v_iy, v_fx, v_fy, a_x, a_y, Δt. By my count, the problem gives you three of them, and you can figure out the a_x and a_y because they're the same for every projectile motion problem. From there, I recommend seeing if you can use one of the kinematic equations to find Δt; once you know Δt, you should be able to find the answer.