SFi=0

1
?????
???,?????????????????
???????,????????????????? ???????????,???(?)?????
??,?????????????????(???)?????
2
??????????????????????,?????????????????????? SF
i0 S ti0
?????????????,???????????????,????????????????????
?????????????????????????????,????????????????????
???
3
?????40N??????,??????800N?????350N????(?????)????
????????????
?????????,?? n-800N-350N-40N 0 n
1190N
4
????????,?????
??????????,???????????? (800N)(1.0m) - (350N)x
0? x 2.29 m
???????????????????,????????????? n(1.0m) -
(40N)(1.0m) (350N)(1.0m x ) 0 ? x
2.29 m
5
?????50N???,??????????????????????????????3.00cm,?
?????35.0cm????????????,?????????????(?)??
6
?????,?????????????????(????)???600N?????????,????
???????????????????
7
???l??????50N???????????????,?????????????
???????????????????
8

9
?????????????????h???(????)??????????????????????
mg1400N, r 30cm, h 10cm
???? (2r-h)
10
??????????????????????????????????????,?????C?????
??7200N???????????
11
?A(?E)???
12
??????????C
????,?????????????????
13
???
?????? ??(stress)?,???????(strain)???????,????
????,??????????(modulus of elasticity)??
Stress modulus strain
14
Stress, Strain, and Elastic Moduli
?? Stress ?? Strain ????Elastic Moduli
Hookes law
15
Youngs modulus
pressure in a fluid
16
?????
1 Pascal 1 Pa 1 N/m2
1 psi 1 lb/in2 6895 Pa and 1 Pa 1.450
10-4 psi
17
Bulk Stress and Strain
bulk modulus
18
Shear Stress and Strain
shear modulus
19
????????????940N,????10m??????????,????????????0.5
??,??????????
20
??????????,?????????????,???????????????????????,?
???????????(?)?????????(shear stress)????(?)????(s
hear strain)??(?)???????????????2.0?108
N/m2,????????
21
(????????(shear modulus)?8.4?1010 N/m2,??shear
strain??
Since ?S tan?, this corresponds to an
angular deformation ?0.02o?
?????????????????????,?????
22
?????(brass)?????1.0?105 N/m2????,?????2.0?107
N/m2????????????????????0.5m3,??????????????
????????(brass)?bulk modulus?6.1?1010 N/m2,?
23
????(Bending Strength)
?????????(mechanical structures)??,???????(stress)
??????????,?????????(deformation)??????????????(cr
oss-sectional area)???????????????????????????,???
????????
24
????l,?????a?b?????????
l
a
R
??????????????????,???????????R???,???????
IA area moment of inertia
E ?????
25
Area moment of inertia???????????????,????????r???
,?????(stress)??
26
????????????,????????
IA a3b/12R
27
Minimizes both stress and weight
28
???????????????area moments of inertia 1?????
2????? 3?I??
29
??????????,????2cm by 6cm???????,????????????????
??6cm???,???????2cm???,??????????????
30
???????
???????????????????????,??????????????????????lc
r2/3?c34.9????
From T. McMahon Science 179, 1201 (1973)
31
???????
????????????????,???????????????????h,???r???,???
??(?????????),????????????????????????????
w ?????,d???(??????????????????)?
32
?????????,????????????????,?????????
33
????????????????????,????????????????????,???????
??
34
???(Twisting Torque)
????l???r??????,??????????????????,???????????dr??
???????????????????????l??r???dr????,???????????x
r????????????????
a
35
???????????????????????
???Ip??????IA,?????polar moment of
inertia??IA???????????(??),?Ip??????????(??)?
http//www.dynomed.com/encyclopedia/encyclopedia/k
nee_and_lower_leg/Fracture_of_Tibia.html
36
????????????????-??(Tibia)???????????????3.4o,???
?100 Nm????????????????30cm??,????????,???????????
???????,???????????????,?????????????????