devil physics the baddest class on campus pre-dp physics

DEVIL PHYSICSTHE BADDEST CLASS ON

CAMPUSPRE-DP PHYSICS

GIANCOLI LESSON 9-4 TO 9-5APPLICATION TO MUSCLES AND JOINTSSTABILITY AND BALANCE

Objectives

Apply the concepts learned in static equilibrium to problems involving the muscles and joints of the human body

Calculate the force required by muscles to perform different functions

Understand the physics behind why humans are susceptible to lower back pain

Know the meaning of stable equilibrium, unstable equilibrium and neutral equilibrium

Introductory Video:How the Body Works - Muscles

Muscles and Joints

Muscles attached to two different bones

Attachment points called insertions Bones attached at joints Muscles can only contract and relax Muscles are normally paired to

extend (extensor muscles) and to contract (flexor muscles)

Example Problem What force must be applied: (a) to hold a

15-kg dumbbell at your side, (b) to hold it at a 45° angle to your body, and (c) at a 90° angle to your body? Neglect the mass of the arm. The person is 1.8m tall.

Example Problem What force must be applied: (a) to hold a

15-kg dumbbell at your side, (b) to hold it at a 45° angle to your body, and (c) at a 90° angle to your body? Neglect the mass of the arm. The person is 1.8m tall.(a) With the dumbbell at your side, tension

in your arm and shoulder support the weight of the dumbbell. There is no torque generated.

NgmF dbarm 147

Example Problem

(b) With the dumbbell at a 45° angle, there is a moment arm for both the insertion and the arm, but less than that for an arm at 90°.

NF

gmF

arm

dbarm

1012

45cos8.1100

1.432.6245cos05.

Example Problem

(c) With the dumbbell at a 90° angle, the moment arm for both the insertion and the arm is at a maximum.

Will the force at 90° be greater than, less than, or the same as the force at 45°?

Example Problem

(c) With the dumbbell at a 90° angle, the moment arm for both the insertion and the arm is at a maximum. The force is the same as at 45°.

NF

gmF

arm

dbarm

1012

8.1100

1.432.6205.

Lower Back Pain Using the diagrams below,

calculate the magnitude and direction of the force on the fifth lumbar vertebra.

Lower Back Pain Using the diagram on the right,

calculate the magnitude and direction of the force on the fifth lumbar vertebra.

First we need to find the force of the muscles using Στ.

18°30°

60°12

°FM


calculate the magnitude and direction of the force on the fifth lumbar vertebra. First we need to find the force of the

muscles using Στ. Find the perpendicular components of

the weights wx60°

CWww

ww

xx

x

x

60sin

60sin



muscles using Στ. Find the perpendicular components of

the weights Now find the perpendicular component

of FM18°30°

60°12

°FM

CWww xx60sin CCWFF

FF

MM

M

M

12sin

12sin



muscles using Στ.

CWww xx60sin CCWFF MM

12sin

wwF

wwwFwwwF

rF

M

M

M

37.210.0237.0

044.0050.014.010.060sin72.060sin48.060sin36.012sin48.0 123


calculate the magnitude and direction of the force on the fifth lumbar vertebra. Now we need to find Fv by finding its

components. First find vertical and horizontal components

of FM

18°30°

60°12

°FM

18cos

18cos

18sin

18sin

MxM

M

xM

MyM

M

yM

FF

FF

FF

FF

wFM 37.2

FM-

xFM-

y



components. First find vertical and horizontal components

of FM

18°30°

60°12

°FM

wFFF

wF

FF

xM

MxM

yM

MyM

25.218cos

732.0

18sin

wFM 34.2

FM-

xFM-

y



components. Now find vertical and horizontal components of FV

through ΣFx and ΣFy

18°30°

60°12

°FM

FM-

xFM-

y

wF

wF

xM

yM

25.2

732.0

FV-

xFV-

y FY



components. Now find vertical and horizontal components

of FV through ΣFx and ΣFy

wF

wF

xM

yM

25.2

732.0

wF

wwwFF

FwFF

F

yV

yMyV

y

xMxV

x

38.1

025.2

0

321

FV-

xFV-

y FY



components. Pythagorize and Tangentiate

wFwF

yV

xV

38.125.2

FV-

xFV-

y FY

5.31tan

tan

64.2

1

22

xV

yV

xV

yV

V

yVxVV

FFFFwF

FFF

Lower Back Pain This is why you should lift with your legs and your

back straight. While the vertebra and discs still have to support the weight, it drastically reduces required back muscle force.

18°30°

60°12

°FM

FM-

xFM-

yFV-

xFV-

y FY

wx60°

wx

60°

Personal Testimony

Stability and Balance A body in static equilibrium will not

move if left undisturbed What happens if it is disturbed? Depends on Balance and Stability

Stable Equilibrium – body returns to its original position

Unstable Equilibrium – body continues to move in the direction of displacement and may accelerate

Neutral Equilibrium – body stays in its displaced position

Stability and Balance

Stable equilibrium


Unstable equilibrium


Neutral equilibrium


Three Cases


Instability occurs when the center of gravity is no longer above its base of support

Potential for instability increases as the distance between CG and base increases

Potential for instability increases as the size of the base decreases


General Rule: Stability can be increased if you can lower the center of gravity and/or increase the size of the base of support

Stability and Balance Reading Activity

Question: Tilt the boxes and truck backward until the center of mass is over the axle of the hand truck. The hand truck is then supporting all of the weight of the boxes and itself.


Which is more stable, a human or a dog?

Stability and Balance Which is more stable, a human or a

dog?

Does this imply that most humans are unbalanced? You be the judge.

Σary Review Can you apply the concepts learned in

static equilibrium to problems involving the muscles and joints of the human body?

Can you calculate the force required by muscles to perform different functions?

Do you understand the physics behind why humans are susceptible to lower back pain?

Do you know the meaning of stable equilibrium, unstable equilibrium and neutral equilibrium?

QUESTIONS

#34-42 (skip #41)Homework

Muscles of the Body

devil physics the baddest class on campus pre-dp physics

Documents

body works muscles muscles

moment arm

extensor muscles

lumbar vertebra

y lower

weightswx60 lower

perpendicular components

example problemc