4. STATICS OF THE BODY

Question 1

1. What is Statics?

Answer 1

• it is the study of Forces in Equilibrium
• the study of bodies that are not accelerating

Question 2

2. What is Static Equilibrium?

Answer 2

• it is an equilibrium that is not in motion
• the physical state of the system, in which the components of the system are at rest and the net force acting on a system should be zero

Question 3

3. What does it mean when a system is in Dynamic Equilibrium?

Answer 3

• it is in equilibrium and also in motion
• they system is travelling at a constant velocity
• or it is rotating at a constant rate

Question 4

4. What can each force be resolved into?

Answer 4

• it can be resolved into components in the x,y and z directions
• these directions are also known as planes
• the vertical, horizontal and perpendicular planes

Question 5

5. What are these components of F?

Answer 5

• Fx
• Fy
• Fz

Question 6

6. What is the sum of the forces (F) in each of the x,y and z directions when the object is in the static condition/equilibrium?

Answer 6

• they are zero

Question 7

7. Can you write down the scientific equation to show that the sum of the Forces acting in the x,y and z directions are zero at static equilibrium/condition?

Answer 7

• Σ Fx= 0
• Σ Fy= 0
• Σ Fz= 0

Question 8

8. Where can the forces be in balance with regards to the object/body?

Answer 8

• they can either be in balance for the entire body
• or for any part of it

Question 9

9. What is torque?

Answer 9

• it is a physical quantity
• it is not a force in the Newtonian sense
• it is a moment
• it is the tendency of a force to rotate an object about an axis, a fulcrum or a pivot
• it is the physical quantity which causes an object to begin to rotate or move in a circle
• or to change its rate of rotation

Question 10

10. What is another name for torque?

Answer 10

• the moment of a force

Question 11

11. What is a fulcrum?

Answer 11

• it is the point at which the lever pivots

PIVOT=the central point on which an object turns

Question 12

12. What is the symbol for torque?

Answer 12

Question 13

13. What components can torque be resolved into?

Answer 13

• it can be resolved into components in the x, y and z directions
• this is written as:
  • τx
  • τy
  • τz

Question 14

14. What happens to the torques about the x,y and z axes when the body/object is in the static condition/equilibrium?

Answer 14

• the torques about the x,y and z axes sum up to zero
• all the torques acting on the system are equal to zero in each plane
• this is true for the entire body/object and for any part of the body/object

Question 15

15. Can you write down the scientific equation to show that the sum of the Torques about the x,y and z directions are zero at static equilibrium/condition?

Answer 15

• Σ τx= 0
• Σ τy= 0
• Σ τz= 0

Question 16

16. When is a system said to be in Equilibrium?

Answer 16

• when the net force on that system is zero
• and the net torque on the system is also zero

Question 17

17. When is a system said to be in Static Equilibrium?

Answer 17

• when it is in equilibrium
• and also stationary

Question 18

18. What are three examples of Static Equilibrium?

Answer 18

• a perfectly balanced seesaw
• the ankle: when a person stands on their tip toes
• the elbow joint: when a person hold an object

Question 19

19. When is a system in stable equilibrium?

Answer 19

• when it returns to equilibrium after it has just been subject to a small displacement
• when the net force and net torque acting on the object are ZERO

NET= sum of

Question 20

20. When is a system in an unstable equilibrium?

Answer 20

• when it does not return to its original equilibrium after having been subject to a small displacement

Question 21

21. When is a system in Neutral Equilibrium?

Answer 21

• when the object is displaced and then moves into a new position
• when its equilibrium is independent of displacements from its original position

Question 22

22. When an object with a much narrower top and a broader base is placed on its top, is it more stable or unstable?

Answer 22

• more unstable

Question 23

23. When an object with a much narrower top and a broader base is placed on its base, is it more stable or unstable?

Answer 23

• more stable

Question 24

24. Which of these is in unstable equilibrium.
    Explain.

Answer 24

• Object one is in unstable equilibrium
• it has been slightly displaced
• it is unable to return to its original position
• it does not return to its original equilibrium

Question 25

25. Which of these is in stable equilibrium. Explain.

Answer 25

- Object two is in stable equilibrium - it has been slightly displaced - it has returned to it's original position - it has returned to it's original equilibrium

Question 26

26. When is stability essential to human beings?

Answer 26

- during standing - during any type of motion

Question 27

27. What is the criteria for overall stability during standing?

Answer 27

- the centre of mass has to be over the area spanned by the feet

Question 28

28. What does the vertical line (centre of gravity) passing through the centre of mass of the person have to pass through to ensure stability?

Answer 28

- it has to pass through the area of the support base - WITH REGARDS TO PEOPLE: this would be the area of ground in between the person's feet

Question 29

29. If a person is to be standing on the ground and spread their legs further apart, is there stability increased or decreased? WHY?

Answer 29

- their stability is increased - because their base of support is increased

Question 30

30. What would happen to a person if the centre of gravity did not pass through the area of their support base and their centre of mass was not above the area spanned by their feet?

