L7 - Angular Kinetics

Angular kinetics

branch of kinetics that deals with the causes of angular motion

Torque

• a force that causes rotation (vector)

• force is applied at a distance from the pivot point/axis of rotation

Moment of inertia

• quantity that describes angular inertia

• an object resistance to change in angular momentum

Moment of inertia represents the resistance to angular acceleration based on both mass and the distance the mass is distributed from the axis of rotation

True

It is more difficult to speed up/slow down the rotation of an object with less angular inertia

False, more angular inertia

For any one axis of rotation, only one moment of inertia is associated with that axis

True

A rigid object has many different moments of inertia because it may have many axes of rotation

True

Why is human’s moment of inertia about any axis variable

• Human body is not a rigid object

• Limbs move relative to each other

• Movements may change the distribution of mass about the axis of rotation

• Allows manipulation of moment of inertia

Distribution of mass along a vertical axis is large

False, small

Distribution of mass along a transverse axis is large

True

Human motion often requires multiple segments to be rotating

True

Propulsive torque

increases the velocity of the rotation in the direction of the rotation

Breaking torque

decreases the velocity of the rotation in the direction of the rotation

Torque will always be in the same direction as the motion of the rotating body

False, will not always

Net torque

the effect of the sum of all torque vectors acting on a body is proportional to the change in angular velocity

Positive net torque

increase in angular velocity

Negative net torque

decrease in angular velocity

Zero net torque

no additional movement occurs (torque is balanced)

Angular momentum

the quantity of angular motion possessed by a body (moment of inertia x angular velocity)

How do you determine the direction of angular momentum

right hand thumb rule

The angular momentum of a system/object remains constant unless acted on by an external torque

True

What happens when annual momentum is conserved

• there is a trade off between moment of inertia and angular velocity

• these two quantities can interact while conserving angular momentum

Increasing the mass moment of inertia will increase the angular velocity at which an object is spinning

False, will decrease velocity

Angular impulse

product of torque and the time over which it’s applied = change in angular momentum

When two objects interact, the torque exerted by object A on object B is counteracted by a torque equal and opposite exerted by object B on object A

True

Center of mass

• the point about which the mass is evenly distributed

• the balancing point of the body

• the point about which the sum of torques equal zero

Center of gravity

the point about which the mass is evenly distributed in the vertical direction

If a force is applied at the axis of rotation, the object will not rotate

True

What is a torque regularly referred to as

moment/moment of force

Moment arm

the perpendicular distance from the pivot point/axis of rotation to the line of action of the force

The greater the lever/moment arm, the greater the mechanical disadvantage

False, greater advantage

Force couple

• pair of equal and opposite forces that produce torque in the same direction

• both have equal magnitude and their effects on the body are added together (even though they’re on opposite sides of the axis of rotation)

Examples of torque in the human body

• The product of muscle tension and muscle moment arm produces torque at the joint crossed by the muscle

• Moment arm for a muscle is the perpendicular distance from the muscle line of action to the joint centre

• Moment arm for a muscle will change as the segment moves through the range of motion

Joint torques

• produce movement of body segments

• muscles pull on points across a centre of rotation (joint) which creates torque

Agnostic and antagonistic muscles work so the change in joint position is a result of net torque

True

Joint torques can be measured directly or indirectly

True

How to measure a joint torque directly

place a strain gauge in the muscle

How to measure a joint torque indirectly

• electromyography (EMG)

• isokinetic device

Lever

A rigid body that is used in conjunction with a pivot point, or axis of rotation, to multiply the force applied to another body

Levers increase the mechanical advantage and allow us to apply relatively small force to move a greater resistance

True

In a lever system, a force further away will have a mechanical advantage over the other force because either creates greater torque for the same amount of force

True

Components of a lever system

• fulcrum: axis of rotation of system (rotation occurs around fulcrum)

• applied force: amount of force applied to lever (used to rotate some resistance around fulcrum)

• resistance: amount applied to lever system that opposes applied force (weight)

Moment arm

• perpendicular distance between location of applied forecasting and axis

• shortest distance from axis of rotation to line of action

• distance from any force that produces torque about the axis of rotation

Resistance arm

distance from the axis of rotation (fulcrum) to the point of resistance application

Describe how lever systems are used in the human body

• joint = fulcrum (axis of rotation)

• bones = rigid segment that rotates about axis (hold, push, pull on object)

• muscles = contract to apply force to system

What are the different functions of lever systems

• balance 2 or more forces

• change direction of applied force

• favour speed and ROM

• favour force production

All lever systems will have an axis (A), resistance (R), and force (F) in three different arrangements

True

First class lever

• arrangement: FAR

• function: balance two forces and change direction of applied force

The cervical spine and cranium us an example of a second class lever

False, first class

Second class lever

• arrangement: ARF

• function: favours force production (bc force arm id always greater than resistance arm)

Which lever class is least common in the body

second class

The talocrural joint, used in planar flexion and dorsiflexion, is an example of a second class lever

True

Third class lever

• arrangement: RFA

• function: favours speed and ROM

Which lever class is most common in the body

third class

Which of these joints is NOT an example of a third class lever: humeroulnar, talocrural, tibiofemoral, coxofemoral

talocrural

A ratio of force arm to resistance arm greater than 1 is mechanically advantageous

True

A ratio of force arm to resistance arm less than 1 is mechanically disadvantageous

True

A ratio of force arm to resistance arm equal to 1 is mechanically advantageous

False, it’s a balanced lever system

Example of lever system with a ratio of force arm to resistance arm equal to 1

first class

Example of lever system with a ratio of force arm to resistance arm greater than 1

• first class

• second class

Example of lever system with a ratio of force arm to resistance arm less than 1

third class

Static equilibrium

• systems at rest or moving at a constant velocity

• all torques in system must sum to zero

Stability

resistance to both linear and angular acceleration

Balance

the ability of an individual to assume and maintain a stable position

Angular impulse is equal to the change in angular momentum

True

Angular power

the angular work done per unit of time