Newton's 2nd law:
F = d(mv)/dt
and can be expressed as:
F = m dv/dt + v dm/dt
In a question like what is the force/lift on a holicopter when the mass is
"pushed" down at speed of v m s-1 and at a rate of m kg s-1, most texts have
used:
F = v dm/dt
Isn't the above equation for constant vel but changing mass?
My understanding is that the vel is accelerated by the rotor from a speed of
0 to v and is hence not constant.
If I use impulse = change in momentum
to find the answer, it is definitely more convincing,
since
F dt = final mom - initial mom
= mv -0
So F = mv/dt
where the mass per unit time is known.
I am confused, as both cases will still give the same answer. Can someone
help?