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MECANIQUE RATIONNELLE

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UMBB Boumerdès, Faculté des sciences, Département de physique<br />

Cours exercices, Mécanique Rationnelle : TCT et LMD-ST sem :3<br />

A.KADI<br />

Calculons d’abord les moments cinétiques des solides ( S 2<br />

) et ) en G en utilisant la<br />

formule de transport :<br />

→<br />

→<br />

−−−→ →<br />

G1 G<br />

G<br />

0<br />

σ ( S2<br />

/ R0<br />

) = σ<br />

2<br />

( S2<br />

/ R0<br />

) + G1G2<br />

∧ m2<br />

V (<br />

2<br />

)<br />

⎛<br />

⎜<br />

0<br />

⎞<br />

⎟<br />

⎛<br />

⎜<br />

( S 3 1<br />

→<br />

• →<br />

•<br />

•<br />

•<br />

→<br />

2 2<br />

2<br />

G1 ( S<br />

2<br />

/ R0<br />

) = A2<br />

α x0<br />

+ ⎜ bsinα<br />

⎟ ∧ m2<br />

⎜ bα<br />

cosα<br />

⎟ = ⎜ A2<br />

α+<br />

m2b<br />

α(cos<br />

α − sin α)<br />

⎟ x0<br />

•<br />

σ<br />

⎛<br />

⎝<br />

⎜ ⎟<br />

⎝−<br />

bcosα<br />

⎠<br />

→<br />

•<br />

•<br />

→<br />

2<br />

G1( S<br />

2<br />

/ R0<br />

) = ⎜ A2<br />

α + m2b<br />

α cos 2α<br />

⎟ x0<br />

σ<br />

→<br />

→<br />

−−−→ →<br />

G1 G<br />

G<br />

⎞<br />

⎠<br />

0<br />

⎞<br />

⎟<br />

⎜ ⎟<br />

⎝−<br />

bα<br />

sinα<br />

⎠<br />

0<br />

σ ( S3<br />

/ R0<br />

) = σ<br />

3<br />

( S3<br />

/ R0<br />

) + G1G3<br />

∧ m3<br />

V (<br />

3)<br />

⎛ 0 ⎞<br />

⎜ ⎟<br />

⎟<br />

⎜ ⎟<br />

⎝−<br />

2b<br />

cosα<br />

⎠<br />

⎛<br />

⎜<br />

→<br />

• →<br />

•<br />

•<br />

•<br />

→<br />

2 2<br />

G1( S3<br />

/ R0<br />

) = −A3<br />

β x0<br />

+ ⎜ 2b<br />

sinα<br />

∧ m3⎜2bα<br />

cosα<br />

⎟ = ⎜−<br />

A3<br />

β + 4m3b<br />

α cos α ⎟ x0<br />

σ<br />

⎛<br />

⎝<br />

→<br />

•<br />

•<br />

→<br />

2 2<br />

G1( S3<br />

/ R0<br />

) = ⎜−<br />

A3<br />

β + 4m3b<br />

α cos α ⎟ x0<br />

σ<br />

⎞<br />

⎠<br />

⎜<br />

⎝<br />

Les moments dynamiques se déduisent facilement par dérivation des deux expressions:<br />

→<br />

d 0<br />

σ<br />

1(<br />

S<br />

2<br />

/ R0<br />

) ⎛<br />

⎞<br />

G<br />

dt ⎝<br />

α<br />

⎠<br />

→<br />

••<br />

••<br />

•<br />

→<br />

2<br />

2<br />

= ⎜ A2<br />

α+<br />

m2b<br />

( α cos 2α<br />

− 2α<br />

sin 2 ) ⎟ x0<br />

d 0<br />

σ<br />

1(<br />

S3<br />

/ R0<br />

) ⎛<br />

⎞<br />

G<br />

dt ⎝<br />

α<br />

⎠<br />

••<br />

••<br />

•<br />

→<br />

2 2<br />

2 2<br />

= ⎜−<br />

A3<br />

β + 4m3b<br />

α cos α − 8m3b<br />

α sinα<br />

cos ⎟ x0<br />

⎛<br />

••<br />

••<br />

•<br />

→<br />

2 2<br />

2 2<br />

= ⎜−<br />

A3 β + 4m3b<br />

α cos α − 4m3b<br />

α sin 2α<br />

⎟ x0<br />

⎝<br />

Le moment dynamique du système est la somme des deux expressions :<br />

→<br />

δ<br />

G1<br />

(<br />

∑<br />

/ R<br />

0<br />

⎛<br />

••<br />

) = ⎜ A2<br />

α+<br />

m2b<br />

⎝<br />

⎛<br />

••<br />

+ ⎜−<br />

A3<br />

β + 4m3b<br />

⎝<br />

⎛<br />

2<br />

••<br />

•<br />

2 ⎞<br />

( α cos 2α<br />

− 2α<br />

sin 2α<br />

) ⎟ x<br />

⎠<br />

••<br />

2<br />

2<br />

α cos α − 4m b<br />

3<br />

2<br />

0<br />

0<br />

→<br />

⎞<br />

⎟<br />

⎟<br />

⎠<br />

0<br />

⎛<br />

⎝<br />

⎛<br />

⎝<br />

•<br />

2 ⎞<br />

α sin 2α<br />

⎟ x<br />

⎠<br />

→<br />

•• ••<br />

••<br />

•<br />

••<br />

•<br />

→<br />

2<br />

2<br />

2 2 2<br />

G1 ( ∑ / R0<br />

) = ⎜ A2<br />

α−<br />

A3<br />

β + m2b<br />

( α cos 2α<br />

− 2α<br />

sin 2α<br />

) + 4m3b<br />

( α cos α −α<br />

sin 2α<br />

) ⎟ x0<br />

δ<br />

⎝<br />

→<br />

0<br />

⎞<br />

⎠<br />

⎞<br />

⎠<br />

⎞<br />

⎠<br />

⎞<br />

⎠<br />

314

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