Chapter 4 - Biomechanics

Question 1

1st Law: Law of Inertia

Answer 1

a body remains at rest or in constant linear velocity except when compelled by an external force to change its state
- force is required to start, stop or alter linear motion

Question 2

static equilibrium

Answer 2

velocity is zero

- state of equilibrium

Question 3

dynamic equilibrium

Answer 3

velocity is not zero, but constant

- acceleration is zero

Question 4

inertia

Answer 4

related to the amount of energy required to alter the velocity of a body
- directly proportional to the mass of an object

Question 5

center of mass

Answer 5

a point about a body where its mass is evenly distributed

• the point where the acceleration of gravity acts on the body
• just anterior to 2nd sacral vertebra(anatomic position)
• COM of body segment cannot change with movement
• COM of an entire extremity can change with movement

Question 6

Mass of moment of intertia

Answer 6

a quantity of a body that indicates its resistance to a change in angular velocity

• depends on mass and the distribution of its mass with respect to an axis of rotation
• reducing MMOI results in increased angular velocity
• ex: running(flexion of knee and hip to decrease MMOI)

Question 7

2nd law: Law of Acceleration

Answer 7

acceleration of a body is directly proportional to the force causing it

• acceleration of a body takes place in the same direction as the force causing it
• acceleration of a body is inversely proportional to the mass of the body

Question 8

2nd law applied to rotary motion

Answer 8

a torque produces an angular acceleration of the body that is proportional to and in the rotary direction of the torque
- angular acceleration is inversely proportional to the mass moment of inertia of the body
Sum of T = I(MMOI) x a(angular acceleration)

Question 9

momentum

Answer 9

the quantity of motion possessed by a body
M = mass x change in velocity
F x t = m x change in velocity

Question 10

Impulse

Answer 10

what is required to change the momentum of a body
Impulse = F x t
- how long a force is being applied

Question 11

Angular momentum and impulse

Answer 11

Angular momentum = I x change in angular velocity

Angular impulse = torque x time

Question 12

Reduce injuries with impulse/momentum relationship

Answer 12

want to decrease force on landing when jumping, so want to increase time

• ex: gastroc/soleus eccentrically contract prior to heel hitting ground
• fat pad underneath calcaneous prolongs time before ground force connects to bone
• insoles
• motorcycle helmet

Question 13

Work-Energy relationship

Answer 13

W(linear) = F x distance(displacement)
W(angular) = T x degrees(angular displacement)
- Energy the fuel available to the system to perform work

Question 14

Power

Answer 14

the rate of work, the speed at which work is performed
Power = W/t
Power(linear) = (F x d)/ t or F x v
Power(angular) = T x w

Question 15

3rd law: Law of action-reaction

Answer 15

for every action there is an equal and opposite reaction

Question 16

joint reaction force

Answer 16

one joint pushes back against another joint surface

• cumulative effect of forces transmitted from one segment to another
• caused by activation of muscle, passive tension in stretched ligaments and gravity(body weight)
• decrease by: decreasing muscle contraction, decreasing weight through joint, decreasing walking speed