## swin2013 Group Title 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? one year ago one year ago

1. hubertH

@ swin u need to know what plane u are working on is it an incline plane or an horizontal plane/

2. hubertH

coefficient of static friction=static frictional force/ normal rection

3. hubertH

I assume the plane is horizontal: the normal reaction=weight=mg ur

4. hubertH

got it buddy?

5. swin2013

@hubertH it doesn't say. so i assumed it was horrizontal

6. hubertH

so u knw how to get to the answer right?

7. swin2013

but is static friction only involved in inclined plane?

8. swin2013

No.. then I wouldn't be asking lol

9. hubertH

No! static friction is the force required to set the body in motion in our case it 's given as 258.0N

10. swin2013

Well they're asking from the coefficient of static friction?

11. hubertH

coefficient of static friction=static friction force/normal reaction

12. hubertH

for horizontal plane normal reaction=mg

13. swin2013

|dw:1351982204492:dw|

14. swin2013

$mumg?$

15. hubertH

|dw:1351982369737:dw|

16. hubertH

do u now have an idea of what u're doing?

17. swin2013

so i say that Fnet = $\mu _{s} - Fg = ma?$ a little do you?

18. hubertH

remember u're given the Fs and the mass only

19. hubertH

there is no Fa, no Fnet

20. swin2013

well if it's starting to move wouldn't it be Ff = Mu(s) Fn?

21. hubertH

$mus =\frac{ ?Fmus }{Nr ? }$

22. swin2013

I have never used Nr in my FBD before?

23. hubertH

Nr is ur normal reaction=mg

24. swin2013

Ok so it's just normal force (Fn)

25. swin2013

so it's Ff/Fn

26. hubertH

exactly!

27. swin2013

and since it's horizontal Fnet = 0N

28. hubertH

what is Fnet standing fo?

29. swin2013

The vector pointing upward

30. swin2013

Ok i got it, Mu(s) = .66

31. hubertH

that's it bro congrats and keep on going forwad

32. swin2013

If they ask for acceleration and give kinetic friction is it still the same equation except it's Mu(k) - Fn = ma?

33. hubertH

no! kinetic friction is the smallest force that keeps the system moving @ constant speed

34. swin2013

Ok...

35. hubertH

if u have more questions on science don't hesitate to massage me.....

36. swin2013

alright because i have a big physics test and I don't really understand forces lol

37. hubertH

we'll try to make it easier together bye for now