Doc Brown's Physics exam study revision notes on: explaining pressure situations like camel's feet, syringe
needle, skis, drawing pin, scissors where a large of small surface area of
contact pressure is very important.

**6.2 ****
Pressure created by standing objects, calculations, P = F/A
formula including where pressure is deliberately increased or decreased when applied to an object**

**Pressure is defined as force per unit area
and is calculated from the simple formula**

**pressure = force normal to the surface ÷ area
of that surface,**

**
P = F / A, F = P x A, A
= F / P**

**P**, pressure in pascals (**Pa**);
**F**, contact force in newtons (**N**); **A**, area on which
force acts in square metres (**m**^{2})

A force of **1 N**
acting on **1 m**^{2} creates a pressure of **1 Pa**

**
A variety of situations to increase or decrease pressure in various situations**

**Often by changing the
surface area of contact between an object and other material**

F = weight on applying the P = F/A equation

Camels have feet of large surface area, this reduces
the F/A ratio (reducing pressure), so their feet do not sink to
deeply into sand.

Similarly, skis or snow shoes have a relatively
large area to reduce the F/A ratio, hence decreasing the impact
pressure on snow, so the wearer does not sink to deeply into the
snow.

Scissors have a sharp edge to create high F/A ratio
to create a high pressure to cut cleanly through materials.

Drawing pins and hypodermic needles have very sharp
end points to create a very high F/A ratio, so the high 'impact'
pressure enables them to easily pass into materials when pushed.

**Similar arguments apply to** ...

Vehicles with large wide tyres or wide
caterpillar tracks to minimise sinking in soft ground.

Crampons are sharp to penetrate ice or snow to
get a good grip, but the rest of the broad surface area of the
boot prevents the wearer from sinking too deeply into the snow.

Snow shoes the size of tennis racquets stop you
sinking into snow by increasing the surface area and reducing
the pressure compact the snow - this also reduces the effort you
have to put walking across deep snow.

**
Pressures caused by standing objects - some simple calculations, if occasionally
painful !**

Any solid object standing on a solid
surface will, due to the force gravity, create a pressure on the surface due
to its weight.

This is a normal contact force,
balanced by the compressed atoms of the solid surface pushing back up.

(This complies with
Newton's 1st Law of Motion)

Ignoring the different
weights of people (the weight 'force'), you should realise from the formula why it
is better to be trodden on by a broad shoe sole than a stiletto heel !!!! (CLUE
!!! P = F / A, no exam pressure here !!!)

**In moving around the particles of a fluid collide with
each other and with any surface they are in contact with.**

Although the mass of an individual particle
is minute and each collision involves the transfer of an equally minute amount of kinetic
energy, collectively the** trillions of collisions cause a pressure** to be
exerted in both gases and liquids.

The combined effects of these particle
collisions produces **a net resultant force at right angles to the surface
of contact with an object or side of a container**.

e.g. the pressure of gases in a container
or the pressure from the atmosphere of air pressure around you.

It is the same anywhere in a liquid,
pressure is exerted against the side of a container wherever the liquid
is in contact and it is the same for an object immersed in a liquid.

**Remember that the force of the
pressure acts in all directions in a fluid.**

The maximum pressure exerted in a fluid is
considered to be due to the collective force of the particle collisions acting
at right angles (normal, 90^{o}, perpendicular) to the surface on which
the collisions take place i.e. any surface in contact with the fluid.

**For specific gas calculations see**
P-V-T pressure-volume-temperature gas
laws and calculations (in my GCSE level chemistry notes).

**
Examples of pressure calculations based on P = F/A**

**
Q1** If a weight of fluid of 200 N acts on a surface of 5 m^{2},
calculate the pressure created.

Worked out ANSWERS to questions

**
Q2** What force must be applied to a surface area of 0.0025 m^{2},
to create a pressure of 200,000 Pa?

Worked out ANSWERS to questions

Q3 In a
hydraulic lift system, what must the surface area of a piston be in cm^{2} if a pressure
of 300 kPa is used to give a desired upward force of 2000 N?

Worked out ANSWERS to questions

**Q4**
The average standard rectangular building brick has a mass of 3.10 kg and
dimensions of 225 mm x 112 mm x 75 mm.

The gravitational field constant g =
9.8 N/kg.

(a) Calculate the pressure the brick
creates when standing on its surface of smallest area.

(b) Calculate the pressure the brick
creates when standing on its surface of greatest area.

Worked out ANSWERS to questions

**
Q5** A stiletto heel has a surface area of approximately 2.5 cm x 2.0
cm.

Worked out ANSWERS to questions

**Q6**
A person is standing on both feet, with flat trainers, each of which has
an ground contact area of 0.025 m^{2}.

If the person weighs 800 N, **what pressure is created on the ground by each foot when the person is standing still**?

Worked out ANSWERS to questions

**Q7**
An elephant is standing on all of its feet, each of which has an ground
contact area of 0.08 m^{2}. If the elephant weighs 50000 N, **what pressure is created on the ground**?

Worked out ANSWERS to questions

**Q8**
A ski design team has to take into account the pressure created by the
skier on the surface of deep snow.

They need to the **calculate the effects of the variables** which are (i) area of one ski, (ii) weight of skier, (iii) pressure created by the skier on the snow and (iv) the pressure the snow can take without the skier sinking in too much!

If the maximum acceptable snow pressure is 5000 N/m^{2}, for a single ski surface area of 0.15 m^{2},
**what is the maximum weight the skier can be**?

Worked out ANSWERS to questions

**Q9
**A ski design team has to take into account the pressure created by
the skier on the surface of deep snow.

They need to the **calculate the effects of the variables** which are (i) area of one ski, (ii) weight of skier, (iii) pressure created by the skier on the snow and (iv) the pressure the snow can take without the skier sinking in too much!

If the maximum acceptable snow pressure is 4000 N/m^{2}, what is the
**minimum single ski surface area acceptable, for a skier of weight 800 N**?

Worked out ANSWERS to questions

**Q10
**A bag of sugar has a base of 6 cm x 10 cm. If it weighs 18 N, **what pressure does it create**
standing on a shelf?

Worked out ANSWERS to questions

**Q11
**A stiletto heal has a base area of 3 cm^{2}.

If the woman weighs 750 N, **what pressure does she create on the floor when standing on one heel**?

Worked out ANSWERS to questions

**Q12
**A waste skip a base of 2m x 4m. If it weighs 10000 N when full, **what pressure does it create**
when standing on the road?

Worked out ANSWERS to questions

**Q13** A brick has a base of 10 cm x 25 cm and weighs 30 N. **what pressure does a stack of ten bricks create**
simulating the pressure created by a low wall?

Worked out ANSWERS to questions

**See also Part 6.3 **

Pressure in a liquid - density, depth factors and
calculations

**For specific gas pressure calculations see**
P-V-T pressure-volume-temperature gas
laws and calculations (in my GCSE level chemistry notes).

Index physics Forces notes 6. Forces & pressure in
fluids, calculations

**
Keywords, phrases and learning objectives for forces involving **
**pressure situations**

Be able to explain in terms of forces the pressure created by standing objects.

Be able to solve problems and do calculations using
the formula **pressure = force/area** with the appropriate units.

Be able to describe and explain situations where pressure is deliberately increased or
decreased by changing surface area when applied to an object or
context e.g. camel's feet, snow shoes, syringe needle, skis, drawing pin
and scissors.

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Index physics Forces notes 6. Forces & pressure in fluids, calculations