Pressure and Moments

Pressure and Moments Lesson 1: Force and Pressure Mr Kueres

Lesson Objectives • To know what a force does and about balanced and unbalanced forces – especially air resistance and friction • To know that pressure depends on the size of a force and the area over which it is exerted Mr Kueres

What is a Force? What Can a Force Do? A force can change the: • Size • Shape • Speed • And Direction of an object Mr Kueres

Balanced Forces If the forces on an object are balanced, they will have no effect on the motion of an object. I.e. The person (opposite) will remain still Or a car moving at 30mph with balanced forces acting on it, will continue to travel at 30mph Mr Kueres

Unbalanced Forces Ifthe forces on an object are unbalanced, they will have some effect on the object’s motion (either change the direction or cause the object to speed up or slow down. Will the car below speed up or slow down? Drag (Resistive forces) Thrust (driving force) Mr Kueres

Pressure Pressure tells us about the effect of a force compared with the area over which it acts. Mr Kueres

Things to Do • Answer the questions on the worksheet ‘Big Pressure, Small Pressure’ – 10 minutes • (H) Draw a table, as shown below, illustrating examples of large area/low pressure and small area/high pressure • (F) Draw examples of things that will exert a high pressure because of its small surface area and a low pressure because of its large surface area. Mr Kueres

Homework Complete Worksheet ‘But Why?’ DUE IN: Next lesson If you have any questions about the homework, come and see me before it is due in. Mr Kueres

Pressure and Moments Lesson 2: Calculating Pressure Mr Kueres

Lesson Objectives • To recall and use the formula for pressure • To know the units for pressure, force and area • To be able to estimate area reliably Mr Kueres

Pressure Pressure is dependent on f and a. If the f applied gets larger, the p will be h. If the a becomes larger, the pressure will be s. Mr Kueres

Pressure Pressure is dependent on force and area. If the force applied gets larger, the pressure will be higher. If the area becomes larger, the pressure will be smaller. Mr Kueres

Formula for Pressure Pressure = Force Area P = F A Force is measured in N , N. Area is measured in s m , m2. Pressure is measured in Newtons per square metre, N/m2 or P , Pa. Mr Kueres

Formula for Pressure Pressure = Force Area P = F A Force is measured in Newtons, N. Area is measured in squaremetres, m2. Pressure is measured in Newtons per square metre, N/m2 or Pascals, Pa. Mr Kueres

Finding the Pressure we Exert on the Floor APPARATUS • cm Squared Paper • Bathroom Scales METHOD • Draw around your foot on the piece of squared paper. • Count the squares and estimate the area of your foot (More than half a square counts as 1, less than half a square doesn’t count). • Use the bathroom scales to find your body WEIGHT. Remember: weight is a force and measured in NEWTONS) (1kg ~ 10N). • Calculate the pressure exerted by each foot, then both feet together. Mr Kueres

Calculation for One Foot Pressure exerted by one foot = Half body weight Area of foot = = N/cm2 Mr Kueres

Calculation for Both Feet Pressure exerted by one foot = Body weight 2 x Area of foot = = N/cm2 Mr Kueres

Evaluation Give one reason why you think your experiment was accurate and one reason why you think your experiment was not very reliable Mr Kueres

Things to Do (H) Answer the questions on the Pressure Practice Worksheet in your exercise books. (F) Complete the worksheet 9La/3: Pressure Points Mr Kueres

Pressure and Moments Lesson 3: Compression of Liquids and Gases Mr Kueres

Lesson Objectives • To review the formula for pressure • To know whether liquids and gases can be compressed • To know that liquids transmit pressure • To know that pressure increases with fluid depth Mr Kueres

Pressure Formula As the force increases, pressure increases. If the surface area increases, pressure will decrease. Pressure = Force Area P = F A Mr Kueres

Pressure = force/area. There is a direct relationship between pressure and force – increasing the force the pressure (provided that the area stays the same). There is an relationship between force and area – increasing the area decreases the pressure (provided that stays the same). If force is measured in , N, and area is measured in metres, m2, then the unit of pressure is the . The atmosphere exerts pressure. Outside the Earth’s , there is no air and no atmospheric pressure. Water also exerts pressure. Water exerts more pressure at greater . Mr Kueres

Pressure Pressure = force/area. There is a direct relationship between pressure and force – increasing the force increases the pressure (provided that the area stays the same). There is an inverse relationship between pressure and area – increasing the area decreases the pressure (provided that force stays the same). If force is measured in Newtons, N, and area is measured in square metres, m2, then the unit of pressure is the Pascal. The atmosphere exerts pressure. Outside the Earth’s atmosphere, there is no air and no atmospheric pressure. Water also exerts pressure. Water exerts more pressure at greater depth. Mr Kueres

Compression in Liquids and Gases In a gas, the are a long way apart and it’s not too hard to push them closer . In a , it’s very hard to particles much closer together. So, the particle model can explain the different of gases and liquids under . Mr Kueres

Compression in Liquids and Gases In a gas, the particles are a long way apart and it’s not too hard to push them closer together. In a liquid, it’s very hard to push particles much closer together. So, the particle model can explain the different behaviour of gases and liquids under pressure. Mr Kueres

