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Musashiaharon
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A while ago, pinkybear posted the following:
A fire hose for use in urban areas must be able to shoot a stream of water to a maximum height of 35 m. The water leaves the hose at ground level in a circular stream 3.5 cm in diameter.
What minimum power is required to create such a stream of water? Every cubic meter of water has a mass of 1.00*10^3 kg.
P=W/t
W=Fd
V(of cylinder)=pi*r^2*h
r=.0175m
v=pi*(.0175)^2*35
=.033674 m^3 then, mass= 33.67kg
w=(33.67)*g*35=11550 N
but then I'm stuck because I don't know what t is...
I tried v=v(0)+at, from which I got
P=m*a^2*D/v(0)
but now, I don't know how to get v(0)...
I was not satisfied with the sole response given, since it involved unnecessary approximation (basically, run through the calculations for a small, but finite amount of water). Here is my calculus-based approach:
It is useful to remember the following:
vf2 - v02 = 2a*Δy
We can calculate the flow rate, which is the change in volume (capital V) over time, or dV/dt:
dV/dt = dy/dt * A
Then, using the density of water, we can convert this to the change in mass over time, dm/dt.
We noted above that power is:
P=1/2*d/dt(mv2).
Since v is constant:
P = 1/2v2 * dm/dt
This is enough to answer the question. But we could go further and calculate the force of the water pushing back on the hose. We'll just use a couple more equations:
F=d/dt (mv) = v * dm/dt
Now we have the force, too!
Pressure, then, is just dividing the force by the sectional area of the nozzle.
Homework Statement
A fire hose for use in urban areas must be able to shoot a stream of water to a maximum height of 35 m. The water leaves the hose at ground level in a circular stream 3.5 cm in diameter.
What minimum power is required to create such a stream of water? Every cubic meter of water has a mass of 1.00*10^3 kg.
Homework Equations
P=W/t
W=Fd
V(of cylinder)=pi*r^2*h
The Attempt at a Solution
r=.0175m
v=pi*(.0175)^2*35
=.033674 m^3 then, mass= 33.67kg
w=(33.67)*g*35=11550 N
but then I'm stuck because I don't know what t is...
I tried v=v(0)+at, from which I got
P=m*a^2*D/v(0)
but now, I don't know how to get v(0)...
I was not satisfied with the sole response given, since it involved unnecessary approximation (basically, run through the calculations for a small, but finite amount of water). Here is my calculus-based approach:
It is useful to remember the following:
- Velocity: v=dy/dt
- Kinetic energy: K=1/2mv2
- Power: P=dK/dt=1/2*d/dt(mv2)
vf2 - v02 = 2a*Δy
We can calculate the flow rate, which is the change in volume (capital V) over time, or dV/dt:
dV/dt = dy/dt * A
Then, using the density of water, we can convert this to the change in mass over time, dm/dt.
We noted above that power is:
P=1/2*d/dt(mv2).
Since v is constant:
P = 1/2v2 * dm/dt
This is enough to answer the question. But we could go further and calculate the force of the water pushing back on the hose. We'll just use a couple more equations:
- Force (Newton's original definition): F=dp/dt
- Momentum: p=mv
F=d/dt (mv) = v * dm/dt
Now we have the force, too!
Pressure, then, is just dividing the force by the sectional area of the nozzle.
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