Midterm

Torque

Tendency for an object to rotate about an axis caused by force

• Also called moment of force

• Rotary force

• Even if object doesn’t move, torque may be present

• Dependent of perpendicular distance from line of action of force to axis of rotation

Definition of torque

Result of force applied a certain perpendicular distance from an axis of rotation

• torque is dependant on size of force and where it is applied

What is torque inflicted by

• line of action of the force

• Point of application

• Magnitude

Fixed axis of rotation

Axis does not move

• like on door hinge the forearm to your elbow axis

Free axis of rotation

Can move freely

• when dealing with the axis of rotation of your whole body

What is a lever used for

Multiple the technical force than can be applied to another object

Force arm

Perpendicular distance from line of action of applied force and fulcrum

Resistance arm

Perpendicular distance from line of action of the object force and fulcrum

Linear motion

Uniform motions of all parts at same speed

Angular motion

Rotation around an axis

• parts undergo same angular displacement but same linear displacement

General motion

Combination of linear and angular motion

• straight line - rectilinear

• Curved line - curvelinear

Mechanical advantage

• kinetic advantage

• Less force is needed to hold object up if force is applied further from fulcrum

  • Decrease in magnitude of force is directly proportional to the distance that the force was moved away from fulcrum

• More force is needed to hold object up if force is applied closer to fulcrum

First class lever

Applied force and resistance are on either side of fulcrum

• ex. Neck

Second class lever

The applied force and resistance are on same sides of forum

• force is further from fulcrum than resistance

• Kinetic advantage

• Ex. Wheelbarrow, ankle

Third class lever

Applied force and resistance are on same sides of fulcrum

• force is closer to fulcrum than resistance

• Kinematic advantage

• Ex. Most joints

Torque and bicycle gears

• low gear results in greater force transmitted to ground over smaller distance

• High gear results in less force is transmitted to the ground over greater distance

  • Larger ratio of front to rear gear

Levers and muscular force

Movement of limb segments is caused by muscular contraction attached a distance away from joint

• tendons indicated the line of action of muscles

• Bicep is a third class lever

• Muscle force closer to axis of rotation than resistance due to centre of mass of forearm

• Small displacement closer to the axis causes a much greater displacement farther away from axis

Moment arm

Perpendicular distance form line of action of force to axis of rotation of the object

Applying force through axis of rotation

If line of action passes through axis of rotation there is no perpendicular distance and no movement arm

• no torque

Determine direction of torque

Clockwise is negative

Counterclockwise is positive

Adding torque

If torque acting in opposite directions are equal object will not rotate

Static equilibrium

Forces are zero for a system as well as torque

Torque in human movement

Used whenever an implement or body is caused to or haas tendency to rotate

• reminds us of importance of knowing how much force to exert and where to apply

• Created by muscles to cause rotation of body about joint axis

• Movement about certain joints can cause movement of other joints by kinetic linking

• Forces and torques produced about a port of body will be transferred through other joints and segments

Muscular torque

• Movement of limb can cause change in orientation of muscle force vector relative to limb and change moment arm