Answer 30

- their torques would not be balanced - they would be unstable - they would topple over

Question 31

31. When the centre of the mass is above the area spanned by the feet, what is the result?

Answer 31

- the right foot causes a - torque - the left foot causes a + torque - they torques cancel out - the net torque is therefore ZERO - the person is stable

Question 32

32. What happens to the person when their centre of mass is to the left of the area spanned by the feet?

Answer 32

- both feet now cause negative torques - the torques are unbalanced - the net torque is not equal to zero - the person is unstable

Question 33

33. What happens to the person when their centre of mass is to the right of the area spanned by the feet?

Answer 33

- both feet now cause positive torques - the torques are unbalanced - the net torque is not equal to zero - the person is unstable

Question 34

34. When are we most stable?

Answer 34

- when the line/centre of gravity is near the centre of the support base

Question 35

35. What happens to the line/centre of gravity when we lean?

Answer 35

- it can pass through the outside of the base - or through the outer limits/ edge of the base

Question 36

36. What is the amount of turning/torque produced by the force applied on the object dependent on?

Answer 36

- the magnitude of the Force (F) - the length of the object - the product of these 2 factors gives the torque

Question 37

37.What is the formula to work out Torque?

Answer 37

τ= Force x distance (distance from the pivot point) τ= Fxd τ=Fd - torque is proportional the length of the object - torque is proportional to the force on the object

Question 38

38. What is the unit that torque is measured in?

Answer 38

- Nm - Newtons x metres

Question 39

39. What does the torque provide us with?

Answer 39

- a useful way to measure the turning effect (the tendency to cause rotation) of the force applied to the object - this is called the moment arm - it can also be called the lever

Question 40

40. What does an increased distance from the point at which the force is applied to the axis of rotation (longer lever arm) result in?

Answer 40

- it makes it easier for the object to rotate - it increases the torque NB: this distance defines the rotating ability of the force

Question 41

41. What kind of motion causes a positive torque?

Answer 41

- motion in the counter-clockwise direction - a force that causes a counter-clockwise rotation about the pivot point causes a positive torque

Question 42

42. What kind of motion causes a positive torque?

Answer 42

- motion in the anticlockwise direction - a force that causes a clockwise rotation about the pivot point causes a negative torque

Question 43

43. What three components can the equilibrium equations be reduced to?

Answer 43

- Σ Fx= 0 - Σ Fy= 0 - Σ τz= 0

Question 44

44. What are the 3 types of levers we get?

Answer 44

- 1st Class Levers - 2nd Class Levers - 3rd Class Levers

Question 45

45. Is there a net torque in a system in static equilibrium? Why or why not?

Answer 45

- there is not - all the torques are balanced

Question 46

46. When there is no net torque, what happens to the system with regards to motion?

Answer 46

- it remains motionless - this is because there is no tendency to rotate

Question 47

47. What is the Principle of Moments?

Answer 47

- at equilibrium, the sum of the clockwise moments are equal to the sum of the counter clockwise moments

Question 48

48. What is the formula to represent the Principle of Moments?

Answer 48

the sum of all the negative torques= the sum of all the positive torques

Question 49

49. What are the characteristics of a First Class Lever?

Answer 49

- the weight (load) and the muscle (effort) act on opposite sides of the fulcrum - the fulcrum (pivot point) is in the middle of the weight (load) and the muscle (effort) - the weight (load) and the muscle (effort) act in the same direction NB: the weight (load) can sometimes be referred to as the Resistance NB: the muscle (effort) can sometimes be referred to as the Applied Force

Question 50

50. Is a First Class Lever very common in the body?

Answer 50

- NO - it is the least common

Question 51

51. Give me two examples of First Class Levers?

Answer 51

- a seesaw - a head atop the spinal cord

Question 52

52. What are the characteristics of a Second Class Lever?

Answer 52

- the muscle (effort) and the weight (load) act on the same side of the fulcrum - the weight (load) is in the middle of the fulcrum and the muscle (effort) - weight (load) and muscle (effort) act in opposite directions

Question 53

53. How common is a Second Class lever?

Answer 53

- it is the second most common lever in the body

Question 54

54. Give me 2 examples of a Second Class Lever?

Answer 54

- a person wheeling a wheel barrow - going on your tip toes

Question 55

55. What are the characteristics of a Third Class lever?

Answer 55

- the muscle (effort) and the weight (load) are on the same side of the fulcrum - the muscle (effort) is in the middle of the fulcrum and the weight (load) - the weight (load) and the muscle (effort) act in opposite directions

Question 56

56. How common is the Third Class Lever?

Answer 56

- it is the most common lever in the body

Question 57

57. Give me 2 examples of a Third Class Lever?

Answer 57

- a tennis racket - holding a weight at a 90 degree angle to work your arm muscles

Question 58

58. What are the levers, fulcrums and efforts in the human body?

Answer 58

- Bones: lever arms - Joints: fulcrums - Muscles: provide effort

Question 59

59. Answer the following question.

Answer 59

- A

Question 60

60. Answer the question.

Answer 60

- D

Question 61

61. Answer the question.

Answer 61

- C

Question 62

62. Answer the question.

Answer 62

- B

Question 63

63. Read through the summary. Does everything make sense?

Answer 63

- yes

Question 64

64. Read through the summary. Does everything make sense?

Answer 64

- yes

Question 65

65. Read through the summary. Does everything make sense?

Answer 65

- yes

Question 66

66. Read through the summary. Does everything make sense?

Answer 66

- yes

Question 67

67. Read through the summary. Does everything make sense?

Answer 67

- yes

Question 68

68. Provide a definition for the centre of mass?

Answer 68

- this is the point at which the total mass of the body may be considered to be concentrated (for many proposes) - this is used when analysing the motion - the centre of mass does not require a gravitational field

Question 69

69. Provide a definition for the centre of Gravity?

Answer 69

- this is the point where the total weight of the material body is concentrated. - in a uniform gravitational field, the centre of gravity is equal to the centre of mass