Question

A cannon ball is fired so that it hits the wall at the top of a 10m tower.If the energy needed to destroy the wall is 49000 F and the mass of the cannon ball is 10 k, the minimum initial velocity of the cannon ball required to break the tower wall is

A 100 m/s
b 200 m/s
c 300 m/s
d 400 m/s

my way:
WD against friction + PEgain =KEloss
49000 + 10*10*10=1/2*(10)(u)^2 - 0

but is that way right?


who can told me the right one?

and if mine is correct,what is the principle behind?

why energy needed is equal to wd against friction?

Answer 1

You have done it well, the principle is conservation of energy: the energy at the top of the wall (PE+KE) is equal the the original energy.

Answer 2

but if i do that,then does it mean that i only supply 49000J to only overcome the work done agaist friction?

Answer 3

the initial KE goes into raising the cannonon ball to the height of 10 m, and then giving it energy to break the wall. Yes, at the top it has KE to break the wall of 49000J.