Pressure and Pivots

Pressure P = F / A

Fluid pressure

P = ρ g h

The Manometer

Pivots

Moments

Pressure

Why do predators often have SHARP teeth and claws?

Why does wearing skis stop you SINKING into the snow?

Pressure – Stick notes in your book

• What pressure is the force per unit area exerted by an object on a surface with which it is in contact with.

• How to calculate it

• Pressure = Force / Area

• P = F / A

• Units N / m2

• Special unit the pascal Pa

• 1000 Pa = 1kPa kilopascal

Exercise 1: words to use surface area , weight , pressure, sink

1. Why do tractors have such large wheels?

2. Why do camels have such large feet?

The large wheels have a large surface area in contact with the ground. Tractors have a large weight (force) and often have to work on soft ground like mud and soil. The large area of the tyres means that they put less pressure on the ground and they are therefore less likely to sink.

Camels are often found in deserts. The camels have a large weight (force) The large surface area of their feet on the ground means that they put less pressure on the sand and are less likely to sink into the sand.

Camels have large feet

Exercise 2: Pressure

Pressure tells us over what sort of area a _____ is acting. Pressure depends upon two factors: the force that is being applied and the ____ over which the force is acting.

The larger the force, the greater the ________. The ______ the force, the lower the pressure.

The smaller the area, the _______ the pressure. The larger the area, the smaller the pressure.

greater area pressure force smaller

force

area

pressuresmaller

greater

Pressure FormulaThe pressure formula allows you to calculate the pressure, force or area for a problem given two of the factors.

Pressure = Force

Area

Pressure measured in Pascals (Pa).

Force measured in Newtons (N).

Area measured in metres squared (m2).

1 Pa = 1 N/m²

Example

A force of 10 N acts over an area of 2 m2. What pressure is created by the force?

Pressure = Force ÷ Area

Pressure = 10 ÷ 2 Pa

Pressure = 5 Pa or 5

N/m2

Pressure formula and triangle• Pressure = Force / Area

• P = F / A • Units N / m2 special unit the pascal Pa

Exercise 3: Hammer and Nail

A hammer is used to drive a nail into a wooden floor. The hammer is brought down with a force of 200 N. The area of the top of the nail is 0.5 cm2. What pressure is put upon the top of the nail by the hammer blow?

Pressure = Force ÷ Area

Pressure = 200 N ÷ 0.5 cm2

Pressure = 400 N/cm2

Exercise 4: Pressure Calculations 1

1. What are the units of pressure, force and area?

2. A force of 500 N acts over an area of 10 m2. What pressure is created by this force?

3. A girl of weight 400 N has feet of area 100 cm2. What pressure does she put on the ground?

4. A car of weight 6 400 N has four wheels. Each wheel has an area of 80 cm2. What pressure does the car put on the ground?

Pressure measured in Pascals.

50 Pa.

4 N/cm2.

20 N/cm2.

Force measured in Newtons.Area measured in metres squared.

Exercise 5: Pressure Calculations 2

1. A force of 300 N creates a pressure of 4 N/m2. Over what area is the force acting?

2. A pressure of 200 Pa is created over an area of 5 m2. What force acts to create this pressure?

3. A tractor has tyres of area 3 m2. It has a weight of 12 000 N. What pressure does the tractor put on the ground?

4. A truck has tyres of area 5 m2. It puts a pressure of 15 000 N/m2 on the road. What is the weight of the truck?

75 m2.

1 000 N.

4 000 Pa.

75 000 N.

Fluid Pressure

In FLUIDS (LIQUIDS and GASES) the pressure acts IN ALL DIRECTIONS.

Also, in fluids, the PRESSURE INCREASES WITH DEPTH.

DAMS are WIDER AT THE BOTTOM

than at the top because pressure

increases with depth in water.

Pressure in fluids can be used to TRANSFER FORCES.

