1. TEST! Next Monday Topic 2 “Matter and Forces”
Use the “Can you…..?” sheet to help you
We’ll do some practice questions on Friday.

2. Can you continue the pressure questions you started last lesson?
Do now!
Can you continue the pressure questions you started last lesson?

3. This lesson – 2.4 Pressure
Relate (without calculation) pressure to force and area.
Recall and use the equation P = F/A

4. Oh no!
YouTube - falling through ice
YouTube - Jason Falls Through Ice

5. Spread the force over a greater area

6. Camels

7. Snow shoes
Yoost

8. Caterpillar tracks

9. House foundations

10. My sofa!

11. I had a dream

12. Death?

13. Sharp = small surface area

14. Drawing pins

16. Pressure N
Pressure = Force
Area
N/m2 or Pa m2

17. Can you copy this please?
Pressure F N
Pressure = Force
Area P A x
Can you copy this please?
N/m2 or Pa m2

18. An example
A woman of weight 600N has a total shoe area of 150 cm2 and a man of weight 750 N has a total shoe area of 360 cm2. What is the pressure beneath their feet?

19. An example
A woman of weight 600N has a total shoe area of 150 cm2 and a man of weight 750 N has a total shoe area of 360 cm2. What is the pressure beneath their feet?
Angelina pressure = force/area = 600/150 = 4 N/cm2
Brad pressure = force/area = 750/360 = 2.1 N/cm2

20. Bunny Suicides involving pressure

21. Bunny Suicides involving pressure

22. Bunny Suicides involving pressure

23. Bunny Suicides involving pressure

24. Calculating your own pressure
Can you do the calculations carefully and neatly on the graph paper to show your pressure (don’t forget you have two feet!) and stick it in your book

25. Pressure under a car

28. Pressure under a car Mass = 2000kg Weight = mg = 2000x10 = 20000N
Area = ?
Pressure = ?

29. Let’s try some questions!

30. Gas pressure
Phet

31. Temperature and pressure
With a fixed volume and mass of gas, the pressure increases with temperature
Can you copy this please?

32. Atmospheric pressure

33. Atmospheric pressure

34. Atmospheric pressure

35. Atmospheric pressure

36. Can you copy this please?
Atmospheric pressure
The atmospheric pressure caused by air particles colliding with objects is about Pascals (this is called ‘1 atmosphere’)
It is this pressure that pushes drink up a straw or causes suction cups to work.
Can you copy this please?

37. U571 YouTube - Ocean Odyssey - Ocean Pressure
YouTube - Styrofoam cup vs. deep sea
YouTube - U-571 Trailer

38. Pressure and depth Pressure acts in all directions
and pressure increases with depth.

39. That’s it!
Let’s remind ourselves of what we have done in this unit.

40. Gravity
Gravity is a force between ALL objects!
Gravity

41. Gravity
The size of the force depends on the mass of the objects. The bigger they are, the bigger the force!
Small attractive force
Bigger attractive force

42. Gravity
The size of the force also depends on the distance between the objects.

43. Gravity
The force of gravity on something is called its weight. Because it is a force it is measured in Newtons.
Weight

44. Gravity 800 N On the earth, Mr Porter’s weight is around 800 N.
I love physics!
800 N

45. Gravity
On the moon, his weight is around 130 N.
Why?
130 N

46. Mass
Mass is a measure of the amount of material an object is made of. It is measured in kilograms.

47. Mass
Mr Porter has a mass of around 77 kg. This means he is made of 77 kg of blood, bones, hair and poo!
77kg

48. Mass
On the moon, Mr Porter hasn’t changed (he’s still Mr Porter!). That means he still is made of 77 kg of blood, bones, hair and poo!
77kg

49. Weight (N) = mass (kg) x gravitational field strength (N/kg)
Calculating weight
To calculate the weight of an object you multiply the object’s mass by the gravitational field strength wherever you are.
Weight (N) = mass (kg) x gravitational field strength (N/kg)

50. Forces
Force is measured in Newtons

51. Friction opposes motion!

52. Newton’s 1st Law
Remember?
If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero).

53. Newton’s 1st law
If there are no forces, or the forces are balanced – constant velocity
Pushing force
friction
NO RESULTANT FORCE

54. Newton’s 2nd law
There is a mathematical relationship between the resultant force and acceleration.
Resultant force (N) = mass (kg) x acceleration (m/s2)
F = ma

55. An example What will be Mr Porter’s acceleration?
Mass of Mr Porter and bike = 100 kg
Pushing force (100 N)
Friction (60 N)

56. An example Resultant force = 100 – 60 = 40 N FR = ma 40 = 100a
a = 0.4 m/s2
Mass of Mr Porter and bike = 100 kg
Pushing force (100 N)
Friction (60 N)

57. Density (g/cm3) = mass(g) volume(cm3) Or using the formula triangle; m
You have to LEARN this!
kg/m3 kg
Density (g/cm3) = mass(g)
volume(cm3)
Or using the formula triangle; m
D x V m3

58. Density of regular shapes
volume = length x width x height
density = mass/volume
mass using a scale
height
width
length

59. Density of liquids Volume Mass of liquid Mass of cylinder
Mass of liquid and cylinder
Density = mass/volume

60. Density of irregular shapes (1)
mass
Difference in level gives the volume of the shape
Density = mass/volume

61. Density of irregular shapes (2)
mass
Displacement can
volume of object
Density = mass/volume

62. Adding forces
For example;
Resultant force
2 N
6 N
4 N 62

63. Hooke’s law (F = kx) Force (N) Extension (x) (cm)
Limit of proportionality
Force (N)
The extension of a spring is proportional to the force applied (until the limit of proportionality is reached). The gradient of the graph is equal to k, the spring constant.
Extension (x) (cm)
Hooke's Law and springs - Physics – YouTube 63 63

64. What does k mean in F=kx?
k is called the spring constant and is a measure of the stiffness of the spring or material
It has units of N/m (newtons per metre)
The higher the k the stiffer the spring
Materials with a high k need a large force to for a given extension

65. More info on Hooke at http://www.roberthooke.org.uk/
A material is said to obey Hookes Law if its extension is directly proportional to the applied force
More info on Hooke at

66. Steel, glass and wood!
Force
Even though they don’t stretch much, they obey Hooke’s law for the first part of the graph
Extention 66 66

67. Rubber
Force
Extension 67 67

68. Pressure N
Pressure = Force
Area
N/m2 or Pa m2

69. Learning profile/Test
Use the learning profile to help you study!
Test on Tuesday 29th November

70. Let’s try some practice questions