Question

You throw a ball downward from a window at a speed of 2.5 m/s. how fast will it be moving when it hits the sidewalk 2.1 m below?

Answer 1

List out all the variables that you do know; 

acceleration=-9.8 ms⁻¹ (this remains constant on Earth)
Final velocity=?
Displacement (s)= -2.1 m 
Initial Velocity(u)=2.5 ms⁻¹

v²=u²+2as 
v²=(2.5)²+2(-9.8)(-2.1)
v²=47.41 
v=√47.41 
v=6.88549 ≈ 6.9 ms⁻¹ 

Hope I helped :)