Less pressure

Higher pressure

Pressure in Liquid

To calculate the pressure at a depth h and a density ρ

Volume of liquid = base area x depth = Ah

Mass of liquid = density x volume = ρ Ah

Weight of liquid = mass x g = ρ g Ah g = 10 N/kg

Force on base = ρ g Ah

This force is acting on area A

Pressure in a liquid

Force on base = ρ g Ah

This force is acting on area A

Pressure = force / area = ρ g Ah = ρ g h

A

Pressure in a liquid If the density of water is 1000kg/m3,what is the

pressure due to the water at the bottom of a swimming pool 2m deep?

Pressure in LiquidThis equation allows us to calculate the pressure at

any point below the surface of a liquid due to the liquid alone.

To determine the actual pressure at this point, we must add on the pressure value at the water surface which is due to the atmosphere (so is called atmospheric pressure).

Atmospheric pressure varies slightly over the earth's surface and changes due to weather conditions –

The value used for calculations is

100 000 Pa = 1.0 x 105 Pa

Questions

Please do question 1 – 3

Exam questions…..

Remember about atmospheric pressure

Diver questionExample(a)Calculate the pressure due

to water (density = 1 000 kg m-3) at a point 20 m below the water surface.

(b) Calculate the actual pressure a diver would experience if she was at this

point.

Manometer and Barometer

Complete sheet

Rotating Forces

The DISTANCE (d) of the force from the pivot. The GREATER THE DISTANCE of the FORCE FROM THE PIVOT, the GREATER THE TURNING EFFECT.

Which factors govern the TURNING EFFECT of a force?

The SIZE of the FORCE (F). The LARGER THE FORCE, the GREATER THE TURNING EFFECT.

If an object is fixed at one end and a force is applied then the object will rotate around the fixed end. We call this fixed end the PIVOT.

We call the turning effect of a force the MOMENT of the force.

Fd

Turning effect

Moments FormulaThe moments formula allows you to calculate the moments, force or distance for a problem given two of the factors.

Moment = Force x Perpendicular distance of force from pivot

Moments measured in Newton metres (Nm).

Force measured in Newtons (N).

Distance measured in metres (m).

Example:

A force of 8 N acts 3 m away from a pivot. What moment is created by the force?

Moment = Force x distance

Moment = 8 x 3 Nm

Moment = 24 Nm

Exercise 6: Spanner and Nut

A spanner is used to undo a nut. The spanner has a force of 350 N applied to it at a distance of 0.2 m from the nut. What is the moment of the force?

Moment = Force x distance

Moment = 350 N x 0.2 m

Moment = 70 Nm

Exercise 7: Moment Questions

Moment = 900 Ncm

Moment = 37.5 Ncm

Moment = 125 Nm

15 cm

7.5 cm

0.25m

60 N

5 N

500 N

Calculate the moments for the diagrams below and give the direction of the moment.

a)

b)

c)

Exercise 8: Moment Questions 2

1. What is the formula for calculating moments?

2. What are the units for moments, force and distance?

3. A force of 40 N is applied 0.4 m from a pivot. What is the moment created by the force?

4. A moment of 15 Nm is created by a force of 60 N. What is the perpendicular distance of the force from the pivot?

5. A moment of 200 Ncm is created by a force acting 4 cm from a pivot. What is the size of the force?

Moment = Force x perpendicular distance of force from pivot

Moments – Newton metres, Force – Newtons, Distance - metres

16 Nm

0.25 m

50 N

Principle of MomentsIf the turning forces on a system are BALANCED then there must be the same CLOCKWISE MOMENTS as there are ANTICLOCKWISE MOMENTS. This is known as the PRINCIPLE OF MOMENTS.

F

F

CLOCKWISE MOMENTS = ANTICLOCKWISE MOMENTS

Exercise 9: Principle of Moments

1. What is the principle of moments?

2. A girl of weight 300 N is sat 3.0 m away from the centre of a see-saw. Where must a boy of weight 450 N sit on the opposite side to balance the see-saw?

3. A metre ruler is supported by a string in the centre at the 50 cm mark. A weight of 30 N is placed on the ruler at the 65 cm mark. Where should a weight of 10 N be located to balance the ruler?

4. What will happen to a system if the turning forces on it are not balanced?

Clockwise moments = anticlockwise moments

2.0 m away from the centre of the seesaw

At the 5 cm mark

The system will rotate