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BMSc - Applied Ortho Semester 1 > Mechanics/ biomechanics - unit 1 deck 3 > Flashcards

Mechanics/ biomechanics - unit 1 deck 3 Flashcards

1
Q

Define what is meany by the term statics in mechanics

A

This is the branch of mechanics which deals with forces and moments acting on bodies which are at rest or moving with constant velocity

==> by static we mean that the bodies are not accelerating; they may be moving with a constant linear &/ or angular velocity

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2
Q

Define what is meant by the term body in mechanics

A

This means the object of interest, it does not refer necessarily to the human body

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3
Q

What type of quantity is force ?

A

It is a vector quantity ==> has a magnitude and direction in which it acts

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4
Q

What are the two effects a froce will have on a body?

A
  • Firstly it will tend to change the bodys position in space
  • Secondly it will tend to deform the bodys shape
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5
Q

Which effect of a force will we only consider in this module and what are we therefore assuming ?

A

We will only consider the first effect - a force will tend to change a bodys position in space

==> assuming that the bodies under consideration are rigid i.e. they cannot be deformed - known as rigid body mechanics

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6
Q

In reality do all bodies deform under the action of an applied force?

A

Yes - but we dont need to worry about that

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7
Q

State one way of measuring a force and explain how it works

A

By using a calibrated spring balance

This works as the spring stretches in proportion to the applied force. The spring balance’s scale shows the magnitude of force acting parallel to the spring

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8
Q

What is the SI unit of force ?

A

Newtons (N)

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9
Q

Define both mass and weight

A
  • Mass is the quantity of matter of which a body is composed
  • Weight is the force of gravity acting on a body

e.g. On the moon the astronauts weigh less than they do on Earth. (Their mass however remains the same as it did on Earth.) The reason for the astronauts’ light weight on the moon is that the moon’s gravitational field is approximately 1/6th the magnitude of that on Earth. In outer
space an astronaut can be described as being weightless as the gravitational field is
negligible. However in space, on the moon and on earth an astronaut is still made up of
the same quantity of matter; his or her mass

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10
Q

State the SI unit of mass and the SI unit of weight

A
  • SI unit of mass = kilogram (kg)
  • SI unit of weight = Newton (N)
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11
Q

What are the symbols used to represent both mass and weight

A
  • Mass - m
  • Weight = W
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12
Q

Define what density of a material is

A

This is the mass per unit volume

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13
Q

State the equation for calculating density

A

ρ = m / V

  • ρ = density of the materal
  • m = mass of the body made from the material
  • V = volume of the body made from the material
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14
Q

What is the SI unit of density?

A

kilograms per meter cubed (kg m-3)

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15
Q

Describe the relationship of density, mass and volume of a material

A

The density of a material always remains constant

Mass and volume are therefore proportional to each other i.e. if the mass increases then the volume must also increase as the density always remains constant

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16
Q

Do SAQ 1 pg. 3 of force in mechanics binder

A
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17
Q

Define what gravity is

A

This is the acceleration due to the gravitational attraction between two bodies

e.g. The gravitational attraction between the moon and the Earth keeps the moon in orbit about the Earth. On Earth, the gravitational attraction between two bodies is negligible in comparison to their attraction to the Earth. (Two cups of coffee sitting on a glass table top don’t move towards each other however smooth the table’s surface, yet, if they were placed too near the edge, they would both “fall” off the table due to the gravitational attraction between them and the Earth.) Therefore, gravity accelerates bodies if they are allowed to freefall

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18
Q

What causes the gravitational attraction between two bodies to increase?

A

It increases as the mass of the bodies increase

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19
Q

Gravity accelerates a body if it is allowed to do what?

A

Free-fall

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20
Q

What does gravity cause a body to have ?

A

Weight

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21
Q

What is the equation for calculating weight ?

A

W = mg

  • W = the bodies weight
  • m = the bodies mass
  • g = the acceleration due to gravity
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22
Q

What is the earths gravity ?

A

9.81 m s-2

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23
Q

Define what the centre of mass of a body is

A

This is the point where all the mass of a body can be assumed to act

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24
Q

What does the symbol in the pic represent ?

A

This is the symbol used to represent the centre of mass of a body

25
Q

What does the position of the centre of mass of a body depend on ?

A

It depends on how the mass is distributed in a body

26
Q

How does the centre of mass in a rigid, symmetrical body of uniform density (i.e. a body with an even mass distribution) compare to the centre of mass in a complex system (e.g. the human body)

A

In a rigid, symmetrical body of uniform density (i.e. a body with an even mass distribution) it will have a centre of mass lying at its geometric centre (e.g. a cylinder and a sphere)

However in a complex system (e.g. the human body) it will not have its centre of mass coinciding with its geometric centre, since the mass is not distributed evenly. This uneven distribution may be due to varying densities within the body (e.g. muscle has a higher density than fat) or due to the structure of the body (e.g. a golf club or boomerang)

27
Q

Does the centre of mass of a body need to lie within the body itself?

A

No

e.g. When someone stands upright, his or her centre of mass will be positioned in the lower abdomen. However, when bending over it will move to lie outside the body in front of the pelvis

28
Q

What can the human body be considered constructed of to make up the whole body?

