Mechanics (1 Viewer)

[Affinity]

I seem to have problems setting out Mechanics solutions, I either write more than required or leave out something (eg quoted a formula, instead of deriving it )

are there any general guidelines, such as which formulae could be assumed.

 

[nerdd]

i'm no expert but the first thing i do is the F=mx (double dot)
and minus/add the forces for whether it goes up or down.

that's generally the first thing i write down.

 

[Affinity]

what about:

a= -rw^2 = v^2r w-> omega (angular speed) for centripetal acceleration

can I use the concept of 'centrifugal' acceleration in questions?

 

[ND]

When doing questions out of the book i usually just quote the formula. Maybe in the exam there will be a lead-in q. asking you to derive it or something? I'm sure a mod (or oldman) will be able to give us a definite answer.

 

[OLDMAN]

centripetal acceleration v^2/r could be assumed unless specifically asked to derive; if asked to derive you should be ready with a standard textbook derivation, I personally prefer the one that assumes sin@--->@ as @--->0 as it is quick and simple.

 

[Affinity]

What about the equations for projectile motion?

 

[OLDMAN]

projectiles: derive from scratch eg. x"(double dot) =0 , y"=-g and proceed from these to get the equations of motion.

I guess it is quite similar to asking whether you can just quote the ellipse tangent eqn. as xx_1/a^2+yy_1/b^2=1, have seen hundreds of worked solutions and they do work it out, so no you can't. Or de Moivre's, yes you can assume it of course. My advise will be as you do more practise questions, you'll have a better feel for what the examiner requires.

 

[Affinity]

thanks very much OLDMAN , I will do some more practice questions