KIN 321 Biomechanics of Human
Movement Exam 2
Inertia - Answer--Property of an object that resists changes in motion
Angular Inertia (rotary inertia) - Answer--Property of an object to resist changes in its angular motion
-Directly proportional to the mass of an object
+Ex: Think about who flips easier- gymnast or football lineman?
-In addition, it is affected by how mass is distributed relative to the axis of rotation
+Ex: Think about what is easier to stop rotating- a baseball bat by itself or a bat with a weight on the
end?
**depends on mass and distribution of mass**
Moment of inertia - Answer--The quantity that describes angular inertia
-Unit: I
-Equation: Ia= ∑miri^2 OR Ia= ∑mka^2
,Manipulation of moment of inertia for a human: - Answer--The moment of inertia of a human can be
manipulated by changing mass distribution which can be done by changing limb position.
-We manipulate moment of inertia for sports and performance
-Ex: Gymnast doing pike, tuck, and layout. In sports, such as skier's equipment is modified- longer
downhill skiers have longer skis than slalom for stability.
Radius of Gyration - Answer--Length measurement representing how far from the axis of rotation all of
the object's mass is concentrated to create same resistance to change in angular motion as the object
had in its original shape
Moments of inertia and linear velocity relationship - Answer--Longer implements mean larger linear
velocity at the striking end. Therefore, as length increases, the moment of inertia increases.
Angular momentum - Answer--The product of angular analog of MASS (moment of inertia) times the
angular analog of linear VELOCITY (angular velocity)
-Equation: Ha= Iawa
**Direction of angular momentum is the same as the direction of angular velocity that defines it**
**Angular momentum depends on the moment of inertia and angular velocity -for rigid objects, only
angular velocity can change**
Angular momentum of the human body - Answer--If the body is a rigid object, all segments rotate with
the same angular velocity
-Some limbs may rotate at different angular velocities, therefore, the sum of the angular momenta of all
body segments gives the approximate angular momentum of the entire body
-Ex: Running- Arms and legs are rotating in opposite direction and cancel each other out and trunk
doesn't rotate. So the total angular momentum of the body is zero
, Angular impulse - Answer--The change in angular momentum
**In most sports, the athlete causes a change in angular momentum of the entire body or implement or
individual body part. Larger external torque acting over longer duration causes a larger change in
angular momentum. Larger torques come from longer moment arms or longer time application of
torque**
Manipulation of Angular impulse and Angular momentum - Answer--Changing the angular momentum
and the angular impulse is necessary to many sport skills and is achieved by the manipulation of the
entire body, individual body parts, or implements.
-In the case of a gymnast or dancer performing a spin, the athlete must extend their foot (limb) away
from the body (axis of rotation/the opposite foot) and utilize it to create a torque via frictional force
between the foot and the floor. In essence, the larger the distance between the axis of rotation (planted
foot) and the extended foot (limb), the larger the moment arm and thus, creating an immense torque.
-Torque is prolonged when the moment of inertia is large. The large moment of inertia slows the spin,
resulting in a large impulse and change in momentum. Once the dancer or gymnast removes their
torque-initiating foot from the floor, they can reduce their moment of inertia to increase angular
velocity.
**In most sports, the athlete causes a change in angular momentum of the entire body or implement or
individual body part. Larger external torque acting over longer duration causes a larger change in
angular momentum. Larger torques come from longer moment arms or longer time application of
torque**
Newton's first law of motion: Angular interpretation - Answer--The angular momentum of an object
remains constant unless a net external torque is exerted on it
-Example: When the aerial skier adducts her arms during the helicopter 1080 trick, she reduces her
moment of inertia and increases her angular velocity about the longitudinal axis. Therefore, speeding up
her rate of spin, since she is airborne and angular momentum is conserved. During the abduction phase,
right before the landing, she increases her moment of inertia, thus decreasing her angular velocity along
the longitudinal axis. This will allow her to slow down her rate of spin and improve her chances of not
falling when she lands.
