1. Lesson 1: Force and Pressure
Pressure and Moments
Lesson 1: Force and Pressure
Mr Kueres

2. 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

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

4. 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

5. Unbalanced Forces
If the 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

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

7. Things to Do Large Area/Low Pressure Small Area/High Pressure
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.
Large Area/Low Pressure
Small Area/High Pressure
Mr Kueres

8. 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

9. Lesson 2: Calculating Pressure
Pressure and Moments
Lesson 2: Calculating Pressure
Mr Kueres

10. 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

11. 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

12. 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

13. 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

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

15. 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

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

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

18. 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

19. 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

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

21. 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

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

23. 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

24. 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

25. 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

26. 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

27. 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

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

29. 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

30. 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

31. 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

32. 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

33. 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 liquid the particles are already close together and it’s very hard to make them closer.
Mr Kueres

34. 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

35. Depth of water in metres 30 Water pressure in kilopascals10
100 20
200
Depth of water in metres 30
Water pressure in kilopascals
300 40
400 50
500
Mr Kueres

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

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

38. 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

39. 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 water pressure trebles. In fact, whenever the depth changes, the water pressure always changes by the same proportion.
Mr Kueres

40. Pressure & Moments
Lesson 4: Moments
Mr Kueres

41. 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

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

43. 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

44. 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

45. 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

46. Moment = Force Exerted x Distance from the Pivot
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

47. Moment = Force Exerted x Distance from the Pivot
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 Newton metres, Nm (or Newton centimetres if the distance is measured in that unit).
Mr Kueres

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

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

50. Things to Do
Complete the Moments Example Sheet 2
Mr Kueres

51. Lesson 5: Balancing Moments
Pressure and Moments
Lesson 5: Balancing Moments
Mr Kueres

52. Lesson 5 - Objectives
To know how to find whether moments are balanced or unbalanced
To be able to balance simple levers
To know that if a lever is balanced, the anticlockwise and clockwise moments are equal
Mr Kueres

53. Moment = Force Exerted x Distance from the Pivot
Moments - Review
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 Newton metres, Nm (or Newton centimetres if the distance is measured in that unit).
Mr Kueres

54. Balancing Levers
How can we alter levers e.g. seesaws, so that they balance?
In which direction would you move the masses to make the lever above balance?
Could you balance the lever by moving the pivot? If so how?
Mr Kueres

55. Balancing Levers We can balance the lever by:
1. Moving the light weight away from the pivot.
Lever moves clockwise
Mr Kueres

56. Balancing Levers We can balance the lever by:
2. Moving the heavy weight towards the pivot.
Lever moves clockwise
Mr Kueres

57. Balancing Levers We can balance the lever by:
3. Moving the pivot towards the heavy weight.
Lever moves clockwise
Mr Kueres

58. Practical – Balancing Levers
Use the ‘Levers’ worksheet to perform the investigation
Record your results in an appropriate table
Mr Kueres

59. Conclusion In order for a lever to balance, the
c moment must e the anti-clockwise m .
Mr Kueres

60. Conclusion In order for a lever to balance, the
clockwise moment must equal the
anti-clockwise moment.
Mr Kueres

61. Total anti-clockwise moment = Total clockwise moment
Anti-Clockwise Moment = Clockwise Moment
Force x Distance from pivot = Force x Distance from Pivot
Mr Kueres