Mechanics/ biomechanics - unit 1 deck 5

Question 1

When considering moments and levers what are we restricting them to acting in ?

Answer 1

To acting in static situations only

Question 2

Define what a the moment of a force is

Answer 2

This is the tendency of a force to produce a rotation about an axis

Question 3

State another name for the moment of a force

Answer 3

Torque

Question 4

What is the moment of a force the product of ?

Answer 4

The applied force and the moment arm (The moment arm is the length of the line that passes through the centre of rotation and is perpendicular to the line of action of the force)

Look at pg. 24 biomechanics figure 22

Question 5

What are the SI units of the moment of a force

Answer 5

Newton metres (N m).

Question 6

Why are moments particularly important in biomechanics ?

Answer 6

Because the action of muscles produce moments about the centres of joints

Question 7

State the equation for calculating a moment

Answer 7

M = F d

• M = the moment
• F = the force
• d = the moment arm

Question 8

What can the positive direction of a moment be determined by?

Answer 8

• The right hand grip rule - by gripping the arrow shaft (axis your working along) with the R-hand, with the thumb pointing in the direction of the arrowhead, your fingers will then point in the +ve moment direction
• In physics the counterclockwise direction is defined as +ve and the clockwise direction is -ve for rotational values

Question 9

Appreciate this:

Think of a force acting on an arm which lies along the x-axis, it will cause flexion or extension which is a +ve or _ve rotation (moment) in the z-axis

Question 10

Describe how it is easier to move a wheelchair with a large diameter handrail compared to a wheelchair with a large diameter handrail despite the size of the wheels remaining constant

Answer 10

• For a small diameter handrim the moment arm is small. The moment arm is greater for the larger handrim. The magnitude of the moment produced when the same force is applied by the hand is therefore greater for the larger handrim. Thus it is easier to move the wheelchair with the larger diameter handrim.

Simply put the handrail is a further diameter away from the point which the rotation is being made (centre of the wheel) therefore the moment arm to the centre is bigger with a handrail which is right on the edge of the wheels diameter (refer to pic)

Refer to the pic on pg. 25 of biomechanics

Question 11

State the conditions for true static equilibrium to occur

Answer 11

The sum of all the external moments and forces acting on a body must be equal to zero

Question 12

State the first and second condition of static equilibrium (both of which need to occur for true static equilibrium to occur) and then state the specific name for each condition

Answer 12

The first condition of static equilibrium is that the sum of all external forces acting on a body is zero - this is known specifically as translational equilibrium

The second condition of static equilibrium is that the sum of all external moments acting on a body must be equal to zero - this is known specifically as rotational equilibrium

Question 13

What will happen if the sum of all external moments acting on a body do not equal to zero?

Answer 13

The body will angularly accelerate

Question 14

State for a rectangular reference frame what the sum of all the external moments in each axis equal

Answer 14

ΣM_x = 0; ΣM_y = 0; ΣM_z = 0

ΣM_x is the sum of all the external moments acting about the x-axis ΣM_y is the sum of all the external moments acting about the y-axis ΣM_z is the sum of all the external moments acting about the z-axis.

Question 15

Do worked example on pg. 24&25, forces unit, mechanics binder

Question 16

Do SAQ 16 pg. 25, forces unit, mechanics binder

Question 17

Describe what a lever system is

Answer 17

It consists of a rigid bar that pivots about a fulcrum (e.g. like a pivot or a hinge) and is acted on by an effort force and a resistance force.

Refer to pg. 26 of biomechancis for pic to illustrate this all

Question 18

Define what the fulcrum is

Answer 18

This is the point against a lever on which is turns

Question 19

What do the effort and resistance forces produce in a lever system?

Answer 19

They produce moments acting about the fulcrum

Question 20

Define what both the effort arm and resistance arm is in a lever system

Answer 20

• Effort arm = the length of the line that passes through the fulcrum and is perpendicular to the effort force (length of it is from fulcrum to the point at which effort force acts)
• Resistance arm = the length of the line that passes through the fulcrum and is perpendicular to the resistance force (length of it is from fulcrum to the point at which resistance force acts)

Question 21

Give an example of a lever system

Answer 21

The use of a crowbar to lift a heavy object (or force open a door)

Question 22

Describe lever systems present in the body

Answer 22

Muscles act (the effort force) to move or prevent the movement of a limb (the limb is the rigid bar) by overcoming external forces (the resistance force) such as gravity.

