Defining Torque

Pushing in the center causes what kind of motion?

Translation

Pushing off center causes what kind of motion?

rotation and curvilinear motion

the more off center you push

more rotation, less translation

Diff between center of mass and center of gravity

COM is the point around which the masses are balanced

• weighted average of the mass distribution

COG is the point where the gravitational force (weight) acts on the body

What is CoG? What does it determine?

The point where the gravitational force (weights) acts on the body

The position of the CoG determines how the body responds to external forces (i.e. control) 

Is COG a fixed position? Can i be outside of the physical body?

No, COG is not a fixed position as it can be outside of the physical body. 

Where’s COG when you raise your arms? When you bend over?

Raise hands = COG shifts up

Bend over = COG comes outside the body

How can we measure COG?

1. Using force plates

2. Using a balance board

What is centric force? What type of motion do they produce?

Forces that go thru COG = centric force

Centric forces will only do translation to object

i.e. centric forces result in rectilinear motion only

What are eccentric forces? What type of motion will eccentric forces produce?

Any force acting off center = eccentric forces 

Eccentric forces result in rotation (change in orientation) and translation (change in position) 

• Rotation and curvilinear motion

When does a pure moment occur?

When 2 ecc forces, equal in magnitude and opposite direction, act equidistant from the COG, there is a pure moment 

What is a couple?

Two ecc forces that cause rotation ONLY ie pure moment

There is net torque, but no net force

How does a force (linear) create rotation (angular)?

Two things needed for this to happen:

1. Need eccentric forces to produce rotation

2. Needs an anchor point (fulcrum, axis of rotation, pivot point)

Force is a linear concept, but force is acting off anchor point. Anchor point holds system in a place. So the linear force will cause the object to simply rotate about it’s axis

Define torque. What are some other names for it?

If force creates angular movement, we call that torque

It is the tendency of a force to rotate an object about an axis 

Other names: 

1. Rotational force

2. moment of force 

3. Moment

What are the three factors that determine the effect a torque will have?

1. Magnitude of force 

2. Point of application

3. Direction

Torque factor: Magnitude of force

The bigger the applied force, the bigger the rotation

Torque factor: Point of application

What’s the motion like as point of application increases

The further the distance between the force and the axis of rotation, the more rotation and less translation there is

Torque factor: Direction

The perpendicular distance from the force arm/line of force to the axis of rotation

Force is made up of vert and horizontal component. Only one of the forces will have an impact: the force that acts perpendicular to the system

Torque formula. Torque UoM. Scalar or Vector

Torque = Force x Moment arm

UoM: Nm 

Torque is vector or scalar? 

Torque is a vector quantity

Direction is defined by angular movement created:

• Negative = clockwise 

• Positive = Counterclockwise 

Define moment arm

The perpendicular distance from the axis of rotation to the line of action of the force

Where should you grasp the wrench?

The further away the force is applied (longer the point of application: distance between the force and axis of rotation), the more effective you are at using the wrench, the more rotation is produced by the torque 

How can you affect the output of force? Give a sports example

By changing the length of the moment arm

e.g. in squash, hold racket towards the end to increase power;

hold racket towards the head for more control 

What do PTs use torque for?

To assess strength and pain-free ROM

Explain doorknob positioning

Doorknob closer to hinges (axis of rot):

• Smaller moment arm → Less rotational force → harder to swing door

Doorknob farther from hinges (axis of rot)

• Longer moment arm → greater rotation force → easier to swing door open

What is a lever?

Lever is a tool used to magnify effect of force.

What does a lever consist of?

A lever consists of:

1. rigid body: the system being moved

2. fulcrum: the axis of rotation

3. effort: the applied force

4. resistance: output force

Define mechanical advantage. How is it measured?

The effectiveness of the lever is determined by the mechanical advantage.

• MA is a number that determines what sort of lever you have and benefit you get 

• Output force: input force 

It is measured by distance between the forces to the axis of rotation

Mechanical Advantage = Force Arm / Resistance Arm

What is a force arm

The distance between the force applied and axis of rotation

What is a resistance arm

Distance between where the resistance is placed and the axis of rotation

Mech advantage increase means? Decrease means?

Mechanical advantage increasing:

• Strength increases

• Sacrifices speed

Mechanical advantage decreasing: 

• Speed and ROM increases

• Sacrifices strength

What can a force do when the FA is larger than the RA?

A force can balance a larger resistance when the FA is longer than the RA

What can a force do when the FA is shorter than the RA?

A force can move a resistance through a larger ROM when the FA is shorter than the RA

Types of Lever: First-Class Lever

Mechanical advantage: None (Can be > or < than 1)

Position: Effort - Fulcrum - Resistance/Load

Practical example: seesaw, scissors

Anatomical example: Head-neck joint

Types of Lever: Second-Class Lever

Mechanical Advantage: Always > 1

• Makes you stronger 

• FA > RA 

Position: Fulcrum - Load/Resistance - Effort

Practical example: wheelbarrow

Anatomical example: Calves/ankle joint

Types of Lever: Third-Class Lever

Mechanical Advantage: Always < 1

• Prioritizes speed and ROM over force

• RA > FA

Position: Fulcrum - Effort - Load/resistance

Practical example: stapler, shovel

Anatomical example: Elbow joint

• Mechanically disadvantaged. Found in parts of the body used for fast movements

What type of force causes both rotation and translation?

Eccentric forces

Define mechanical advantage. What does mech advantage tell you about a system?

Mech advantage is used to determine the effectiveness of a the lever 

It is determined by the ratio of the output force (resistance/load) to the input force, and the moment arm (perpendicular distance btwn the force arm and axis of rotation) 

Mech adv > 1: 

• Lever is built for strength 

• Fulcrum is in the middle 

Mech adv < 1: 

• Lever is built for ROM and speed 

• Fulcrum - Effort - Resistance/Load

• RA > FA 

What advantages do third-class levers provide

Third-class levers magnify speed and ROM, sacrifice force 

FA < RA so force can carry resistance through a larger ROM and amplifies speed, reduces strength