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Yr 2 - Biomechanics > W9 - Angular Kinetics - Centre of Gravity > Flashcards

W9 - Angular Kinetics - Centre of Gravity Flashcards

1
Q

Compare Newtons 1st law for linear & angular motion

A

L = Objects remains at rest or constant velocity unless acted on by an external force.

A = Rotating object will continue to do so at constant angular momentum unless acted on by an external force

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2
Q

Equation for Newtowns second law of motion

A

F = ma

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3
Q

Apply newtons 2nd law to LINEAR motion

A

A force applies to an object will cause the object to accelerate in proportion to that force.

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4
Q

Apply Newtons 2nd law to ANGULAR motion

A

An external moment produces an angular acceleration that is:

  • Proportional to the moment
  • In the direction of the moment
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5
Q

Apply newtons 3rd law to LINEAR motion

A

Every action has an equal + opposite reaction

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6
Q

Apply newtons 3rd law to ANGULAR motion

A

For every applied moment there is an equal + opposite reaction moment

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7
Q

Define moment/torque

A

Tendency of a force to cause rotation about a specific axis

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8
Q

Is a moment/torque a force?

A

NO

It’s the effect of a force in causing a rotation.

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9
Q

What is moment/torque also known as?

A

The measurement of a turning effect

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10
Q

What is the moment/torque a product of?

A

Magnitude of a force

+ perpendicular distance from line of action of force to axis of rotation.

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11
Q

How is moment calculated?

A

Mass x distance

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12
Q

Define eccentric force

A

Force whose line of action doesn’t pass through the centre of gravity

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13
Q

What does eccentric force always cause?

A

Rotation therefore a moment.

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14
Q

Eccentric force always causes rotation + therefore a moment.

What can it also cause?

A

Translation

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15
Q

What does Centric force have?

A

A line of action passing through axis of rotation/CoG

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16
Q

What is a couple?

A

2 equal + opposite parallel forces

When rotation occurs about body centre of gravity

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17
Q

What happens to the translation effects in a couple?

A

T effects fo each force cancel each other out = no translation

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18
Q

What is the moment effect of a couple?

A

Turning effect prod by a force couple

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19
Q

What does the amount of rotation in the moment effect of a couple depend on?

A

Magnitude of the forces
-But only need the mag. of 1 of the forces.

Perpendicular distance between lines of action.

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19
Q

Equation for the moment of a couple

A

M = F1 x distance

20
Q

What is inertia in the linear case?

A

Objects tendency to resists a change in motion

Mass

21
Q

What is inertia in the angular case?

A

Objects tendency to resist a change in motion

Moment of inertia

22
Q

What are the 2 components of moments?

A

Magnitude of force

Perpendicular distance from pivot point to the line of action of the force

23
Q

Units for moment

A

N.m

24
Q

What is the resultant moment?

A

Sum of all moments about a defined point

25
Q

What are clockwise moments conventionally?

A

-ive in direction

26
Q

What are anti-clockwise moments conventionally?

A

+ive in direction

27
Q

Define equilibrium

A

When all parts of the body are at rest or moving w/ the same constant velocity

28
Q

What is the sum of moments when theres equilibrium?

A

0

29
Q

What is the centre of gravity?

A

Point about which the gravitational force is balanced.

Where mass is EQUALLY distributed + through which the resultant force of gravity acts.

30
Q

What are the 3 methods that can be used to ESTIMATE the location of an objects Centre of gravity?

A

Suspension Method

Reaction board method

Segmental method

31
Q

Limitations to the suspension method

A

Not very practical

Cardboard cut outs only represent shape, not mass

32
Q

What does the reaction board method employ?

A

Theory of equilibrium

33
Q

How does the reaction board method employ the theory of equilibrium?

A

System is in equilibrium when nobody is on the board + when there is.

So, the forces + moments introduced by the person lying on the board must be in equilibrium.

34
Q

Segmental method

What is required to calculate an objects centre of gravity location?

A

Coordinates of each segment endpoint

Segment weights

Locations of segment centres of gravity

35
Q

What are the 3 main ways that segment data have been obtained for use with human participants in the past?

A

Cadaver data

Mathematical modelling

Scanning

36
Q

Limitations to using cadaver data

A

Are the sampled population relevant?

How does cadaver storage influence characteristics?

Approach of finding centre of mass for 2 segments means the segment is considered 2D rather than 3D

37
Q

Explain the mathematical modelling

A

Divides body into 17 segments

Differentiates between M + F

Accounts for varying densities

Allows subject-specific estimates

38
Q

Scanning

A

Use of DXA to estimate segment properties

39
Q

Limitations to scanning

A

Invasive

Costly

Time heavy

40
Q

What is kinetics about?

A

Forces acting on the body

41
Q

What can the forces acting on the body cause?

A

Rotation

Translation

or both

42
Q

What does the segmental method theory make use of?

A

The fact that:

Resultant moment = Sum of moments

(Resultant: Moment of entire body acting around whole body CoG)

(Sum of moments: Each segment of body has a moment which we want to sum)

43
Q

What do we calculate in the segmental method theory?

A

The moment for each segment:

Moment = force x distance

44
Q

What is required to calculate an objects centre of gravity location?

A

Coordinates of each segment endpoint

Segment weights

Locations of segment centres of gravity

45
Q

Do practise questions on sway slides

A

done?

46
Q

What do moments about OY axis provide?

A

Horizontal location of CoG

X co-ordinate

47
Q

What do moments about OX axis provide?

A

Vertical location of CoG

Y co-ordinate

Yr 2 - Biomechanics (7 decks)

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