Movement Exam 2
Inertia - Answer--Property of an object that resists changes in motion
Angular Inertia (rotary inertia) - Answer--Property of an object to resist changes in its angular motion
-Directly proportional to the mass of an object
+Ex: Think about who flips easier- gymnast or football lineman?
-In addition, it is affected by how mass is distributed relative to the axis of rotation
+Ex: Think about what is easier to stop rotating- a baseball bat by itself or a bat with a weight on the
end?
**depends on mass and distribution of mass**
Moment of inertia - Answer--The quantity that describes angular inertia
-Unit: I
-Equation: Ia= ∑miri^2 OR Ia= ∑mka^2
,Manipulation of moment of inertia for a human: - Answer--The moment of inertia of a human can be
manipulated by changing mass distribution which can be done by changing limb position.
-We manipulate moment of inertia for sports and performance
-Ex: Gymnast doing pike, tuck, and layout. In sports, such as skier's equipment is modified- longer
downhill skiers have longer skis than slalom for stability.
Radius of Gyration - Answer--Length measurement representing how far from the axis of rotation all of
the object's mass is concentrated to create same resistance to change in angular motion as the object
had in its original shape
Moments of inertia and linear velocity relationship - Answer--Longer implements mean larger linear
velocity at the striking end. Therefore, as length increases, the moment of inertia increases.
Angular momentum - Answer--The product of angular analog of MASS (moment of inertia) times the
angular analog of linear VELOCITY (angular velocity)
-Equation: Ha= Iawa
**Direction of angular momentum is the same as the direction of angular velocity that defines it**
**Angular momentum depends on the moment of inertia and angular velocity -for rigid objects, only
angular velocity can change**
Angular momentum of the human body - Answer--If the body is a rigid object, all segments rotate with
the same angular velocity
-Some limbs may rotate at different angular velocities, therefore, the sum of the angular momenta of all
body segments gives the approximate angular momentum of the entire body
-Ex: Running- Arms and legs are rotating in opposite direction and cancel each other out and trunk
doesn't rotate. So the total angular momentum of the body is zero
, Angular impulse - Answer--The change in angular momentum
**In most sports, the athlete causes a change in angular momentum of the entire body or implement or
individual body part. Larger external torque acting over longer duration causes a larger change in
angular momentum. Larger torques come from longer moment arms or longer time application of
torque**
Manipulation of Angular impulse and Angular momentum - Answer--Changing the angular momentum
and the angular impulse is necessary to many sport skills and is achieved by the manipulation of the
entire body, individual body parts, or implements.
-In the case of a gymnast or dancer performing a spin, the athlete must extend their foot (limb) away
from the body (axis of rotation/the opposite foot) and utilize it to create a torque via frictional force
between the foot and the floor. In essence, the larger the distance between the axis of rotation (planted
foot) and the extended foot (limb), the larger the moment arm and thus, creating an immense torque.
-Torque is prolonged when the moment of inertia is large. The large moment of inertia slows the spin,
resulting in a large impulse and change in momentum. Once the dancer or gymnast removes their
torque-initiating foot from the floor, they can reduce their moment of inertia to increase angular
velocity.
**In most sports, the athlete causes a change in angular momentum of the entire body or implement or
individual body part. Larger external torque acting over longer duration causes a larger change in
angular momentum. Larger torques come from longer moment arms or longer time application of
torque**
Newton's first law of motion: Angular interpretation - Answer--The angular momentum of an object
remains constant unless a net external torque is exerted on it
-Example: When the aerial skier adducts her arms during the helicopter 1080 trick, she reduces her
moment of inertia and increases her angular velocity about the longitudinal axis. Therefore, speeding up
her rate of spin, since she is airborne and angular momentum is conserved. During the abduction phase,
right before the landing, she increases her moment of inertia, thus decreasing her angular velocity along
the longitudinal axis. This will allow her to slow down her rate of spin and improve her chances of not
falling when she lands.