Introduction to Hydraulics Many robots use hydraulics or liquid pressure. There are two cylinders – a master cylinder and a slave cylinder – connected by a sturdy pipe. The master cylinder contains the effort piston and the slave cylinder contains the load piston. The slave does everything that its master tells it to do, but it does it with more force than the master could manage. Hydraulics are also used in jacks to lift up cars. Mr Kueres

Hydraulic fluid is pushed through Increased pressure results in load being lifted The Hydraulic Jack Effort Small piston provides the effort but moves along way Large piston lifts the load but doesn’t move as far X Non-Return Valve Mr Kueres

Hydraulics If the load piston has an area 5 times that of the effort piston, the load lifted is 5 times the effort, BUT moves only 1/5 the distance. Mr Kueres

Questions • Why do we use oil and not water in hydraulic machines? • What is the non-return valve for? • What would happen if there was an air bubble in the pipe work between two cylinders? Mr Kueres

Motor Vehicle Brakes If liquids are useful because they are incompressible and air in hydraulic fluid prevents it working effectively, why do some vehicles have brakes operated by compressed air? Mr Kueres

Hydraulics Review A system can convert a small force into a bigger force. The system is filled with liquid, which is . The force acting on the piston in the master cylinder acts over a small . The is transmitted through the liquid, so the same pressure acts on the piston in the slave cylinder, but it acts over a bigger area. The same pressure acting over a bigger area produces a bigger . A gas is of no use in a hydraulic system because it is . Pressure acting on a gas pushes the closer together. In a the particles are already close together and it’s very hard to make them closer. Mr Kueres

Hydraulics Review A hydraulic system can convert a small force into a bigger force. The system is filled with liquid, which is incompressible. The force acting on the piston in the master cylinder acts over a small area. The pressure is transmitted through the liquid, so the same pressure acts on the piston in the slave cylinder, but it acts over a bigger area. The same pressure acting over a bigger area produces a bigger force. A gas is of no use in a hydraulic system because it is compressible. Pressure acting on a gas pushes the particles closer together. In a liquidthe particles are already close together and it’s very hard to make them closer. Mr Kueres

Deep Sea Diving Why does a diver need to breathe compressed air? • The pressure of the water above him makes it hard to inhale with out the gas helping. • The pressure is due to the mass of water above him. • Pressure = Weight of water • Area of diver • The pressure exerted depends on: • the depth • the weight of the fluid (hence, its density) Mr Kueres

10 100 20 200 Depth of water in metres Water pressure in kilopascals 30 300 40 400 50 500 Mr Kueres

Submarine Pressure Mr Kueres

Things to Do • Answer the Questions on the Submarine Pressure worksheet • Answer the Questions on the Water Pressure worksheet Mr Kueres

Submarine Pressure The bigger the depth of water, the b the water pressure is. For example, if the depth doubles then water pressure also d . And if the depth trebles then w p trebles. In fact, whenever the depth changes, the water pressure always changes by the same p . Mr Kueres

Submarine Pressure The bigger the depth of water, the bigger the water pressure is. For example, if the depth doubles then water pressure also doubles . And if the depth trebles then waterpressure trebles. In fact, whenever the depth changes, the water pressure always changes by the same proportion. Mr Kueres

Pressure & Moments Lesson 4: Moments Mr Kueres

Lesson Objectives • To know that a force can produce a turning effect • To know that the turning effect is called a moment • To know that the moment of a force depends on the size of the force and the distance of the force from the pivot • To know that the pivot is also called a Fulcrum • To be able to calculate the moment of a force Mr Kueres

Turning Effects Write a list of examples of where a force produces a turning effect. Mr Kueres

Demonstration Try opening a door in each of these three places: • A point furthest away from the hinges • A point roughly in the centre of the door • A point close to the hinges At which point is it the most difficult to open the door? Mr Kueres

Demonstration Hold a metre ruler horizontally at arms length. If we apply a 10N weight near your hand, then gradually move the weight along the ruler, what happens? We find that it becomes increasingly difficult to hold the ruler horizontal. This is because the mass exerts a turning effect on the ruler. The turning effect is called a MOMENT and depends on the weight and the distance from the pivot Mr Kueres

Moments The turning effect caused by a force is called a moment and depends on the force (weight) applied and the distance from the pivot. Mr Kueres

Moments Moment = Force Exerted x Distance from the Pivot Moment = F x d UNITS Force is measured in N , N. Distance is measured in m , m (usually c , cm, for the distances we use in class). Moments are measured in N m , Nm (or Newton centimetres if the distance is measured in that unit) Mr Kueres

Moments Moment = Force Exerted x Distance from the Pivot Moment = F x d UNITS Force is measured in Newtons, N. Distance is measured in metres, m (usually centimetres, cm, for the distances we use in class). Moments are measured in Newtonmetres, Nm (or Newton centimetres if the distance is measured in that unit). Mr Kueres

Things to Do • Complete 9Ld/3: Revision Questions • Complete the Moments Example Sheet 1 Mr Kueres

G G Balancing Act We can use the idea of moments to see why an object balances (or overbalances) Centre of gravity acts between the wheels – the bus is balanced Centre of gravity acts outside of the wheels Mr Kueres

Things to Do Complete the Moments Example Sheet 2 Mr Kueres

Pressure and Moments