A

A number of parts or segments

29
Q

What does each individual body segment (e.g. the right thigh) have ?

A

Its own centre of mass

30
Q

What will happen if one body segment is moved to the centre of mass of the whole body?

A

As one segment is moved relative to the others the centre of mass of the whole body will shift accordingly. (human body centre of mass usually lies in the lower abdomen)

An example of the shift in centre of mass is illustrated in Figure 19 Pg. 5 in forces section of mechanics binder, which shows how Biomick’s centre of mass position varies as he performs a long jump

31
Q

Along with a centre of mass what will a body in a gravitational field also have ?

A

A centre of gravity

32
Q

Define what is meant by the centre of gravity of a body?

A

This is the point where the weight of the body can be assumed to act

33
Q

Does the centre of mass and centre of gravity occur at the same point ?

A

Coincidentally they do, but its important to remember that centre of gravity only occurs when a body is in a gravitational field e.g. an astronaut in outer space will have a centre of mass but not a centre of gravity

34
Q

Do SAQ 2 pg. 5 of force unit in mechanics binder

A
35
Q

Define what friction is

A

This is the force arising between two surfaces when they rub against each other

36
Q

What direction does friction act

A

It acts as a tangent (straight line) to the surfaces rubbing against eachother and opposes motion (i.e. acts in the opposite direction to motion)

37
Q

What is the SI unit of friction?

A

Netwton (N) since it is a force

38
Q

What is the maximum magnitude of friction force dependent on ?

A

It is dependent on the textures of the two surfaces and the magnitude of the force pressing them down

39
Q

What is the difference in friction between wet or lubricated surfaces and dry surfaces ?

A

The wet or lubricated surfaces will have a smaller friction force than the dry surfaces

40
Q

As the force pressing two surfaces together increases what happens to the friction force?

A

The friction force increases

41
Q

Define what the coefficient of friction is

A

This is the measure of the maximum friction force between two surfaces

It is the ratio of the friction force to the force acting normally (perpendicular) to press the two surfaces together

42
Q

Define what is meant by the term perpendicular

A

This means a straight line at an angle of 90 degrees to a given line, plane or surface

43
Q

State the equation for calculating the coefficient of friction between two surfaces

A

μ = F / N

  • μ = the coefficient of friction between two surfaces
  • F = the friction force
  • N = the force acting normally to the surfaces
44
Q

What are the 3 different circumstances in which friction can occur ?

A
  1. Static
  2. Sliding
  3. and Rolling friction
45
Q

Does the co-efficient of friction vary between the 3 different circumstances in which friction occurs ?

A

Yes it does - there is three different values for the coefficient of friction for the interaction between the same materials

46
Q

When does static friction occur ?

A

It occurs only when motion is about to occur between two surfaces

47
Q

When static friction is present just before motion is about to occur, what is the static friction force present just sufficient enough to do ?

A

To oppose the applied force that is trying to more the surfaces forces over one another

48
Q

What needs to be exceeded for motion to occur ?

A

The maximum static friction force needs to be exceeded

So essentially as a gradually increasing force is exerted in an attempt to slide the two surfaces apart then the static friction force will increase, matching the applied force until the maximum possible static friction force is reached. Above this maximum static friction force sliding will begin.

Refer to pic on pg. 6 force unit, mechanics binder

49
Q

What is the coefficient of static friction equal to ?

A

It is equal to the ratio of the maximum friction force to the force acting normally to press the two surfaces together (refer to the equation)

50
Q

When does sliding friction occur ?

A

It only exists when sliding occurs between two surfaces

51
Q

What is the measure of sliding friction?

A

This is the coefficient of sliding friction which is (as with static friction) the ratio of the sliding friction force to the force acting normally to press the two surfaces together

52
Q

During sliding what happens to the friction force (sliding) opposing the motion?

A

It remains constant

53
Q

Is sliding friction force greater than static friction force?

A

No! sliding friction is less than static friction; more force is required to start a body moving against a frictional force than to keep it moving

e.g. try sliding your hand along the surface of a table with and without pressing downwards - notice how much more difficult it was to slide your hand when pressing down (as the force acting ormally to press the surfaces together increases, static friction force increases). You may have also noticed that once you’re hand began sliding the resistance was reduced.

54
Q

When does rolling friction occur ?

A

This arises between an object and the surface over which it is rolling

55
Q

Why does rolling friction occur ?

A

It arises because of the deformation of the two surfaces caused by the force acting normally to the two surfaces

e.g. the deformation of a car tyre on a road, the rolling friction force is given by the coefficient

56
Q

What is the rolling friction force given by?

A

The coefficient of rolling friction

57
Q

Is the friction force acting on a rolling object higher or lower than the friction force acting on a sliding one ?

A

Friction force acting on a rolling object is lower than on a sliding one hence the use of ball bearings in machinery to reduce friction

58
Q

Does lubrication lower rolling friction like in static friction?

A

No but it may reduce wear

BMSc - Applied Ortho Semester 1 (52 decks)

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