Mechanics/ biomechanics - unit 1 deck 5 Flashcards
When considering moments and levers what are we restricting them to acting in ?
To acting in static situations only
Define what a the moment of a force is
This is the tendency of a force to produce a rotation about an axis
State another name for the moment of a force
Torque
What is the moment of a force the product of ?
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
What are the SI units of the moment of a force
Newton metres (N m).
Why are moments particularly important in biomechanics ?
Because the action of muscles produce moments about the centres of joints
State the equation for calculating a moment
M = F d
- M = the moment
- F = the force
- d = the moment arm
What can the positive direction of a moment be determined by?
- 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
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
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
- 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
State the conditions for true static equilibrium to occur
The sum of all the external moments and forces acting on a body must be equal to zero
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
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
What will happen if the sum of all external moments acting on a body do not equal to zero?
The body will angularly accelerate
State for a rectangular reference frame what the sum of all the external moments in each axis equal
ΣMx = 0; ΣMy = 0; ΣMz = 0
ΣMx is the sum of all the external moments acting about the x-axis ΣMy is the sum of all the external moments acting about the y-axis ΣMz is the sum of all the external moments acting about the z-axis.
Do worked example on pg. 24&25, forces unit, mechanics binder
Do SAQ 16 pg. 25, forces unit, mechanics binder
Describe what a lever system is
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
Define what the fulcrum is
This is the point against a lever on which is turns
What do the effort and resistance forces produce in a lever system?
They produce moments acting about the fulcrum
Define what both the effort arm and resistance arm is in a lever system
- 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)
Give an example of a lever system
The use of a crowbar to lift a heavy object (or force open a door)
Describe lever systems present in the body
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.
What can be expressed by calculating the mechanical advantage of a lever system ?
The mechanical effectiveness of a lever system
State the equation for calculating the mechanical advatange of a lever system
MA = df / dr
- MA = mechanical advantage
- df = force-fulcrum distance (effort arm)
- dr = resistance-fulcrum distance (rassistance arm)
A lever system can serve one of what two purposes?
To amplify the effort force or to amplify the distance moved through
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?
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)
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?
A mechanical disadvantage
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
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.
What does a mechanical advantage of > 1 and < 1 indicate ?
- 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.
Why do muscles in the human body usually act at a mechanical disadvantage ?
Because they are usually inserted (attached) close to a joint.
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 ?
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
What are the 3 classes of lever systems and what are these classes all dependent on
First, Second and Third class lever systems
These are all dependent on the positions of the two forces relative to the fulcrum.
Define a first class lever system and give some examples
The fulcrum is located between the effort and the resistance
e.g. crowbar, scissors and see-saw
What mechanical effectiveness can a first class lever work at ?
It can work at either a mechanical advantage or mechanical disadvantage.
Define a second class lever and give an example of one
It has the resistance located between the effort and the fulcrum as shown
e.g. the wheelbarrow and the nutcracker
What mechanical effectiveness can a second class lever work at ?
A second class lever always works at a mechanical advantage.
Define a third class lever system and give an example of one
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)
What mechanical effectiveness can a third class lever system work at ?
A third class lever always works at a mechanical disadvantage.
Do SAQ 17 pg.28&29, forces unit, mechanics binder
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
