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 Topic  |
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zeryphex
Radio Wave
USA
8 Posts |
 Posted - 02/25/2003 : 20:38:10
word-for-word problem:
A student wants to determine the coefficients of static friction and kinetic friction between a box and a plank. She places the box on the plank and gradually raises one end of the plank. When the angle of inclination with the horizontal reaches 28.0[degrees], the box starts to slip and slides 2.53m down the plank in 3.92s. Find the coefficients of friction.I already tried going to my professor after his lecture, but he said he isn't available for the whole day, and I'm having trouble understanding force analysis/friction/dynamics. I know the tangent of 28.0[degrees] gives me coefficient of static friction, but can you help me on determining the coefficient of kinetic friction? I also know coefficient of kinetic friction is less than coefficient of static friction, but why am I given distance traveled and time traveled in the problem?
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enigma
PF Advisor
USA
1016 Posts |
Posted - 02/25/2003 : 23:57:27
Seperate the static problem from the kinetic problem completely.You have a block sliding down a 28 degree slope. The forces acting on it are normal force, friction force, and gravity. Sum them up to get total force in terms of coefficient of kinetic friction, and then plug that into your equations of motion to solve for the coefficient. Does that help any, or do you need a little more detail? enigma "Life is the crummiest book I've ever read. There isn't a hook; just a lot of cheap shots, pictures to shock, and characters an amateur would never dream up."
-Bad Religion
Stranger than Fiction
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zeryphex
Radio Wave
USA
8 Posts |
Posted - 02/26/2003 : 01:59:37
Ahh...I know what to do afterwards...frictional force equals (coefficient kinetic)*(normal)...solve for coeff...gotcha...thanks. =)
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zeryphex
Radio Wave
USA
8 Posts |
Posted - 02/26/2003 : 05:20:27
Another problem. =\word-for-word problem:
A piece of ice slides from rest down a rough 33.0° incline in twice the time it takes to slide down a frictionless 33.0° incline of the same length. Find the coefficient of kinetic friction between the ice and the rough incline. I'm not asking for the answer, but can you steer me in the right direction?... ...what correlation is there between time and coefficients?? All the equations having to do with coefficients don't seem to have time in them...do I use the link between dynamics and kinematics with the a =  kg equation?
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enigma
PF Advisor
USA
1016 Posts |
Posted - 02/26/2003 : 20:07:11
It is just a sum of force question.If it takes a longer time, there is a different acceleration acting on the block. In the frictionless example, there is only gravity. In the other case, you have friction opposing motion to add into the equations of motion enigma "Life is the crummiest book I've ever read. There isn't a hook; just a lot of cheap shots, pictures to shock, and characters an amateur would never dream up."
-Bad Religion
Stranger than Fiction
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zeryphex
Radio Wave
USA
8 Posts |
Posted - 02/27/2003 : 09:05:59
Thanks. =) I'll try to work it out.
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russ_watters
Radio Wave
USA
53 Posts |
Posted - 02/27/2003 : 18:37:47
Both of these problems are the same. You find the kinetic friction force by subtracting the acceleration force from the gravitational force. And static friction is equal to the gravitational force. The motion equations are f=ma, s=at, and d=st. Combine them and solve for force.
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