Question

A stone is thrown from ground level at 70 m/s. Its speed when it reaches its highest point is 55 m/s. Find the angle, above the horizontal, of the stone’s initial velocity. Answer in units of ◦ .

Answer 1

at the highest point, vertical velocity Vy = 0. So

if the question says that its speed when it reaches its highest point is 55 m/s, that means Vx = 55 m/s.

here, Vx does not change in projectile motion because there is no force affecting it. On the other hand, gravity affect Vy, that's why Vy changed over time. So we can conclude that Vx = 55 initially and always.

Since we now have Vx and the initial velocity(Vi) 70m/s

we know that

cos(theta) = Vx/Vi

so theta = Arccos(Vx/Vi)

theta = Arccos(55/70) = 38.2 degree