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Pressure and Pivots Pressure P = F / A Fluid pressure P = ρ g h The Manometer Pivots Moments.

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Presentation on theme: "Pressure and Pivots Pressure P = F / A Fluid pressure P = ρ g h The Manometer Pivots Moments."— Presentation transcript:

1 Pressure and Pivots Pressure P = F / A Fluid pressure P = ρ g h The Manometer Pivots Moments

2 Pressure Why do predators often have SHARP teeth and claws? Why does wearing skis stop you SINKING into the snow?

3 CIE IGCSE Pressure Relate pressure to force and area, using appropriate examples Recall and use the equation p = F/A Relate (without calculation) the pressure beneath a liquid surface to depth and to density, using appropriate examples Recall and use the equation p = hρg Describe the simple mercury barometer and its use in measuring atmospheric pressure Use and describe the use of a manometer

4 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 / m 2 Special unit the pascal Pa 1000 Pa = 1kPa kilopascal

5 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

6 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 pressure smaller greater

7 Pressure Formula The 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 (m 2 ). 1 Pa = 1 N/m² Example A force of 10 N acts over an area of 2 m 2. What pressure is created by the force? Pressure = Force ÷ Area Pressure = 10 ÷ 2 Pa Pressure = 5 Pa or 5 N/m 2

8 Pressure formula and triangle Pressure = Force / Area P = F / A Units N / m 2 special unit the pascal Pa

9 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 cm 2. What pressure is put upon the top of the nail by the hammer blow? Pressure = Force ÷ Area Pressure = 200 N ÷ 0.5 cm 2 Pressure = 400 N/cm 2

10 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 m 2. What pressure is created by this force? 3.A girl of weight 400 N has feet of area 100 cm 2. What pressure does she put on the ground? 4.A car of weight N has four wheels. Each wheel has an area of 80 cm 2. What pressure does the car put on the ground? Pressure measured in Pascals. 50 Pa. 4 N/cm N/cm 2. Force measured in Newtons. Area measured in metres squared.

11 Exercise 5: Pressure Calculations 2 – Pressure worksheet 1.A force of 300 N creates a pressure of 4 N/m 2. Over what area is the force acting? 2.A pressure of 200 Pa is created over an area of 5 m 2. What force acts to create this pressure? 3.A tractor has tyres of area 3 m 2. It has a weight of N. What pressure does the tractor put on the ground? 4.A truck has tyres of area 5 m 2. It puts a pressure of N/m 2 on the road. What is the weight of the truck? 75 m N Pa N.

12 Graph skills 3. Find the gradient

13 Graph skills

14 Large Force - Large surface area – lower pressure Large force - weight and large surface area of balloon tyres - means less pressure this works most of the time but sometimes the mud is just to bad. Meanwhile in Siberia, Russia Federation..\..\..\Videos\Balloon Tyres Have Greater Surface Area.flv

15 Fluid Pressure In FLUIDS (LIQUIDS and GASES) the pressure acts IN ALL DIRECTIONS. The pressure in SOLIDS always acts DOWNWARDS because of the PULL of GRAVITY. 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

16 Copy this Pressure in liquids acts in all directions it increases with depth it depends on the density of the liquid it is the same at any depth, it does not depend on the shape or width of the container Demo

17 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 = force / area = ρ g Ah = ρ g h A

18 Pressure in water equation Pressure = density x gravitational field strength x depth p= ρ g h Pa ρ = density kg/m 3 g = gravitational field strength g= 10 N/kg h= depth – height of water – deep m

19 Pressure in a liquid If the density of water is 1000kg/m 3, what is the pressure due to the water at the bottom of a swimming pool 2m deep? p= ρ g h ρ = 1000 g=10 h = 2

20 Pressure in Liquid This equation allows us to calculate the pressure at any point below the surface of a liquid due to the liquid alone. To determine the total 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 Pa = 1.0 x 10 5 Pa

21 Copy this Total Pressure in liquids = Water pressure + atmospheric pressure p= ρ g h Pa Pa Atmospheric pressure is Pa 1 x 10 5 Pa Atmospheric pressure varies slightly over the earth's surface and changes due to weather conditions

22 Diver question Example (a)Calculate the pressure due to water (density = 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. Atmospheric pressure = Pa = 1.0 x 10 5 Pa

23 Questions Please do question 1 – 2 Exam questions….. Remember about atmospheric pressure

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25 Mark scheme

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28 Air pressure demonstration

29 Copy this Air Pressure acts in all direction becomes less as you rise up through it, because there is less and less weight above. Kinetic Theory – in a gas, the molecules are constantly striking and bouncing off the wall of a container. The force of these impacts causes pressure

30 Copy this Air Pressure acts in all direction becomes less as you rise up through it, because there is less and less weight above. Kinetic Theory – in a gas, the molecules are constantly striking and bouncing off the wall of a container. The force of these impacts causes pressure

31 Air Pressure Air pressure is usually N/m2. The can contains air particles and is also surrounded by air particles. They are hitting the sides of the container on the inside and the outside. The vacuum pump removes the air particles from inside the can. The pressure caused by the particles on the outside collapses the can.

32 Manometer A mercury barometer can be made by filling a long glass tube with mercury, and then turning it upside down in a bowl of mercury. The mercury is held up by air pressure. As the air pressure varies every day (depending on the weather), the height of the mercury varies. The height of the mercury can be used to predict the weather. The height of the mercury is usually 760mm and this is called Standard Atmospheric pressure (written 760mm Hg). Show Barometer - aneroid barometer

33 Manometer Questions 1.Stick in book 2.What liquid does a barometer contain? 3.What are barometers used for? 4.What holds up the mercury? 5.What is Hg? 6.What is special about 760mm? Answers 2. Mercury 3. Measuring atmospheric pressure – helps to predict the weather 4. Atmospheric pressure 5 Chemical symbol for mercury 6 It is atmospheric pressure

34 Manometer A manometer is a u-tube containing a liquid, usually water. It can be used to measure the pressure of the gas supply. The higher the pressure the further the water is pushed around the u-tube and the greater height h. Demo manometer

35 Now do multiple choice questions on manometers and barometers

36 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. F d Turning effect

37 Manometer Question 2. What can the manometer be used to measure and how does it work? 3. What is a vacuum? Answer 2. It can be used to measure the pressure of the gas supply 3. A vacuum is a volume of space that is empty of particles

38 Moments Formula The 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

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42 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

43 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)

44 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

45 Principle of Moments If 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

46 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


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