Lecture 9, Defining Torque - Rotary Influences Flashcards
Centre of Gravity
the point around which a system’s weight is equally distributed in all directions
- also referred to as the centre of mass - point around which the masses are balanced (same amount of force above or below, left or right, in front or behind)
- the position of the centre of gravity determines how the body responds to external forces (i.e., control)
- is measurable
- for the average person the centre of gravity is around the belly button
- centre of mass and gravity are the same thing - mass (kg) is not affected by gravity but weight (N) is, if the centre of gravity is where all the forces are balanced then the centre of mass is the point at which all the mass are balanced (just referencing two different things as one is taking into account the quantity of you and how much stuff you are made off and the other is taking into account matter, attraction due to gravity and therefore all the forces acting upon a system)
Locating the Center of (Gravity) Mass
not a fixed position
- may be outside of the physical body
- force platforms - have someone stand on it and find their centre of gravity - usually on the ground is where it is measured but we can estimate it for other areas
- centre of gravity moves as we move as humans - as you move it changes ever so slightly (if you put ur arm up you put more force on top so centre of gravity will go upwards as will)
- not found inside the system necessarily but it can be measured (can be found outside)
a simple balance procedure using a wooden board to locate the position of the centre of gravity
- see where you are level
Forces and Movement (what happens when a force acts directly at the centre of gravity?)
when a force acts directly at the centre of gravity, it will produce translation only
- there is no change in its orientation (will only create rectilinear motion meaning translation)
- centric forces at the centre of a system (goes straight through the centre of gravity)
- centric forces result in rectilinear motion only
Forces and Movement (what happens when a force acts away from the centre of gravity?)
when a force acts away from the centre of gravity, it will produce some translation and rotation
- if you are off centre (does not matter how much) you are going to see some change in position but also in orientation
- eccentric forces act off centre on a system (eccentric muscle action and eccentric forces are both different
- eccentric forces result in rotation and curvilinear action (the more off centre you are the more rotation you will see)
- both centric and eccentric give rise to some kind of translation
Forces and Movement (3)
when two forces that are equal in magnitude but opposite in direction, act equidistant from the centre of gravity, the result is a pure moment (rotation)
- when two forces act on a system (both off centre) in opposite directions at the exact same magnitude will produce rotation
- although there is no net force, the two forces act to produce rotation (no change in position, remain anchored in spot but it is still spinning around centre of gravity)
- a couple is two eccentric forces that cause rotation ONLY
- moment means dealing with rotation (moment = rotation)
Forces and Rotation - any eccentric force will cause rotation but how does a force (linear) create rotation (angular)?
- force is a push or pull
- for that rotation to occur something else must be present
- the system must be “anchored” at a point
- axis of rotation, fulcrum, or pivot point (point when there is another force acting holding the object down or when two objects lock) - spin about that point but it will cause rotation
- a force applied away from the anchor point will rotate the object about the axis (prevents it from changing its position)
Forces and Rotation (torque)
- a force is a push or a pull (that can produce a linear movement)
- when the push or pulling influence acts at a point away from an axis of rotation, it can cause a rotational movement
- a torque is a force that has a tendency, to rotate an object about an axis
- other names for a torque include; rotational force, moment of force, or moment (moment = dealing with rotation)
- a torque can be thought of as a twist to an object
- a force that causes rotation is called torque (for torque to exist we need linear force and axis of rotation)
- a force can create, change or prevent movement
Defining Torque
A torque (or moment) is a force that may produces rotation
- a torque can create, change, prevent rotation
- the effect of the torque is dependent upon:
◦ the magnitude of the force: bigger force bigger rotation/effect - the larger the applied force, the bigger the rotational change (cannot forget anchor point as it is what keeps in locked in place)
◦ point of application: the distance between the force and the axis of rotation
◦ direction: the perpendicular distance from the force arm to the axis of rotation - are you pushing at an angle or attacking it straight one - calculated in a specific way
Calculating Torque (units)
Torque (T)
Torque = Force x Moment arm
T = Fd⟂
units: Nm
d⟂ = moment arm
d⟂ = the perpendicular distance between the line of action of the force and the axis of rotation
- how we calculate a torque takes into account size of force, where that force is and the direction that force is applied
- torque will take into account force, angle at which it acts and the distance (for formula)
Who Gives a Darn About Torque? (squash players)
squash players frequently slide their hand up and down the racket, changing the grip depending on the shot they want to play
- sliding the hand up decreases power but increases control
- sliding the hand down sacrifices control but increase power
- as you increase the distance you magnify the effect you want to see
Who Gives a Darn About Torque? (physiotherapists)
physical therapists use manual resistance to assess injuries
- therapist applies a torque to the elbow
- patient tries the action be generating a counter torque
- therapist can vary torque by changing the position of the hand on the arm
applying torques to assess strength and pain-free range-of-motion
- less resistance when they apply closer to elbow but if they want to exert more they will reach higher up by increasing distance from where the force is applied to the axis or rotation - when you shorten the moment arm there is less resistance
Examples of How Torques Are Used
torque created by a pulling force on the doorknob causes the door to swing open
- what would happen if the doorknob was moved closer to the door hinge (axis of rotation)?
- handle is put on opposite side away from the hinges, by pushing further away from the axis of rotation you magnify the effect of what you do (you can move heavier objects)
Levers
a relatively rigid object that may be made to rotate about an axis by the application of a force
- levers MAGNIFY the effect of a muscle
- an arrangement between a rigid body and a couple of things
- external levers or they can be found inside your body such as bones (rigid segment) - it is attached somewhere (the pivot point)
- the thing that we are moving and where they are applied against the machinery (you or the thing you are trying to move)
- effect can be one of two things - where the components are placed are what changes the effect it has as the components remain the same
Levers (what is consists of?)
there are three kinds of levers: first class, second class, and third class
- classified by the relative positions of the forces to the axis of rotation
each lever consists of:
- the rigid body: the system being moved
- the fulcrum: the axis of rotation or pivot point (rigid body has physical connection to bones or for example you are pushing one end of the table while the other side is pivoted)
- the effort: the input force or the applied force (where the force is applied, the work you are doing)
- the resistance: the output force (what it is you are moving)
the effectiveness of the lever is determined by the mechanical advantage (movement advantage)
- the ratio of the output force to the input force
- measured by the distances between the forces to the axis of rotation
- where the force is applied and resistance is in respect to axis of ration will determine movement advantage (compare both and create ratio)
Mechanical Advantage (big #)
a force can balance a larger resistance when the force arm (the distance between where the force is applied to the axis of rotation) is longer than the resistance arm (the distance between where the resistance is and the axis of rotation)
as mechanical advantage increases the lever is built for strength, but sacrifices speed
- the ratio that we calculate is the force arm over the resistance arm
- force arm/resistance arm = 2/1
- if that # is big then that lever is built for strength (the bigger the number the stronger you become - bigger or further force is from resistance)
- but there is a tradeoff as you can lose speed and range of motion
