Finding mass from friction force and coefficient of friction

[KatieM]

physics-12th grade-forces and friction PLEASE HELP ME!

I'm not really sure about how to go about this problem. It says:

After every full moon, Mr. Sowell likes to slide around the hallways on freshly waxed floors in his socks. If the U* between the socks and the floor is 0.120 and amount of force required to move him 102 N, what was his mass in kg.

If i just knew which formula to use, and what each thing stood for I'm pretty sure i'd be able to solve this. the U* that i wrote is the funny looking greek U calle mue or whatever. Please help me someone!

 

[Doc Al]

Here's a hint: the force of kinetic friction is $F_f = \mu N$, where N is the normal force pushing the surfaces together (the sock and the waxed floor). The normal force is equal to the man's weight. Apply what you know (or should know!) about weight and mass, and what's given in this problem.

 

[KatieM]

thank you for the help...I just wasn't sure what numbers to plug in where...

 

[KatieM]

i still have a question though... i thought i understood it but i guess not

if i use the formula Ff=u*Fn

then which variables stand for Newtons and kg's

 

[Doc Al]

Forces are in Newtons and masses are in kg. The formula I gave just has forces. What's the formula that tells you the weight of given mass?

 

[KatieM]

so i would rearrange the formula and get Fn=Ff
U*

and 102N
.120 = 850kg

is that right?

 

[Doc Al]

You rearranged the formula correctly, but you are interpreting it incorrectly. You solved for the Normal force $F_n = F_f/\mu = 850$. That's the normal force, 850 Newtons, not kg. But, as I told you earlier, the normal force equals the man's weight. So what's the relationship between weight and mass? (Look in your book!) In other words, what is the mass of someone who weighs 850 N?

 

[KatieM]

okay...i think i got this now...Mass=force/acceleration so 850N/9.8m/s^2=86.7 kgs

 

[Doc Al]

Excellent.

It's good to remember that the weight of a mass (on the Earth's surface, that is) is given by w = mg.

 

[KatieM]

thank you for your help :) i should be able to do the rest of my worksheet now