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# Mechanical Comprehension Study Guide 2 for McGraw-Hill's ASVAB (page 4)

By McGraw-Hill Professional
Updated on Jun 26, 2011

### Water Pressure

On the ASVAB, water pressure questions often involve flow through pipes. Keep these principles in mind:

1. Total flow through a pipe system must be the same everywhere because water cannot be compressed.
2. When liquid speeds up, pressure falls.
3. When liquid slows down, pressure rises.

In the diagram, the same amount of water is flowing everywhere in the pipe system. For this to be true, water must be flowing faster at point B than at point A. That means that pressure is lower at point B.

Water in a container also exerts pressure on the bottom of the container. The deeper the water, the greater the pressure. To find the amount of water pressure in a tank, calculate the total weight of the water and divide by the area of the base of the tank.

Example

A tank with a base that measures 2 feet × 4 feet holds 1,600 pounds of water. What is the water pressure at the base of the tank?

2 ft × 4 ft = 8 ft2
1,600/8 = 200 lb/ft2

Remember too that 1 ft2 = 144 in2. To convert pressure between pounds per square inch and pounds per square foot, divide or multiply by 144.

### Filling and Emptying Tanks

The Mechanical Comprehension test often includes problems about filling and emptying tanks. Usually the filling and emptying take place at different rates, as in the following examples.

Example

Water is being piped into a tank at the rate of 2 gallons per second. At the same time, it is being piped out of the tank at the rate of 60 gallons per minute. How many gallons will be added in 5 minutes?

Convert the inflow rate so that you are working only with gallons per minute.

Subtract:

120 gal/min inflow – 60 gal/min outflow = 60 gal/min net gain
The net gain in 5 minutes is 5 – 60 = 300 gallons.

Example

A 100-gallon tank contains 10 gallons of water. Water is added through one pipe at the rate of 3 gallons per minute. It is drained away through another pipe at the rate of 2 gallons per minute. How long will it take to fill the tank?

Find the net gain of water per minute:
3 gal/min – 2 gal/min = 1 gal/min

It will take 100 – 10 = 90 gallons to fill the tank. At the rate of 1 gal/min, it will take 90 minutes to fill the tank.