swin2013
Forces: You discover that it takes 258.0 N to set a 40.0 kg crate in motion on a concrete surface. What is the coefficient of static friction for the crate?
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hubertH
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@ swin u need to know what plane u are working on is it an incline plane or an horizontal plane/
hubertH
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coefficient of static friction=static frictional force/ normal rection
hubertH
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I assume the plane is horizontal: the normal reaction=weight=mg ur
hubertH
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got it buddy?
swin2013
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@hubertH it doesn't say. so i assumed it was horrizontal
hubertH
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so u knw how to get to the answer right?
swin2013
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but is static friction only involved in inclined plane?
swin2013
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No.. then I wouldn't be asking lol
hubertH
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No! static friction is the force required to set the body in motion in our case it 's given as 258.0N
swin2013
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Well they're asking from the coefficient of static friction?
hubertH
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coefficient of static friction=static friction force/normal reaction
hubertH
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for horizontal plane normal reaction=mg
swin2013
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|dw:1351982204492:dw|
swin2013
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\[mumg?\]
hubertH
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|dw:1351982369737:dw|
hubertH
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do u now have an idea of what u're doing?
swin2013
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so i say that Fnet = \[\mu _{s} - Fg = ma?\] a little do you?
hubertH
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remember u're given the Fs and the mass only
hubertH
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there is no Fa, no Fnet
swin2013
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well if it's starting to move wouldn't it be Ff = Mu(s) Fn?
hubertH
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\[mus =\frac{ ?Fmus }{Nr ? }\]
swin2013
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I have never used Nr in my FBD before?
hubertH
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Nr is ur normal reaction=mg
swin2013
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Ok so it's just normal force (Fn)
swin2013
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so it's Ff/Fn
hubertH
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exactly!
swin2013
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and since it's horizontal Fnet = 0N
hubertH
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what is Fnet standing fo?
swin2013
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The vector pointing upward
swin2013
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Ok i got it, Mu(s) = .66
hubertH
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that's it bro congrats
and keep on going forwad
swin2013
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If they ask for acceleration and give kinetic friction is it still the same equation except it's Mu(k) - Fn = ma?
hubertH
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no! kinetic friction is the smallest force that keeps the system moving @ constant speed
swin2013
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Ok...
hubertH
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if u have more questions on science don't hesitate to massage me.....
swin2013
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alright because i have a big physics test and I don't really understand forces lol
hubertH
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we'll try to make it easier together bye for now