Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

[Phys-L] Re: Energy is primary and fundamental?



Since I have been rather fussy about what solutions I would accept to the
question of how do you find the speed of a falling ball after 5 seconds only
using energy, let me propose a solution that I think is acceptable.

mgh + 0.5 m v^2 = constant

Take the derivative of the above.

mg (dh/dt) + m v (dv/dt) = 0

v = - dh/dt

- mgv + mv (dv/dt) = 0

Therefore

dv/dt = g

And

v = gt

So v = 50 after 5 seconds.

M and g are just numbers. (I really don't understand what m means in this
case, or what sets the value of g). But at least there is no overt appeal to
kinematics. There is an implicit one by assuming g = 9.8 :-(

I see this derivation as a correct solution, but there is no way I would
present this to students as a way of understanding free fall motion.

Bob at PC