Unit 305 Session 6 Flow, force and pressure

​Pressure, force and flow of water 

​The SI units of Flow, force and pressure

​However, in plumbing systems, flow rate is usually measured in litres per second (l/s) or (l/m)

Poll

Which of these do you think describes Flow rate?

• Flow rate is the volume of fluid that passes through a given point or surface per unit of time, typically measured in litres per second (l/s) or litres per minute (l/m).

• Flow rate is the pressure exerted by a fluid in a pipe or container, usually measured in Pascals (Pa).

• Flow rate is the total mass of fluid that flows through a system, measured in kilograms (kg) per unit of time, regardless of the fluid's density.

​In plumbing, flow rate is defined as an amount of fluid that flows through a pipe or tube over a given time.

It is usually measured in litres per minute (l/m)
or litres per second (l/s).

​Plumbing systems in the UK must comply with specific building regulations and standards, such as the Water Supply (Water Fittings) Regulations 1999.

These regulations set maximum and minimum flow rates for different types of water fittings (e.g., taps, showers, toilets) to prevent waste and ensure water is used efficiently.

​Force and Pressure

Open Ended

Describe what you think the difference is between force and pressure

​Force is an influence on an object at rest that, acting alone, will cause the motion of the object to change.

If the object at rest is subjected to a force, it will start to move. For example, consider water in a pipe connected to a cistern at one end and a tap at the other.

When the tap is closed, the water is not moving and so is said to be at rest.

When the tap is opened, the force of gravity will move the water out of the tap, causing water to flow.

​The unit of force is the newton.

It is the force required to accelerate a mass of 1 kg at 1 metre per second, every second.

On Earth, that force of acceleration (known as gravitational pull) is 9.81 metres per second per second, or 9.81 m/s2.

Therefore, if we multiply the mass of an object (in kg) by 9.81, the result is measured in newtons.

​Consider the cistern in the picture.

If it contained a mass of water equal to 40 kg, then by multiplying the mass by the force of gravity, the force of the cistern acting downwards can be calculated:

40 × 9.81 = 392.4 N

​40kg

​Force
⋁

​392.4 newtons of force
acting on the tap

​Static pressure of water (head)

The unit of pressure is the pascal.

The pressure exerted by water is due to its mass and is determined by the height of the column of water.

Pressure is defined as force per unit area.

For an object sitting on a surface, the force pressing on the surface is the weight of the object measured in newtons per square metre (N/m2).
However, in different orientations it might have a different area in contact with the surface and will therefore exert a different pressure.

​They both exert the same force on the floor, however as cistern 1 has a smaller area on its base it will exert more pressure.

​Pressure = Force
Area

​Pressure is expressed in pascals
p

​For example, if a cistern measuring 1 m long × 0.5 m wide × 0.7 high was placed in a roof space, then what pressure would it exert if:

​1m

​.5m

​.7m

​● it was placed on its bottom
● it was placed on its side
● it was placed on its end?

​For example, if a cistern measuring 1 m long × 0.5 m wide × 0.7 high was placed in a roof space, then what pressure would it exert if:

​1m

​0.5m

​0.7m

​● it was placed on its bottom

​We first need to calculate the volume,
V = length x width x height
= 1 x 0.5 x 0.7 = 0.35m³
To calculate capacity (How much water is in the cistern) we multiply by 1000.
0.35 x 1000 = 350 litres

​For example, if a cistern measuring 1 m long × 0.5 m wide × 0.7 high was placed in a roof space, then what pressure would it exert if:

​1m

​.5m

​.7m

​● it was placed on its bottom

​So 350 litres = 350kg
We now need to calculate the area of the base,
1m x 0.5m = 0.5m

​For example, if a cistern measuring 1 m long × 0.5 m wide × 0.7 high was placed in a roof space, then what pressure would it exert if:

​1m

​.5m

​.7m

​● it was placed on its bottom

Now we have the weight we can calculate the force the cistern is exerting,
Force = weight x gravity (9.81 n/m²)

350 x 9.81 = 3433.5 n/m²

​For example, if a cistern measuring 1 m long × 0.5 m wide × 0.7 high was placed in a roof space, then what pressure would it exert if:

​1m

​.5m

​.7m

​● it was placed on its bottom

​So pressure = force ÷ area,
3433.5 x 0.5 = 6867 Pascals

Fill in the Blanks

Fill in the Blanks

Fill in the Blanks

​Atmospheric pressure

Atmospheric pressure is the amount of force or pressure exerted by the atmosphere on the Earth and the objects located on it.

The more pressure there is, the stronger that force will be; at sea level, the atmospheric pressure is 101.325 kPa.

This is known as 1 atmosphere (atm).


Atmospheric pressure decreases with height.

​Siphonic action 

Siphonic action happens due to atmospheric pressure; this is how many WCs flush. 

Siphonic action happens due to atmospheric pressure. 

Both cisterns have atmospheric pressure acting on them. 

The lower beaker has greater pressure because it is lower. 

When suction is applied at point C, water will flow upwards from beaker A, over the weir and down to beaker B. 

​Flow rate  

Open Ended

What do you think flow rate is?

​Flow rate  

The amount of fluid that flows through a pipe at a given time. 

Usually measured in litres per minute l/m or litres per second l/s


We can measure flow rate with a wier cup

​Flow rate  

The amount of fluid that flows through a pipe at a given time. 

Usually measured in litres per minute l/m or litres per second l/s

Flow rate can be affected by many factors: 

Open Ended

What do you think can have an effect on the flow rate of water in a plumbing system?

​Flow rate  

The amount of fluid that flows through a pipe at a given time. 

Usually measured in litres per minute l/m or litres per second l/s

Flow rate can be affected by many factors: 

• Changes in direction 

• Pipe size 

• Pressure 

• Length of pipe 

• Frictional resistance 

• Constrictions. 

Multiple Choice

What is the most common unit of flow rate in plumbing systems?

• Metres per second (m/s)

• Litres per second (l/s)

• Cubic metres per second (m³/s)

• Kilopascals (kPa)

Multiple Choice

What is force defined as in physics?

• The amount of water flowing in a pipe

• The mass of an object at rest

• An influence that causes motion to change in an object

• The pressure exerted on a surface

Multiple Choice

What is the unit of force?

• Joules (J)

• Newtons (N)

• Pascals (Pa)

• Watts (W)

Multiple Choice

Pressure is defined as:

• Mass per unit area

• Force per unit area

• Flow rate per unit time

• Force multiplied by area

Multiple Choice

If a cistern measuring 1 m long × 0.5 m wide × 0.7 m high is placed on its bottom, how is the pressure exerted calculated?

• Weight of the cistern divided by 1 m²

• Weight of the cistern divided by 0.5 m²

• Weight of the cistern divided by the area of its bottom (1 m × 0.5 m)

• Weight of the cistern multiplied by its height

Multiple Choice

The pressure exerted by a water column is determined by:

• The diameter of the pipe

• The height of the water column

• The temperature of the water

• The flow rate of the water

Multiple Choice

Dynamic pressure refers to the pressure of:

• Still water

• Water in motion

• Water at rest

• Atmospheric pressure

Multiple Choice

How do changes in the direction of a pipe affect flow rate?

• They increase flow rate

• They have no effect

• They decrease flow rate

• They cause a sudden burst in flow rate

Multiple Choice

What happens to the flow rate when the length of the pipe increases?

• It increases

• It decreases

• It stays the same

• It fluctuates

Multiple Choice

Which of the following would cause the most resistance to flow in a plumbing system?

• A wide pipe

• A short pipe

• A smooth internal bore

• A tap or valve