Can force be expressed as mass flow rate times velocity

In summary, Newton's law states that force is the rate of change of momentum with time. If you have a constant velocity, then force can be simply the rate of change of mass times velocity.
  • #1
waverider
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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?
 
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  • #2
waverider said:
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,
[tex]F = \dfrac{d(mv)}{dt}.[/tex]
Therefore,
[tex] F = m\dfrac{dv}{dt} + v\dfrac{dm}{dt}.[/tex]
So, if you have a constant velocity, then yes, force can be simply the rate of change of mass times velocity.
 
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  • #3
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?
 
  • #4
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 .
 
  • #5
Cool, thanks
 
  • #6
waverider said:
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.
 
  • #7
boneh3ad said:
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) ?
 
  • #8
waverider said:
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

:wink:
 

1. Can force be expressed as mass flow rate times velocity?

Yes, according to the basic equation of force (F=ma), force can be expressed as the product of mass flow rate (m) and velocity (v).

2. How is mass flow rate calculated?

Mass flow rate is calculated by dividing the mass of a substance flowing per unit of time (m) by the time it takes to flow (t). It is expressed in units of mass per unit time, such as kilograms per second or pounds per hour.

3. What is velocity in relation to force?

Velocity is a measure of the speed and direction of an object's motion. In the context of force, it is used to calculate the impact of an object's motion on the amount of force exerted. The faster an object moves, the greater the force it can exert.

4. Can force be expressed as mass times acceleration?

Yes, force can be expressed as the product of mass (m) and acceleration (a). This is known as the fundamental equation of force (F=ma) and is used to calculate the force required to accelerate a mass at a certain rate.

5. How is force related to mass and velocity?

Force is related to mass and velocity through the equation F=mv, where m represents mass and v represents velocity. This equation shows that force is directly proportional to both mass and velocity, meaning that an increase in either of these factors will result in a greater force.

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