# Rate of flow and velocity

1. Aug 9, 2005

### hhh79bigo

Hi

I hope some one will be able to help me.

I have a problem which requires me to find the force exerted on a truck which pumps water from its tank and through a hose at a rate of 10kg/s. The speed that the water exits the hose is 20m/s.

This is all the info i am given,

Regards

hhh79bigo

2. Aug 9, 2005

### FredGarvin

Based on the little bit you are given, I guess you'll have to use Newton's second law and momentum and use:

$$F = \dot{m} V$$ where

$$\dot{m}$$ = mass flow rate in kg/sec

$$V$$ = velocity normal to cross sectional area at exit in m/s

There are a lot of assumptions going on here.

3. Aug 9, 2005

### hhh79bigo

Thanks alot

What are these assumptions,

4. Aug 9, 2005

### HallsofIvy

Staff Emeritus
The problem appears to be saying that, in one second, 10 kg of water is accelerated from 0 m/s (sitting in the tank) to 20 m/s: from 0 to 20 m/s in one second is 20 m/s2 so F= ma= (10 kg)(20 m/s2)= 200 Newtons. Strictly speaking that that is the force the trucks pump applies to the water but, of course, it is the same as the force the water applies to the truck.