## anonymous 4 years ago 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?

1. anonymous

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

2. anonymous

coefficient of static friction=static frictional force/ normal rection

3. anonymous

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

4. anonymous

got it buddy?

5. anonymous

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

6. anonymous

so u knw how to get to the answer right?

7. anonymous

but is static friction only involved in inclined plane?

8. anonymous

No.. then I wouldn't be asking lol

9. anonymous

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

10. anonymous

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

11. anonymous

coefficient of static friction=static friction force/normal reaction

12. anonymous

for horizontal plane normal reaction=mg

13. anonymous

|dw:1351982204492:dw|

14. anonymous

$mumg?$

15. anonymous

|dw:1351982369737:dw|

16. anonymous

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

17. anonymous

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

18. anonymous

remember u're given the Fs and the mass only

19. anonymous

there is no Fa, no Fnet

20. anonymous

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

21. anonymous

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

22. anonymous

I have never used Nr in my FBD before?

23. anonymous

Nr is ur normal reaction=mg

24. anonymous

Ok so it's just normal force (Fn)

25. anonymous

so it's Ff/Fn

26. anonymous

exactly!

27. anonymous

and since it's horizontal Fnet = 0N

28. anonymous

what is Fnet standing fo?

29. anonymous

The vector pointing upward

30. anonymous

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

31. anonymous

that's it bro congrats and keep on going forwad

32. anonymous

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

33. anonymous

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

34. anonymous

Ok...

35. anonymous

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

36. anonymous

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

37. anonymous

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