W9 - Angular Kinetics - Centre of Gravity

Question 1

Compare Newtons 1st law for linear & angular motion

Answer 1

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

Question 2

Equation for Newtowns second law of motion

Answer 2

F = ma

Question 3

Apply newtons 2nd law to LINEAR motion

Answer 3

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

Question 4

Apply Newtons 2nd law to ANGULAR motion

Answer 4

An external moment produces an angular acceleration that is:

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

Question 5

Apply newtons 3rd law to LINEAR motion

Answer 5

Every action has an equal + opposite reaction

Question 6

Apply newtons 3rd law to ANGULAR motion

Answer 6

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

Question 7

Define moment/torque

Answer 7

Tendency of a force to cause rotation about a specific axis

Question 8

Is a moment/torque a force?

Answer 8

NO

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

Question 9

What is moment/torque also known as?

Answer 9

The measurement of a turning effect

Question 10

What is the moment/torque a product of?

Answer 10

Magnitude of a force

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

Question 11

How is moment calculated?

Answer 11

Mass x distance

Question 12

Define eccentric force

Answer 12

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

Question 13

What does eccentric force always cause?

Answer 13

Rotation therefore a moment.

Question 14

Eccentric force always causes rotation + therefore a moment.

What can it also cause?

Answer 14

Translation

Question 15

What does Centric force have?

Answer 15

A line of action passing through axis of rotation/CoG

Question 16

What is a couple?

Answer 16

2 equal + opposite parallel forces

When rotation occurs about body centre of gravity

Question 17

What happens to the translation effects in a couple?

Answer 17

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

Question 18

What is the moment effect of a couple?

Answer 18

Turning effect prod by a force couple

Question 19

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

Answer 19

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

Perpendicular distance between lines of action.

Question 19

Equation for the moment of a couple

Answer 19

M = F1 x distance

Question 20

What is inertia in the linear case?

Answer 20

Objects tendency to resists a change in motion

Mass

Question 21

What is inertia in the angular case?

Answer 21

Objects tendency to resist a change in motion

Moment of inertia

Question 22

What are the 2 components of moments?

Answer 22

Magnitude of force

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

Question 23

Units for moment

Answer 23

N.m

Question 24

What is the resultant moment?

Answer 24

Sum of all moments about a defined point

Question 25

What are clockwise moments conventionally?

Answer 25

-ive in direction

Question 26

What are anti-clockwise moments conventionally?

Answer 26

+ive in direction

Question 27

Define equilibrium

Answer 27

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

Question 28

What is the sum of moments when theres equilibrium?

Answer 28

0

Question 29

What is the centre of gravity?

Answer 29

Point about which the gravitational force is balanced. Where mass is EQUALLY distributed + through which the resultant force of gravity acts.

Question 30

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

Answer 30

Suspension Method Reaction board method Segmental method

Question 31

Limitations to the suspension method

Answer 31

Not very practical Cardboard cut outs only represent shape, not mass

Question 32

What does the reaction board method employ?

Answer 32

Theory of equilibrium

Question 33

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

Answer 33

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.

Question 34

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

Answer 34

Coordinates of each segment endpoint Segment weights Locations of segment centres of gravity

Question 35

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

Answer 35

Cadaver data Mathematical modelling Scanning

Question 36

Limitations to using cadaver data

Answer 36

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

Question 37

Explain the mathematical modelling

Answer 37

Divides body into 17 segments Differentiates between M + F Accounts for varying densities Allows subject-specific estimates

Question 38

Scanning

Answer 38

Use of DXA to estimate segment properties

Question 39

Limitations to scanning

Answer 39

Invasive Costly Time heavy

Question 40

What is kinetics about?

Answer 40

Forces acting on the body

Question 41

What can the forces acting on the body cause?

Answer 41

Rotation Translation or both

Question 42

What does the segmental method theory make use of?

Answer 42

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)

Question 43

What do we calculate in the segmental method theory?

Answer 43

The moment for each segment: Moment = force x distance

Question 44

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

Answer 44

Coordinates of each segment endpoint Segment weights Locations of segment centres of gravity

Question 45

Do practise questions on sway slides

Answer 45

done?

Question 46

What do moments about OY axis provide?

Answer 46

Horizontal location of CoG X co-ordinate

Question 47

What do moments about OX axis provide?

Answer 47

Vertical location of CoG Y co-ordinate