Static/kinematic friction trouble

[afg_91320]

A man is trying to move a couch with a mass of 200 kg, across a rug covered floor.
He must use an exert force 0f 700 N to get it to barely move. His daughter has estimated that once it moves, then it accelerates at 1.10 m/s^2

determine:
a) coefficient of static friction

b) coefficient of kinetic friction between couch and the rug.

I don't even know what units i should use and what formula is generally given. the section for friction gives like 20 diff units to use but doesn't tell which to use.

please help me understand-i don't want to know the answer-i want to know how to do it!

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also another one-how do you find an object's acceleration if you are given only one angle and force (ie-object of 10kg is sliding upward on a vertical wall with force of 60 and angle at 60. what is the normal force and the object's acceleration)

^^ for the normal force exerted i used:
F cos 60 = 60 cos 60 = 30 N <--does this apply to the latter question??

thanks alot!

:D

 

[J Hill]

Have you done free-body diagrams in your class?

If not, think of the couch as a point mass and label all of the forces on it.
...^
...| Fn
...|
fs <----o----> Fa
...|
...| W
...v
(sorry, just did this quickly)
These help sort out which forces you're looking at.

Secondly, the coefficients of friction are unitless-- it's only a ration between two forces:
$$\mu=\frac{f}{F_N}$$

To find the coeficient of static friction is pretty easy to find, because the couch is not accelerating in either the x nor the y direction, so:
$$f_s=F_{applied}$$
and
$$F_N=W$$, so you can substitute these into the above equation.

The only difference with the kinetic friction is you have to use Newton's second law to find the force of kinetic friction.

 

[afg_91320]

im looking at the graph and it still doesn't make sense.

f = Fapplied <---this would be the 700 N correct?
FN = W <--- the 200 kg??

so coefficiant would be 700/200 = 3.5 <---no units?

 

[LowlyPion]

im looking at the graph and it still doesn't make sense.

f = Fapplied <---this would be the 700 N correct?
FN = W <--- the 200 kg??

so coefficiant would be 700/200 = 3.5 <---no units?

Doesn't weight involve gravity as in m*g?

Yes the coefficient of friction is dimensionless. It is a ratio of like quantities. Newtons/Newtons.

 

[afg_91320]

^^ok so i fixed my FN, which is now:
FN = W = 200 kg(9.80m/s^2) = 1960

= f / FN = 700/1960 = 0.357 <---correct answer??
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now how do i use Newton's second law to find the coefficiant of kinetic friction bw the rug and couch??

 

[LowlyPion]

now how do i use Newton's second law to find the coefficiant of kinetic friction bw the rug and couch??

F = m*a

You have m, you have a, so what force is resulting in acceleration?

Given that 700N was needed to overcome friction and now you know hopw much force is freed up to accelerate it, how much is still being taken by the kinetic friction? Then figure μ_k in the same way as before.

 

[afg_91320]

^is there a formula i have to use?

 

[LowlyPion]

^is there a formula i have to use?

Draw a picture and identify the force elements.

If you try to remember a formula without understanding it, you will likely be heading for problems.