Week 3

Question 1

Human motion study begins with the

Answer 1

classifying of the types of movement.

Question 2

Standing analysis starts with

Answer 2

discussion of overall and local stability and friction.

Question 3

Kinematics -

Answer 3

branch of mechanics dealing with the motion of bodies, without considering mass or force.

Question 4

Kinematics involves (2):

Answer 4

1. tracking the motion of the center of mass of the body
2. tracking angles (θknee,θthigh, θankle) versus time and versus each other during a walking or running cycle

Question 5

criterion for overall stability during standing -

Answer 5

for the center of mass to be over the area spanned by the feet

Question 6

When are we most stable?

Answer 6

when the line of gravity is near the center of the support base

Question 7

stance phase -

Answer 7

from foot strike to toe off

Question 8

swing phase -

Answer 8

from toe off to foot strike

Question 9

stance phase in walking:

Answer 9

longer than the swing, 60-65%

Question 10

stance phase in running:

Answer 10

shorter than swing, 40%

Question 11

swing phase in walking:

Answer 11

shorter, 35-40% of the cycle

Question 12

swing phase in running

Answer 12

longer, 60% of the cycle

Question 13

difference b/w running & walking (2):

Answer 13

• leg becomes almost straight during walking, but it never does during running
• during walking each foot is on the ground for more than half the time, Sometimes both are on the ground at the same time. during running, each foot is on the ground for less than half the time, Sometimes neither foot is on the ground

Question 14

How high you can jump, depends on

Answer 14

how fast you can take off

Question 15

kinetic energy at takeoff is all converted

Answer 15

into potential energy at the top of free flight

Question 16

How are the jump height and takeoff time related?

Answer 16

you can decrease the extension time τ, you can jump higher

Question 17

Momentum -

Answer 17

vector quantity defined as the product of mass and velocity
SI units of momentum are kg m/s

Question 18

Elastic collision -

Answer 18

collision of bodies in which the total kinetic energy is the same before and after the collision

in totally elastic or partially elastic collisions, the objects bounce back

Question 19

Inelastic collision -

Answer 19

collision of bodies in which total kinetic energy is not conserved

Question 20

Sticky inelastic collision -

Answer 20

inelastic collision where the two objects are stuck together after the collision. Sometimes known as a totally (or perfectly) inelastic collision

Question 21

what happens to linear momentum and E in a collision?

Answer 21

IN A COLLISION, LINEAR MOMENTUM AND ENERGY ARE CONSERVED. However, if the collision is not elastic, kinetic energy is not conserved and heat and/or object deformation is produced

Question 22

UPON COMPRESSION BONES TYPICAL BREAK WHEN SUBJECTED TO A FORCE PER AREA ABOVE - ?

What is this damage threshold called?

Answer 22

170 MPa

This damage threshold is called the ultimate compressive stress

Question 23

height from which jumping is dangerous?

Answer 23

stresses on the tibia exceed UCS (ultimate compressive stress), so jumping from 1 m and landing on one stiff leg will break your leg

Question 24

how to reduce the effect of the impact on the bones (2)?

Answer 24

1. increasing time of collision
   and/or
2. increasing the area of contact A

Question 25

The collision time can be increased by (3):

Answer 25

1. using better padding at the site of the collision
2. increasing the duration of the impact by adjusting your body during actual impact
3. rolling over

Question 26

Collisions are survivable depending on:

Answer 26

1. what part of the body makes contact
2. what type of surface it hits
3. the deceleration time

Question 27

HUMAN COLLISIONS ARE USUALLY NOT SURVIVABLE FOR DECELERATIONS OF

Answer 27

175– 200 G OR GREATER

Question 28

During a very slow step the forces on the foot

Answer 28

slowly build up from 0 to mbg

Question 29

During running the forces on the foot

Answer 29

reach a peak of approximately 2.0–3.6 mbg

Question 30

Fracture due to direct blow (2):

Answer 30

• low E => 2 pieces (transverse)
• high E => many pieces

Question 31

Fracture due to indirect blow (4):

Answer 31

• traction => transverse fracture perpendicular to F
• bending => transverse w/ butterfly fragment
• torsion => spiral fracture
• bending & torsion => oblique fracture