Chapter 7, 8, & 9: Angular Kinetics, Fluid Mechanics, & Mechanics of Biological Materials

Angular Kinetics

Looks at the cause of motion.
Answers questions to why we perform certain movements.
-Why a discuss thrower spins.
-Why a long jumper "runs" in the air.

Angular Inertia

Property of an object to resist change in its angular motion.
Is directly related to the mass of an object.
-Heavier objects are harder to start or stop swinging.
Location of the mass of an object is in relation to the axis of rotation.
-The further out t

Radius of Gyration

Length that shows how far the axis is from CoG.
Radius of Gyration = K
Out of the two factors that influence angular inertia,
-This K is more important than mass.
--A longer bat is harder to stop/start than a heavier one.

Inertia and the Human Body

Human beings can move their limbs relative to each other, we can change our weight distribution around a longitudinal axis (our CoG).
-Figure skater video example.

Inertia and Linear Velocity

The longer the radius of gyration of the striking surface of an object, the more linear velocity is generated.
-Longer golf club
Remember linear velocity is caused by Ang. Acceleration(radius).
-However, the longer the implement, the more inertia it has (

Angular Momentum

Remember, linear momentum is the product of mass and velocity.
Angular momentum is the product of angular inertia (which is a product of mass) and angular velocity.

Angular Impulse

Remember, linear impulse is the product of force and amount of time of that force.
-Therefore, angular impulse is the product of force (in this case a torque) and the time of application.
Impulse impacts momentum.
-So the longer we can apply a torque, the

Fluid Mechanics

The forces that fluids exert on objects moving through them.
-We are most concerned with water and air.

Fluid Forces

-Buoyant Force
-Dynamic Force

Buoyant Force

1 o 2 types of fluid forces.
Force exerted on an object by a fluid environment being immersed in.
This force is related the size of the object and how much is immersed in the fluid.
Pressure- results from the force applied to the object immersed in the fl


The weight of the water causes pressure (force) on the object.
-It's a reaction force caused by the object displacing the fluid.
-Every meter equates to around 9800N of weight, equating to 9800N/m.

Buoyancy of the Human Body

Density- is a ratio between mass to volume (fat display).
-The density of water is around 1000kg/m.
In order to be buoyant, we must be less dense than water.
-Muscles and bones have densities greater than 1000kg/m.
-Fat is less than 1000kg/m.
-These densi

Dynamic Force

1 of 2 types of fluid forces.
Force due to an objects motion in a fluid.
Force due to motion.
Relative to the:
-density of the fluid (water or air)
-surface area of the object
-velocity of the object traveling in the fluid
Has to do with the principle of

Relative Velocity

The difference between the object's velocity and the fluid velocity.

Wind Assistance in Sports

A tail wind pushes the object/runners forward (+)
A head wind pushes the object/runners backwards (-).
Field Goal attempt
Running events
-Maximum legal wind assistance(2m/s)

Drag Force

1 of 2 components of dynamic fluid force.
Works in opposition to the relative motion of the object.
-It typically slows the velocity of the object.
Produced by two categories:
-Surface drag
-Form drag
Reducing this in sports can be done two ways:
-Make th

Surface Drag

1 of 2 categories of drag force.
Its known as skin friction.
-It's the friction between the fluid and surface area of the object.
Oftentimes we can not change the fluid (air/water).
-We focus on reducing skin friction.
---Materials of clothing

Form Drag

1 of 2 categories of drag force.
AKA shape drag.
The drag associated to how the objects shape interacts with the fluid it travels through.
-Aerodynamic or streamline shapes have less form drag acting on them.

Lift Force

1 of 2 components of dynamic fluid force.
Component of dynamic force that acts perpendicular to the motion of the object in relation to the fluid.
-Instead of opposing the motion of the object, it tries to change its direction.
Lift can be in any directio

Magnus Effect

A lift force caused by a spinning ball.
-The spinning ball creates resistance to the air, resulting with lift force pushing the ball down.
-Basketball shot
-Golf swing
-Tennis (downward)
-Soccer (side spin)
-Baseball pitch
-Table tennis video
-Bruce L

Stresses and Strains on the Body

Internal forces act on the body that
-Impact movement
-Impose strain (joints, tendons, bones, etc...)
Understanding these interactions can help prevent and evaluate the causes of injury.


When external forces act on the body.
-These forces are resisted by internal forces and cause deformations of the body.
Principle stresses include:


AKA normal stress.
Occurs when a force pulls apart in the direction of the force.
-This leads to deformation of the object.
Large tensile loads can lead to torn ligaments, muscles, etc...
Component of bone primarily responsible for its tensile strength?


This type of stress results when a load or force pushes the object more tightly together.
-The object will deform by shortening.
Component of bone primarily responsible for compressive strength?
Bone is strongest in resisting this type of stress


Force that acts perpendicular to the object in question (not a normal stress).
The object will deform by changing the orientation of the object's sides.
-Blisters on the skin, joint dislocations or shear fracture of a bone.

Bending Loads

One side compresses, one side stretches.
-The thicker the object, the more it can withstand bending.
The shear stress depends on the location of the forces relative to the center of the object.


This type of load is a type of multiple loads.
-Results from torques acting on the long axis of the object.
Shear stress occurs between the torques.
-Torn ligaments of the ankle and knee. (AstroTurf)


The quantification of the deformation of a material.
Two types:
-Linear strain.
-Shear strain.

Linear Strain

The result in the change of the object's length.
Produced from either compressive or tensile stresses.
Where is linear strain produced?
-Throwing a ball?
-Landing after jumping?

Shear Strain

Change in the orientation of an object as a result of the molecules sliding past each other.
Produced by either torsion or bending stresses.
Measured by the change in angle relative to the direction of the object.

The Stress-Strain Relationship

How far can a bone bend?
How much energy can a ligament absorb before breaking?
How far can a tendon stretch before it becomes permanent?
-Ligaments are less stiff and slightly weaker than tendons.
-Elastic Behavior
-Plastic Behavior

Elastic Behavior

Material that stresses under load but returns to original shape.

Plastic Behavior

After too much stress an object may experience some permanent deformation.

Which component of bone is primarily responsible for its compressive strength?


Which component of bone is primarily responsible for its tensile strength?


Bone is strongest in resisting what type of stress?

Compressive Stress

Bone is weakest in resisting what type of stress?

Shear Stress

Ligaments and tendons stiffness and strength comparison

Ligaments are less stiff and slightly weaker than tendons.