Calculating Kinetic Energy of a Ball at Different Points

[Alu]

Homework Statement

A 0.04-kg ball is thrown from the top of a 30-m tall building (point A) at an unknown angle above the horizontal. As shown in the figure, the ball attains a maximum height of 10 m above the top of the building before striking the ground at point B. If air resistance is negligible, what is the value of the kinetic energy of the ball at B minus the kinetic energy of the ball
at A , that is calculate (K(B) – K(A))?

Homework Equations

The Attempt at a Solution

I think K(B)- K(A)=K(B)=mgh is this correct?

 

[Feldoh]

K(b)-K(a) = mgh however that doesn't tell you what K(a) is. You assumed K(a) = 0 but how can it be zero if it is moving upward initially?

 

[Alu]

yeah right, I assumed that K(a)=0 which is not. But at least we can find K(b) which is equals to mgh where h is 30+10 m

 

[Feldoh]

Err actually if you're just finding the difference in kinetic energy, it's just the change in potential energy so you are correct and you don't need to know K(a). I mean K(a) =/= 0 but it doesn't matter anyways.

 

[Alu]

yeah exactly, which leads to: K(b)-K(a)=mgh where h is only 30m(the difference in potential energy). which is equals to 0.04*10*30=12J