Posted by ju on Thursday, November 11, 2010 at 3:10pm.
I keep geting this question wrong and cant figure out what i am doing wrong. Help please.
When you push a 1.75 book resting on a tabletop, it takes 2.23 to start the book sliding. Once it is sliding, however, it takes only 1.45 to keep the book moving with constant speed.
What is the coefficient of static friction between the book and the tabletop?
What is the coefficient of kinetic friction between the book and the tabletop?

physics  MathMate, Thursday, November 11, 2010 at 4:22pm
What did you get, and how?

physics  ju, Thursday, November 11, 2010 at 5:38pm
i tried it many different ways. but basically i used fk=ukN . forexample for static friction (uk) the acceleration= o
so unN=ma
us(1.45)=1.75kg(0)
us=1.45 and i got the wrong

physics  ju, Thursday, November 11, 2010 at 5:42pm
...answer**
then for kinetic friction, i am clueless how to find the acceleration since there is no speed given in the equation. i know there is somehting i am missing or that i am looking at this question in the wrong way.
the varible i think are 1.75kg for the weight of the book
and 2.23N to start the books motion and 1.45N to get it at constant speed
just clearning that up from the question

physics  MathMate, Thursday, November 11, 2010 at 6:04pm
The question does not ask for acceleration. It only reduced the pushing force once it starts so that the book moves at constant speed.
You do not indicate units, which can be a major cause of the problem.
Force and can be expressed in newtons (N), and mass in kg.
Weight due to a mass of m (kg) is mg (N).
Fk=μk*N ...(1)
is correct, except that take care that N is expressed in newtons using
N=mg
so
μk = Fk/N = Fk/(mg)
Fs=μs*N
μs = Fs/N = Fs/(mg)
See if you find the correct answer with the above information.

physics  mathhelp, Thursday, November 11, 2010 at 6:35pm
ok got it. thank you.
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