1. Forces II
Return of the forks

2. Weight vs. Mass Mass is the amount of matter in us
900kg
900kg
Mass is the amount of matter in us
Same on Earth and Space
9000N
ZeroN
Weight is the pull of gravity on us
Different on Earth and Space

3. Newton’s Cannon

4. Weight vs. Mass
Earth’s Gravitational Field Strength is 10N/kg. In other words, a 1kg mass is pulled downwards by a force of 10N. W G M
Weight = Mass x Gravitational Field Strength
(in N) (in kg) (in N/kg)
What is the weight on Earth of a book with mass 2kg?
What is the weight on Earth of an apple with mass 100g?
Dave weighs 700N. What is his mass?
On the moon the gravitational field strength is 1.6N/kg. What will Dave weigh if he stands on the moon?

5. Balanced and unbalanced forces
Reaction
Consider a camel standing on a road. What forces are acting on it?
These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere.
Weight

6. Balanced and unbalanced forces
Reaction
What would happen if we took the road away?
Weight

7. Balanced and unbalanced forces
What would happen if we took the road away?
The camel’s weight is no longer balanced by anything, so the camel falls downwards…
Weight

8. Balanced and unbalanced forces
1) This animal is either ________ or moving with _____ _____…
2) This animal is getting _________…
3) This animal is getting _______….
4) This animal is…

9. Balanced and unbalanced forces
Fnet=200N
F=-100N
F=200N
Fnet=100N
F=-200N
F=200N
Fnet=0N
F=-200N
Fnet=-200N

10. Galileo’s Falling Balls

11. Gravity all bodies have gravity we feel it only from planet sized objects
T=0 v=0m/s
Acceleration due to gravity is 10m/s2
That means every falling body gets 10m/s faster every second
T=1s v=10m/s
T=2s v=20m/s
T=3s v=30m/s

12. All bodies fall at the same rate

13. Gravity is from the earth

15. More force means more Extension Force is proportional to extension
Hookes Law
Force 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Extension
More force means more Extension Force is proportional to extension

16. Hookes Law Calculation1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Force
=24N
Length=17cm
Ext.
=12cm
Force=0N
Length=5cm
Ext.=0cm
Force=6N
Length=8cm
Ext.=3cm
Force=12N
Length=11cm
Ext.=6cm

17. Length/m
2.4
3.4
4.6
5.8 7
Weight/N 1 2 3 4

18. Stopping a car… Tiredness Too much alcohol Thinking distance
Too many drugs
Poor visibility
Wet roads
Icy roads
Stopping distance
Tyres/brakes worn out
Driving too fast

19. Work done Work done = Force x distance moved W D F
When any object is moved around work will need to be done on it to get it to move (obviously).
We can work out the amount of work done in moving an object using the formula:
Work done = Force x distance moved
in Joules in Newtons in meters W D F

20. Work Done Lifting
Lift a mass of 80kg a height of 6m. How much work is done?
Work done = Force x distance moved
Work done = 800N x 6m
Work done = 4800 Joules

21. Power The rate at which work is done POWER = Work Done time taken
Example A microlite takes 20s to climb to 100m. If the microlite has mass 2000kg find the work done and the power?
Work Done = Force x Distance = 2000x10 x = Joules
Power = Work Done / Time = Joules / 20
= Watts

22. Question
On a school trip a pupil of mass 50kg climbs the Eiffel Tower. If the tower is 320m high and he takes 50mins (3000s) find his work done and power.

23. Pressure F A P Pressure depends on two things:
How much force is applied, and
How big (or small) the area on which this force is applied .
Pressure can be calculated using the equation: F A P
Pressure (N/m2 or Pa) = Force (in N)
Area (in m2)
Pa is Pascals

24. Calculating Pressure
To use the triangle we have to cover the letter we do not know
F=PxA
Find the force that we need to have for a pressure of 10Pa over an area of 2m2?

25. Balloon Pressure Expt
Spread it out share the weight man

26. A circus elephant weighs 10,000N and can stand on one foot
A circus elephant weighs 10,000N and can stand on one foot. This foot has an area of 0.5m2. How much pressure does he exert on the floor (in Pa)?
Pressure=Force/area
= 10000N/ 0. 5m2 = Pa

27. A 50kg woman copies the elephant by standing on the heel of one of her high-heeled shoes. This heel has an area of 1cm2. How much pressure does she exert on the floor?
Pressure=Force/area
= 500N/ m2 = Pa

28. Pressure – in Fluids
Pressure increases with depth

29. As the frog goes deeper there is a greater weight of water above it.
Pressure and Depth
As the frog goes deeper there is a greater weight of water above it.

30. Atmospheric Pressure ATMOSPHERIC PRESSURE
The earth is covered with layer of Gas.
We are at the bottom of a gas ocean 200km deep.
The effect of this huge column of gas is 1 Tonne of weight on our shoulders.
This is called
ATMOSPHERIC PRESSURE
Heavy!

31. The Barometer The weight of the air holds up the mercury.
Vacuum
The weight of the air holds up the mercury.
If we use water the column is 10.4m high.
1 Atmosphere is 760mm of Hg.

32. The Altimeter Air is also a fluid
As we go higher there is less air above us.
There is less Atmospheric pressure
We can measure the altitude using a barometer with a different scale.

33. Crushing the can

34. Boiling Point
Increase pressure increase B.P.

35. Before altimeters they used BP to find altitude.
Used to find the source of the Nile.

36. H/W
P310
Q 10-17

37. Center of Gravity
Things stay standing (STABLE) because their Center of Gravity acts through their base.

38. Equilibrium
Things fall over because the center of gravity is outside the base

39. Equilibrium
There are 3 types of equilibrium
STABLE
NEUTRAL
UNSTABLE

40. Center of Gravity
The place where all the mass of the body appears to act.
How do we find the center of gravity of this flat piece of card?

41. of the body appears to act
Center of Gravity of a Lamina We must freely suspend the lamina from three places Then we trace the plumb line onto the paper.
Center of Gravity
Where all the mass
of the body appears to act

42. Internet Demo

43. Levers A rigid body that rotates around a fulcrum
Used to make work easier

45. Moments =Force x Perpendicular distance
Fulcrum
Perpendicular distance
FORCE

46. Moments =Force x Perpendicular distance
= 10N x 5m = 50Nm
Perpendicular distance=5m
FORCE =10N

47. Moments =Force x Perpendicular distance
= 50N x 0.1m = 5Nm

49. More Than Two forces N N N N

50. http://www. absorblearning. com/media/attachment. action

51. Find the following moments
Moments = F x d
=0.4 x 4 = 1.6Nm
Moments = F x d
=0.25x 4 = 1 Nm
Clockwise Moments = Anticlockwise
F x 0.25 = 0.5 x 5
F = 2.5/0.25 = 10N

52. Weight
Moments
Pressure
Atmospheric
C of G
Hooke’s Law

53. H/W p