KINE 3050 : Torque

Centric forces are those that act

through an objects center of mass

What forces will only result in linear changes in position?

Centric Forces

Eccentric Forces are those that are

not directed through a objects center of mass

What force will cause linear and angular(rotational) changes in position

Eccentric Forces

The turning effect of an eccentric force is called

torque

In order for rotation to occur, what type of force is required ?

Eccentric Force

What is torque?

Vector

The rotational effect of an eccentric force

Units: Nm

Moment arm is the

perpendicular distance between the axis and the line of force application

Torque is changed throughout the movement because what changes?

the moment arm

What is a lever

Whenever a movement occurs around an axis

• can be a set of rigid segments with an axis

• Many bones’ primary purpose is to act as one

What are the 3 main components for a lever system?

1. External force, load or resistance (weight of our bodies)

2. Internal Force, Effort or Muscle Force (how we move the load)

3. Fulcrum, Axis, or Pivot Point (often a joint)

What is a class 1 lever?

The internal and external forces are on opposite sides of the fulcrum

• ex) external resistance (weight of head) acting on one side of the fulcrum (alanto-occipital joint) and the internal force (all of the neck extension muscles) acting on the other side

What is a class 2 lever?

have the fulcrum and internal force on opposite sides of the external force

• internal and external forces are on the same side of the fulcrum, but the distance from the fulcrum is greater for the internal force

  

Is it easier to create torque when the distance from the fulcrum is greater for the internal force?

true

Class 3 Lever is

opposite of Class 2

• fulcrum and external force is on either sides of the internal force

• the distance between external force and fulcrum is greater than internal force (makes it harder to overcome)

What is the mechanical advantage?

ratio of the moment arm distance of internal force to the moment arm distance of external force

When does mechanical advantage happen?

when internal moment arm size is > the external arm size

When does mechanical disadvantages happen?

when external moment arm size > internal moment arm size 

What are some advantages of class 3 levers?

• can move lighter loads larger distance

• give ability to throw objects

• run fast

What is the most common lever class system in the human body?

Class 3

What lever class is the push up exercise?

Class 2

What lever class is a calf raise exercise ?

Class 2

What is the most mechanically advantaged lever system?

Class 2

A small child named Jeff, who is 25kg, sits on a seesaw that has a length of 8m and an axis in the middle. There are 4m on each side of the axis. He sits 3m on the right side of the axis. How much torque is he producing?

τ = F× d

Step 1: Find Force

F = mg

F= 25 × 9.8 = 245N

Step 2: Find Distance from axis

d= 3m

Step 3: Solve

τ = 245N x 3m = 735N

Now Jeff has a friend, Shelly, but her weight is greater than his. She weighs 700N and Shelly is also sitting 3m from axis but on the opposite side. What is the net torque on the seesaw now ?

What we have : τJeff​= 735N

Step 1: Find torque produced by Shelly

700 × 3 = 2100N

Step 2: Find the Net Torque

τnet​ = τJeff ​− τShelly

735N - 2100N = -1365N

(it is negative because Jeff was positive/clockwise and Shelly was negative because she was counterclockwise )

Based on the Jeff and Shelly problem, how could we create equilibrium (net torque = 0Nm)

Moving Shelly closer to the fulcrum

Muscles generate

torque to cause movement in humans

Muscle actions create?

tension

Since muscles are not located directly on top of joints(axes), they have what?

moment arms

Muscle tension x muscle moment arms = torque at the joint that the muscle crosses

Muscular torque causes our limbs to

rotate about a joint axis

In the weight room, you pick up a 98.1 N dumbbell (10kg x 9.81) so you can hammer out some curls before hitting bench. The dumbbell in your hand is 0.25m from your elbow joint. Your forearm weighs 60 N and its center of gravity if 0.15m from elbow joint. If your bicep attaches 0.03m from the elbow joint, how much force does it need to produce to counteract the forces applied with you elbow flexed at 90 degrees?

What we have :

Dumbbell : Force (98.1N) Distance (0.25m)

Forearm: Force (60N) Distance (0.15m)

Biceps: Force (?) Distance (0.03)

Step 1: Write equation

Fbicep​⋅dbicep​ = Fforearm​ ⋅ dforearm​ + Fdumbbell​ ⋅ ddumbbell​

Fbicep​⋅0.03 = 60 ⋅ 0.15 + 98.1 ⋅ 0.25

Step 2: Complete Torques

60⋅0.15=9N

98.1 x 0.25 = 24.525N

9+24.525=33.525N

Step 3 : Solve for Fbicep

33.525 / 0.03 = 1117.5N

What may be the bigger factor in humans where we mostly have 3rd class levers?

force

Axis of rotation is the point

around which something rotates

Center of mass is the point where the entire

mass of an object is assumed to be concentrated

Center of gravity is the point where mass is evenly

distributed and where the force of gravity acts 

When standing stationary in the anatomical position, the center of gravity is near?

L2-L3 *roughly 55-57% of height)

• it will move as we move

From a biomechanics perspective, COG is the

point on the body that follows Newtons Laws of Motion

The location of COG plays a role in

stability, mobility, and balance

What influences torque?

Force and Moment Arm