Can force be expressed as mass flow rate times velocity

[waverider]

I know that F=ma which give the units of kg.m/s/s (in SI units) but can force also be expressed as mass flow rate times velocity which also has the same units? Example: water coming out a hose or gas coming out a spray can?

 

[boneh3ad]

I know that F=ma which give the units of kg.m/s/s (in SI units) but can force also be expressed as mass flow rate times velocity which also has the same units? Example: water coming out a hose or gas coming out a spray can?

Newton's law is actually that force is the rate of change of momentum with time,
F = \dfrac{d(mv)}{dt}.
Therefore,
F = m\dfrac{dv}{dt} + v\dfrac{dm}{dt}.
So, if you have a constant velocity, then yes, force can be simply the rate of change of mass times velocity.

 

[waverider]

Thanks. So for water flowing through a pipe (kg/s) at constant velocity, what does the force calculated by m(dot).v represent? is it the frictional force on the pipe?

 

[Nidum]

Mass flow is simply Kg/sec . Actual velocity depends on density and pipe size .

No dynamic force is generated until something happens to accelerate or de-accelerate the flow .

This can be a change in cross sectional area of pipe or a change of flow direction as in a turbine .

Usually better to think of pressures rather than forces when dealing with fluids .

 

[waverider]

Cool, thanks

 

[boneh3ad]

Thanks. So for water flowing through a pipe (kg/s) at constant velocity, what does the force calculated by m(dot).v represent? is it the frictional force on the pipe?

It would be the force experienced by, say a plate that goes sprayed by that water stream if all of the water from that stream was then deflected sideways when it hits the plate, for example.

 

[waverider]

It would be the force experienced by, say a plate that goes sprayed by that water stream if all of the water from that stream was then deflected sideways when it hits the plate, for example.

Thanks, a real life example makes it easier to visualize
I wounder if F=m(dot).v could be applied to a spray can that is spraying gas at the rate of m(dot) and gas velocity v, would the nozzle experience a force, (disregarding the air pressure and friction) ?

 

[boneh3ad]

Thanks, a real life example makes it easier to visualize
I wounder if F=m(dot).v could be applied to a spray can that is spraying gas at the rate of m(dot) and gas velocity v, would the nozzle experience a force, (disregarding the air pressure and friction) ?

See: thrust