| Author | Message | ||
     Ho Ka Wai Username: 971066 Registered: 04-2003 |
Method of calculation:<---Rubbish 1)List all equations(often 2 components, 1 moment, 1 friction-reaction equation,but sometimes mathematic calculation from graphs) 2)Solve them 3)Some questions may ask other things, that needs other technique | ||
| Ho Ka Wai Username: 971066 Registered: 04-2003 |
Example 12  | ||
| Ho Ka Wai Username: 971066 Registered: 04-2003 |
a) Let AB=x In equilibrium, consider the component of forces along plane, ...................equation (1) consider the component of forces perpendicular to plane, ...................equation (2) Take moment about A, ...................equation (3) In limiting equilibrium, F=Rtana....equation (4) 4quations, 3unkown, must be solved My suggested answer: (maybe wrong) F=Wsina R=Wcosa acts on (a-2h)tana-a from edge B | ||
| Ho Ka Wai Username: 971066 Registered: 04-2003 |
b) when overturning, x=0 Take moment about B, ............ P=(2aWsina)/h other method from a), (a-2h)tana-a =0 tana =a/(a-2h) So P= F+W sina=2Wsina=......... | ||
| Ho Ka Wai Username: 971066 Registered: 04-2003 |
Example13  | ||
| Ho Ka Wai Username: 971066 Registered: 04-2003 |
(Question:find when equilibrium is broken) tana=3/4 In equilibrium, consider vertical component of forrces, R2+R1cosa=W+F1sina R2+(4/5)R1=W+(3/5)F1.....(1) consider horizontal component of forces, ..........(2) Take moment about A, .......(3) | ||
| Ho Ka Wai Username: 971066 Registered: 04-2003 |
Case(1) If equilibrium is broken by slidding, in limiting quilibrim, F1=(1/3)R2.......(4) & F2=(1/3)R2......(5) by solving (1)-(5), P=(17/11)W (a very complicated calculation......answer may wrong, so dun trust me) Case (2) If equilibrium is broken by overturning, R2=F2=0 in limiting quilibrim, Take moment about B, ................ P=(16/17)W <(17/11)W so equilibrium will be broken down by overturning about B as P increases to (16/17)W |