[padfoot]

Just doing some very last minute revision, and i have two questions:

1) When a particle/object is moving on a rough/smooth surface is there a normal reaction? Or is it just for when its stationery, in limiting equilibrium etc.?

2) Can take a negative value?... reasoning for this q... was just doing a question where my acceleration was negative (ie. deceleration) so when working out using F=ma, it gave me a negative answer for ... my answer was kind of correct, it was just that it was positive rather than negative...

[KingPin]

1) yes there is as otherwise it'll fall through whatever it was on!

2)if mu was ever negitive you could make an very nice perpetual motion machine, so sadly no it cant be
Im guessing you just missed a sign somewhere, its called doing a louise in my maths class as she always does it

[padfoot]

Originally Posted by KingPin

1) yes there is as otherwise it'll fall through whatever it was on!

2)if mu was ever negitive you could make an very nice perpetual motion machine, so sadly no it cant be
Im guessing you just missed a sign somewhere, its called doing a louise in my maths class as she always does it

lol
thanks

[kashkng]

Originally Posted by KingPin

1) yes there is as otherwise it'll fall through whatever it was on!

2)if mu was ever negitive you could make an very nice perpetual motion machine, so sadly no it cant be
Im guessing you just missed a sign somewhere, its called doing a louise in my maths class as she always does it

yes just as kingpin said, if particle is resting on a table then there is always reaction/ normal and mu can't be -ve

[2710]

wait wait...how do you work out mu with F=ma? You realise the F in that formula is Resultant force, and the formula you want is F=mu x R, where the F there is friction right?

[ALICEMOON]

Originally Posted by padfoot

Just doing some very last minute revision,

Holy crow
last minute? when is your exam the 5th June? ah i have done no revision for it let alone last bloody minute revision.....

[2710]

My mechanics is on 22 May 0_0 Hope I do good

[KingPin]

Originally Posted by 2710

wait wait...how do you work out mu with F=ma? You realise the F in that formula is Resultant force, and the formula you want is F=mu x R, where the F there is friction right?

[u=mu & 0=angle between horizontal]
all you'd have to do is F-uR=ma
where R is the reaction force (mg on a flat surface, mgcos0 on a slope)

[2710]

Yes, but if that was what OP intended, then why would mu be -ve?

[padfoot]

Originally Posted by KingPin

2)if mu was ever negitive you could make an very nice perpetual motion machine, so sadly no it cant be
Im guessing you just missed a sign somewhere, its called doing a louise in my maths class as she always does it

i just checked i didn't miss anything out... i've uploaded how the book's worked it out and they just changed the acceleration from negative to positive....

Originally Posted by 2710

Yes, but if that was what OP intended, then why would mu be -ve?

does anyone know why the change the signs (i marked it on the working out with red arrows) doesn't make sense to me...

the book's working out

[2710]

I think the book's wrong. I think it's confusing Fr and F. Its using Fr as F there in the equation......

EDIT:

OK Ive got it. Basically, here, its an exception, since the Resultant Force IS the Resistive force, as it is the only force acting on it horizontally:

therefore

F = ma
F = mu r

mu r = ma
mu x mg = m x -5/8

....

ok....need to think....

[KingPin]

Originally Posted by padfoot

does anyone know why the change the signs (i marked it on the working out with red arrows) doesn't make sense to me...

The only thing I can think that they've done is to work out the acceleration initally in terms of its vector direction, as there are two balls, and then in the second part reversed the signs as there is only one ball (mathematicians like to keep things positive)

[meatball893]

The reason they changed the sign is because both the frictional force and the acceleration act in the same direction; against the motion of the stone. You always have to take account of the direction a force/other vector acts in.

[padfoot]

Originally Posted by meatball893

The reason they changed the sign is because both the frictional force and the acceleration act in the same direction; against the motion of the stone. You always have to take account of the direction a force/other vector acts in.

Originally Posted by KingPin

The only thing I can think that they've done is to work out the acceleration initally in terms of its vector direction, as there are two balls, and then in the second part reversed the signs as there is only one ball (mathematicians like to keep things positive)

thanx guys makes sense