Mechanics help!!!!!!! (1 Viewer)

[Squar3root]

when you're resolving forces on like a conical pendlum or banked track or something, sometimes one of the componment are negative like, tcos(0) -nsin(0) = mrw^2
[(0) = Theta]. how do you know which one os negative or to subtract from what ??? any help is appreciated.

Thanks,
Shadow

 

[RealiseNothing]

It just depends on which direction you take as positive and negative. If you take up as positive for example, then any forces acting upwards are positive and any forces acting downwards are negative.

 

[obliviousninja]

Generally positive is horizontally towards the centre of the pendulum. Whilst positive is up and gravity is down.

 

[braintic]

I think he is actually asking about why the signs are different, not about sign conventions.

 

[Squar3root]

I think he is actually asking about why the signs are different, not about sign conventions.

Yes, like why are te components sometimes negative?

 

[braintic]

Yes, like why are te components sometimes negative?

This can really only be answered by reference to a force diagram. If one force component acting to the left is considered positive, then one acting to the right would be negative. The same applies to up/down.

If you don't understand, it would be best if you posted a particular question that is confusing you.

 

[Squar3root]

This can really only be answered by reference to a force diagram. If one force component acting to the left is considered positive, then one acting to the right would be negative. The same applies to up/down.

If you don't understand, it would be best if you posted a particular question that is confusing you.

HSC 2011 MX2: Q5)a
HSC 2009 MX2: Q4)b

 

[bangladesh]

When you draw the triangles of forces. You have to give them directions.
mg is ALWAYS Downwards. MRW^2 is ALWAYS outwards meaning the opposite direction as the particle. Then you just resolve forces meaning solve for horizontal and vertical.

I'm sorry but you really should know this as it is the first thing that is taught in circular motion so if you don't know it. i suggest you learn this tonight as everyone is expecting a fair amount of motion questions tomorrow.

Best of luck!

 

[panda15]

When you draw the triangles of forces. You have to give them directions.
mg is ALWAYS Downwards. MRW^2 is ALWAYS outwards meaning the opposite direction as the particle. Then you just resolve forces meaning solve for horizontal and vertical.

I'm sorry but you really should know this as it is the first thing that is taught in circular motion so if you don't know it. i suggest you learn this tonight as everyone is expecting a fair amount of motion questions tomorrow.

Best of luck!

HSC generally takes mrw^2 to act towards the centre of moton ie. inwards.

 

[bangladesh]

HSC generally takes mrw^2 to act towards the centre of moton ie. inwards.

lol...You can take it as both inwards or outwards. But when you're solving forces you take it as outwards. If you want to take it as inwards you have to equate it to -mrw^2. I really don't know how to explain this over the internet. But the forces are equal therefor outwards=-inwards.

Cmon dude. You do physics. I don't even do physics, this should be super simple for you.

 

[Makematics]

lol...You can take it as both inwards or outwards. But when you're solving forces you take it as outwards. If you want to take it as inwards you have to equate it to -mrw^2. I really don't know how to explain this over the internet. But the forces are equal therefor outwards=-inwards.

Cmon dude. You do physics. I don't even do physics, this should be super simple for you.

What are you on about? In motion questions you find the horizontal forces acting inwards and EQUATE them to the centripetal force mrw^2.

 

[panda15]

lol...You can take it as both inwards or outwards. But when you're solving forces you take it as outwards. If you want to take it as inwards you have to equate it to -mrw^2. I really don't know how to explain this over the internet. But the forces are equal therefor outwards=-inwards.

Cmon dude. You do physics. I don't even do physics, this should be super simple for you.

Hence why I said "generally", I know it's equal inwards and outwards. But look at the 2011 HSC. The resolved forces acting horizontally are taken inward, hence why the Nsin0 is negative. And in physics, mrw^2 is taken as positive when acting towards the centre of motion.

 

[bangladesh]

What are you on about? In motion questions you find the horizontal forces acting inwards and EQUATE them to the centripetal force mrw^2.

YES EXACTLY. I'm just bad at explaining it.

 

[bangladesh]

ok so.
say x and y are the inward forces.
x+y=mrw^2 right?
mrw^2 is equated because opposite forces are equal.

using the same equation:
x+y-mrw^w=0 which is the overall horizontal equation.