## catsrule332 3 years ago A 25 kg block is at rest on a horizontal surface. A horizontal force of 75 N is required to set the block in motion. When it is in motion a 60 N forced is required to keep the block moving with a constant speed. Find the coefficients of static and kinetic friction. Draw a force diagram and explain!

1. salini

so catsrule wat have u figured out so far in this question?

2. catsrule332

find normal force: n=mg=(25)(9.8)= 245 N $f=m*\eta$|dw:1333113193720:dw|

3. JamesJ

When the object is not moving, the maximum frictional force on the block is the static friction. Static friction is given by $Fr_S = \mu_S N$ where $$\mu_S$$ is the coefficient of static friction. Now which force in the problem is that equal to?

4. salini

u are in the right track wat is the expression for frictional force with normal reaction? and do u know the difference abt co-efficient of static friction and kinetic friction?

5. JamesJ

On the other hand, once the object is moving, the maximum frictional force is the kinetic friction and that is given by $Fr_K = \mu_K N$ where $$\mu_K$$ is the coefficient of kinetic friction and of course $$N$$ is the normal force. What value is the kinetic friction equal to in this problem?

6. salini

cats rule james has given everything needed for the sum u have to get the answer using that

7. catsrule332

Static friction: fsmax=$\mu *\eta$ Kinetic friction: fk=$\mu * \eta$ 75/(25)(9.8)= .31 N 60/245) =.24 N

8. JamesJ

Yes, the coefficient of static friction is 0.31. Notice that these coefficients are unitless, because you have Newtons on both sides of the equation $$F = \mu N$$ and force of friction $$F$$ and the normal force $$F$$ both have units of Newtons.