Force and Moments

1. Force and Moments

2. Plan for Section • Lesson 1 - OP4 • understand the concept of force • recall that the Newton is the unit of force • describe forces and their effects • Lesson 2 - OP5 • investigate examples of friction and the effect of lubrication

3. Lesson 3 - OP7 • understand that weight is the force of gravity and that weight varies with location • recall that mass in kilograms multiplied by 10 is approximately equal to weight in Newtons on the surface of the earth • Lesson 4 - OP6 • investigate the relationship between the extension of a spring and the applied force

4. Lesson 5 - OP9 • investigate the law of the lever • recall two everyday applications of levers • Lesson 6 - OP8 • find the centre of gravity of a thin lamina • Lesson 7 - OP8 • investigate the role of centre of gravity in design for stability and equilibrium • Lesson 8 – Assessment !!!

5. Lesson 1 • OP4 • understand the concept of force • recall that the newton is the unit of force • describe forces and their effects

6. What is a Force? • A force is anything which causes an object to accelerate, i.e to change its velocity. • Force is measured in newtons (N)

7. Examples of forces • A push or a pull • Weight (the pull of gravity) • Friction (a force between sliding surfaces) • Electric (force between electric charges) • Magnetic (force between magnets)

8. Examples of Forces – their Effects • Push • Changes velocity of an object • Causes it to go faster, slower, or stop

9. Weight • Accelerates objects towards the ground

10. Friction • Slows down moving objects Ice Grass

11. Electric • Causes charged particles to move

12. Magnetic

13. Forces as Pairs • Forces always occur in pairs, so for every force there is an equal and opposite force, e.g. • Gun recoil • Deflating balloon • People on roller skates

14. Forces as pairs – Gun Recoil bullet gun

15. Forces as pairs – Balloon Deflating balloon air

16. Forces as pairs – Roller Skaters • Jane pushes John. • John experiences a force pushing him away from Jane. • John has inertia (mass) and so Jane experiences an equal force pushing her away from John, in the opposite direction. • They go in opposite directions • Because they are on skateboards frictional forces from the ground are relatively small.

17. Lesson 2 • OP5 • investigate examples of friction and the effect of lubrication

18. What is friction? Friction is the force that opposes sliding motion between two surfaces Friction slows down motion of objects sliding over surfaces. 18

19. Friction Slows down moving objects Ice – low friction Grass - high friction 19

20. Note • Friction is a property associated with TWO surfaces that can slide over one another.

21. Examples of friction High friction Car tyres on tarmac Rubber on glass Rubber soled shoes on cement Glass on damp glass • Low friction • Car tyres on ice • Steel on steel (smooth) • Glass on glass (very dry or very wet) • Polythene on polythene 21

22. Lubricants How do we reduce friction? We introduce a lubricant – we lubricate the surfaces causing friction Examples of lubricants are: Oil Grease 22

23. Lubrication reduces friction Use a spring balance How much force does it take to move the block? Put some oil or grease between the sliding surfaces. How much force does it take now? 23

24. Advantages of Friction Allows us to Walk Run Cycle How does friction help? Force of friction works against the push we make against the ground 24

25. Disadvantages of Friction Friction due to gases in atmosphere heats the outside of Space Shuttle. Friction wears Joints in our body Car engines Tyres Is friction all bad ? 25

26. Lesson 3 OP7 understand that weight is the force of gravity and that weight varies with location recall that mass in kilograms multiplied by 10 is approximately equal to weight in newtons on the surface of the earth 26

27. Weight The weight of an object is the force with which it is pulled towards the Earth. It is due to the attraction between the Earth and a body. Weight is a force, so it is measured in newtons (N). 27

28. Do I always weigh the same? No! Why not? 28

29. Where would I weigh less? 29

30. Long Jump Record Why did Bob Beaman’s 1972 long jump record (8.9 metres) last until 1991? The Olympics were held in Mexico city….he weighed less! 30

31. Calculating Weight Weight (N) = Mass (kg) × 10 (m/s2) E.g. 1. Find the force of gravity on a bag of flour of mass 1 kg Solution: F = (1kg)(10 m/s2) = 10 kg m/s2 = 10 N 31

32. Calculating Weight – Example 2 Find the weight of a 250 g bag of biscuits Solution: First change the mass to kg 250 g = 0.25 kg (i.e. 250 ÷ 1000) Now, F = 0.25 kg × 10 m/s2 =2.5 kg/m2 = 2.5 N 32

33. Mass & Weight Mass Measured in kilograms (kg) Is fixed, never changes Is a fixed property of all things, like length or volume Weight Measured in newtons (N) Varies, depends on where you are Is a force or a pull on something 33

34. Lesson 4 OP6 investigate the relationship between the extension of a spring and the applied force 34

35. Hooke’s Law The extension of an elastic material is directly proportional to the applied force producing the extension 35

36. Title: To Investigate Hooke’s Law 36

37. Hooke’s Law Graph Plot your result See how it only obeys the law within the elastic region of the spring. Length Weight 37

38. Lesson 5 OP9 investigate the law of the lever recall two everyday applications of levers 38

39. Turning forces This is a good example of the use of a lever Pushing the below rock just won’t work What else can he try? Pulling the rock isn’t working either! 39

40. The Lever A lever is a rigid body which is free to rotate about a fixed point called the fulcrum fulcrum lever 40

41. Turning effect of a force The turning effect of a force is also called the moment of the force It depends on two things The size of the force (measured in newtons) The distance (measured in metres) from the fulcrum to where the force is applied Moment of a force = (Force) × (Perpendicular distance from force to fulcrum) 41

42. Balancing Moments The metre stick above is not balanced, as there are more turning forces on LHS of fulcrum. If we add 10 N a distance of 40 cm from fulcrum on the RHS, we balance the metre stick. 10 20 30 40 50 60 70 80 90 10N 42

43. Law of the Lever When a lever is balanced, the clockwise and anticlockwise moments are equal Question – is the below lever balanced? `10N 10 20 30 40 50 60 70 80 90 10N • Question – is the below lever balanced? • The lever has been balanced by the 10 N on the right, 40 cm from the fulcrum 43

44. Law of the Lever Examples Mathematically, LHS 10 N * 0.4 m = 4 N m 10 20 30 40 50 60 70 80 90 10N `10N RHS 10 N × 0.4 m = 4 N m 44

45. Law of the Lever Examples How do we balance this lever? Moments on LHS = (10 × 0.4) + (20 × 0.25) =(4) + (5) = 9 N m 0.4m 10 20 30 40 50 60 70 80 90 0.25m 10N `20N 45

46. Law of the Lever Examples The total turning effect on the RHS is 9 N m (i.e. 30 × 0.3) The lever is now balanced 0.4 m 10 20 30 40 50 60 70 80 90 0.25 m 0.3 m 30 N 10 N `20 N 46

47. Everyday examples of levers Door handle • wheelbarrow • Revolving doors 47

48. Lesson 6 OP8 find the centre of gravity of a thin lamina 48

49. Centre of gravity of a thin lamina 49

50. Centre of gravity of a thin lamina 50