• Same force will produce different torque depending on position of arm

  • How we are stronger at certain parts

Projectile motion

Acceleration on an object is constant - uniform acceleration

• acceleration due to gravity is constantly downward

  • Flight path will follow parabola - symmetric

Throwing

Need more force

• lower magnitude when lobbed and increased vertical component

• Higher magnitude when whipped and not as large vertical component

Parabolic path

Once released only force is force of gravity

Looking at x-component: motion will remain constant - never change

Vertical velocity will increase until zero at peak and then negative

• final velocity is negative of positive velocity

Due to downward acceleration and upward velocity

Features of projectile

1. Apex (vert = 0)

   1. Highest point achieved

   2. At ½ total flight time

2. Vertical component at beginning is equal and opposite to end of flight

projectile motion deterministic model

• want to break down all components than can affect projectile motion

  • Cover distance based on force

  • Lower landing height has longer travel time

  • Change velocity at take-off by having to change momentum

Impact of mass

• accelerates at same rate due to gravity but momentum is affected

  • Acceleration is dependant on mass

Angle of release

• determined by direction of velocity vector at take off

  • Determined by direction of force vector

• Can control through force application

• Angle for distance is different than for height

  • Optimal for height is 90º

  • Optimal for distance is 45º

Limitations of human movement

• can alter optimal take-off angle

• Speed vs. take-off angle

• Faster horizontal speed results in shallower take-off angle

Easier to create horizontal velocity than vertical velocity

Relative take off

• affected by changing levels of take-off and landing surfaces

• Change position of centre of gravity

Air resistance

Changed by surface and shape of object

• still acts similarly

Acceleration due to gravity

Changed by position relative to earth core

• more affected by distance than mass

Kinematics

• study of motion exclusive of influences of mass and force

  • Displacement, velocity and acceleration

Linear motion

Uniform motion of all parts going same speed

Angular motion

Rotation around an axis

• parts undergo same angular displacement but same linear displacement

General motion

Combination of linear and angular motion

• curvilinear: curved displacement

• Rectilinear: direct displacement

Deterministic models

• used to consider mechanical forces and how they link to successful performance of a skill

  • Results of performance and factors that produce results

Format of flow chart

• top of model is goal

• Variables at bottom can easily be changed to impact performance

• Model is subjective

• Only incorrect if mechanical relationship does not match

Evaluating performance

• observe and describe how to improve

• Look at every factor of performance and how they can. Be changed to improve performance

• Determine a final list based on what can be changed now

Establish priority

1. Set up guidelines

2. Make biggest improvement in shortest time

3. Knowledge of participant

   1. When to introduce correction

   2. How to instruct performer

   3. What type of practice is required to correct error

Free body diagram

Isolated drawing of an object that is assumed to be rigid

• observational tool

Benefits of free body diagrams

• less distractions

• Can focus on critical joint movement and posture

• Rigid segments where forces act

Limitations of FBD

• lose context - rotation

• Perspective

• Depth

• Range of motion

• Subjectivity (of choosing joint centres)

Vector

• arrow connecting two points

• Direction

• Magnitude

• Point of application

What can vectors represent

• forces

• Motion

• Displacement (vs distance which is scalar)

A vector can be ___ into _____

Broken down

Components

Adding vectors

• add tip to tail - then draw resultant tail to tip

• Break down into components and add those to determine resultant

Force

A push or pull

Force

• push or pull

• Vector quantity

• Can cause change in state of motion of an object (accelerate/decelerate)

• Comes in Paris (action and reaction)

Important qualities of force vectors

• magnitude (length of arrow)

• Direction (relative to an axis)

• Line of action (shaft)

• Point of application (where it acts)

Internal force

• acts within an object or system

Ex. Compressive force when landing

Tensile force: pulling

Tensile force

• internal pulling force

• Tension in soft tissue (muscle) when pulling

Compressive force

Internal pushing force

Ex. Joint resists being compacted when landing

External force

• force acts on an object as a result of the surroundings

• Contact and non-contact

Non-contact forces

• gravity

• Magnetic

• Electrical

Gravity

Non content force that causes us to accelerate towards earth

• we use force to overcome gravity

Contact forces

• occur between objects in contact with each other

• Solid or fluid

• Air resistance and water resistance are fluid

• Normally resolved into one component that is perpendicular to contact surface and one is parallel

Normal force

Reaction force from the surface acting on the block that is equal and opposite to the component of gravity

Parallel surface

Tendency to move object along surface

Friction

Proportional and perpendicular to normal force

F=uN (where N is magnitude of normal)