Question 23

What can be expressed by calculating the mechanical advantage of a lever system ?

Answer 23

The mechanical effectiveness of a lever system

Question 24

State the equation for calculating the mechanical advatange of a lever system

Answer 24

MA = d_f / d_r

• MA = mechanical advantage
• d_f = force-fulcrum distance (effort arm)
• d_r = resistance-fulcrum distance (rassistance arm)

Question 25

A lever system can serve one of what two purposes?

Answer 25

To amplify the effort force or to amplify the distance moved through

Question 26

When the effort arm is longer than the resistance arm the magnitude of the effort force required to overcome a given resistance force is smaller than the magnitude of the resistance force.

In this case the lever system is said to have what and give an example of this?

Answer 26

A mechanical advantage

e.g. the crowbar - which is why we use a crowbar rather than our fingers to move heavy objects

A mechanical advantage means that the effort force is lower than the resistance force (i.e. when the effort arm is longer than the resistance arm then the effort force does not have to be as large as the resistance force)

Question 27

When the resistance arm is longer than the effort arm then the resistance may be moved through a relatively large distance.

In this case the lever system is said to have what?

Answer 27

A mechanical disadvantage

Question 28

In the human body’s musculoskeletal lever systems, muscles are usually at a force disadvantage - what is meant by this statement and explain why this is so

Answer 28

That is to say that the forces produced by the muscles are greater than the forces resisting them.

This is because muscles’ insertion points tend to be closer to the fulcrum than the resistance force.

Question 29

What does a mechanical advantage of > 1 and < 1 indicate ?

Answer 29

• A mechanical advantage > 1 indicates that the lever system is working at a mechanical advantage and that the effort force is less than the resistance force, such as in a crowbar.
• A mechanical advantage < 1 indicates that the lever system is working at a mechanical disadvantage and that the effort force is greater than the resistance force.

Question 30

Why do muscles in the human body usually act at a mechanical disadvantage ?

Answer 30

Because they are usually inserted (attached) close to a joint.

Question 31

Muscles in the human body usually work at a mechanical disadvantage because there insertion is close to the joint, what is the advantage however of this close insertion to a joint ?

Answer 31

The closer a muscle is inserted to a joint then the smaller the change in muscle length required to produce a correspondingly larger limb movement, and the quicker the movement. A compromise is made between strength and mobility

Question 32

What are the 3 classes of lever systems and what are these classes all dependent on

Answer 32

First, Second and Third class lever systems

These are all dependent on the positions of the two forces relative to the fulcrum.

Question 33

Define a first class lever system and give some examples

Answer 33

The fulcrum is located between the effort and the resistance

e.g. crowbar, scissors and see-saw

Question 34

What mechanical effectiveness can a first class lever work at ?

Answer 34

It can work at either a mechanical advantage or mechanical disadvantage.

Question 35

Define a second class lever and give an example of one

Answer 35

It has the resistance located between the effort and the fulcrum as shown

e.g. the wheelbarrow and the nutcracker

Question 36

What mechanical effectiveness can a second class lever work at ?

Answer 36

A second class lever always works at a mechanical advantage.

Question 37

Define a third class lever system and give an example of one

Answer 37

It has the effort located between the fulcrum and the resistance

e.g. the fishing rod and tweezers

(the pics in biomechancis pg. 28 help understand the examples)

Question 38

What mechanical effectiveness can a third class lever system work at ?

Answer 38

A third class lever always works at a mechanical disadvantage.

Question 39

Do SAQ 17 pg.28&29, forces unit, mechanics binder

Answer 39

When tackling these questions draw out the basic lever system so that you can figure out the rotations going on (refer to binder and notes for what diagram I mean by this)

Also always define which rotation you are taking as positive and negative when starting a question