Calculating Kinetic Energy of a Ball at Different Points

AI Thread Summary
The discussion focuses on calculating the difference in kinetic energy of a ball at two points, A and B, during its trajectory from a 30-meter tall building. It is established that the kinetic energy at point B minus the kinetic energy at point A can be determined using the change in potential energy, as air resistance is negligible. The potential energy change is calculated using the formula mgh, where the height difference is 30 meters. The final calculation yields a difference of 12 Joules. The conclusion emphasizes that while K(A) is not zero, it is unnecessary for determining the energy difference.
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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?
 
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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?
 
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
 
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.
 
Last edited:
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
 
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