• acts opposite to parallel force

• Result of interaction of surface molecules

• Not affected by size of contact area

Static friction

Two objects in contact are not moving relative to one another

• force friction = external force until object starts moving

Dynamic friction

Two objects begin to slide relative to one another

Trade off between pressure and area

Pressure = force/area

Force (normal) is dictated by mass of object

Two shoes with same mass, different surface area will have same force of friction (if they have same coefficient of friction/normal force)

Action and reaction force

Equal and opposite hold joint in place

Stress strain relationship

Bones, tendons, ligaments and muscles can withstand forces (stress) and can be deformed (strain) but only to a certain point

• elastic bounces back

• Plastic - permanent change to a structure

• Failure

Stress strain relationship in bone

Strong in vertical compression and weak in torsional

Stress strain relationships in ligaments

Good in tension, not good in compression and tear easily in torsion

Force motion principle

States that it takes unbalanced forces (and the torques they induce) to create or midday our motion

Summation

forces through the lower limbs are transferred and combined with forces in the upper limbs to enable the performer to jump and shoot

• kinetic linking

internal forces in your body can add together to act on an external object to move the object or your body

Kinetics

• concepts of mass, force and energy as they affect motion

Mass

Mechanical relationships depend on mass - measure of matter

Matter

Commonly defined as the substance physical objects are made of - needed for physical interaction

Inertia

Resistance to change in a state of motion

• greater mass = greater inertia

• In motion or at rest

Force

A push or pull acting on a body that causes or tends to cause a change in a motion to the body

Characteristics of force

• magnitude

• Direction

• Point of application

• Line of action

Newtons 1st law

Law of inertia

• an object in uniform motion stays in uniform motion and an object at rest stays at rest unless acted upon by a net external force

When is an object at rest

No motion and net forces acting on it sum to zero

• horizontally or vertically

Static equilibrium in rest

Uniform motion

• object at a constant velocity will maintain its velocity unless accelerated or decelerated

  • Requires unbalanced force

Momentum

Tendency of an object to resist change in motion, during motion and continue to move in its direction of travel

= mass X velocity

Will have magnitude and direction

Conservation of momentum

The momentum in any system is always conserved

• objects that collide

Sum of momentum before = sum after

Types of collisions

• elastic

• Static

Elastic collision

Two objects collide and bounce off of one another

Inelastic collision

Two objects collide and behave as one object after collision

Coefficient of restitution

• bouncyness factor

• Quantifies how elastic collisions are

  • Most collisions are neither perfectly elastic or inelastic

Absolute value of the ration of velocity of separation to the velocity of approach

Perfectly elastic COR=1

Inelastic COR=0

COR is determined by

Nature of both object

• height of rebound can be used to determine COR in balls

Newton’s second law

Law of acceleration

F=ma

• if net force applied, object will accelerate in direction of net force and acceleration will be proportional directly proportional to net external force and inversely proportional to its mass

Force of gravity and weight

• we know that gravity only acts vertically downward so we only need talk about force

  • F=ma

• The force of gravity causes an acceleration of 9.81m/s2

  • F=mg

• The force of gravity is equal to the persons weight

  • W=mg

Force and momentum

Force can change the velocity of an object, so can be used to change the momentum

Impulse

The measure of the amount of time a force is applied

The average net force action over some interval of time will cause a change in momentum of an object

How do we change velocity of an object such as in sports

Use impulse to change momentum

Larger force over shorter time or smaller force over longer time

Newtons third law

The law of action and reaction: for every action there is an equal and opposite reaction

If A exerts force on B, then B exerts the dame force on A but in an opposite direction

(Forces are not necessary cancelled out because they can be on different objects)

Anatomical terms for body position

• proximal and distal

• Superior and inferior

• Medial and lateral

6 potential degrees of freedom

3 translational (heave, surge, sway)

3 rotational (pitch, yaw, roll)

Pitch

Tilting about y-axis

• flexion and extension

Yaw

Tilt about x-axis

• adduction and abduction

Roll

Tilting about z-axis

• rotation

Heave

Up and down - scapula and hips