Posted: 9/21/2005 6:11:46 PM EDT
 Does anyone else feel like a loser when helping their kids with their homework?Daughter's problem: If the coeffient of Friction is .023 what is the force of friction of a 20 Kg object? Patty |
Yea yea just give me the damn answer and pour me a drink, scotch n soda please.  Patty
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Coefficient of friction = Force of friction / normal force / means divided by normal force (N) is the object's weight in Newtons (NOT kilograms) (hint: F=MA, Force=Mass*Accel, Normal force=Mass*Gravity) Simple algebra allows you to rearrange and solve for Force of friction. |
Show off.  Patty needed a drink when she was trying to get the answer. I didn't need one until I read this post. Now my brain hurts. 
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"Give a man a fish today and he will be hungry tomorrow. Teach a man to fish and he will never go hungry." 
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97? 5 billion 2 hundred and 10? Pie? .46 kg? 7000 grains? 2 deciliters? 1/5 of Jack Daniels? What does the magic 8 ball say?  Edit: If it's multiple choice guess "C". |
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If the object is on a level surface. It is on the earth where gravity, g=9.81m/s^2 There is kinetic friction and static friction. Kinetic is when the object is moving, static is before the object starts to slide. Static will ALWAYS be higher than Kinetic. Coefficent of Friction is the greek letter mu. Frictional Force=Normal Force*Coefficent of Friction. The Normal Force is the Mass of the Object(kg)*gravity. Frictional Force=Mass*g*Coefficent of Friction |
Althought I just researched the answer and had no knowledge of it prior to this thread... I will agree with CSM based on what I have read in a the last few minutes. see also: Wikipedia on Friction Determining the Coefficient of Friction |
 M_M you're too funny!OKay let me see if I can figure this out. Hang on - let me get the daughter. Patty |
 Patty
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You are quoting the acceleration of a freeing falling body. 32 feet per second squared or 9.8 meters per secomd squared is that the correct application for this question? I know nothing about this stuff...I'm just asking the question. 
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everything is always under accelleration from gravity. yes it is the correct way. |
My daughter's a freshman in high school and this is for her physical science class. I put Math problem because my feeble brain is able to do most equations IF given a formula. I got my degree in History. They never come to me with a history problem.  Patty |
Ff=mu*F_normal. This means the frictional force is equal to the coefficient of friction, denoted by lower case the greek letter mu, times the normal force. The normal force is always the force perpendicular to the surface. Think of it in terms of Newton's Third Law, every action has a reaction both equal in magnitude and opposite in direction. So if the block is sitting on a plane surface, and the mass of the block is 20kg, then the weight( a force quantity) is w=m*g, or 20kg*-9.8 m/s/s, which is about 196 Newtons. Now if there is a force of -196 newtons applied to the surface, but nothing is in motion, then the system is in equilibrium. This means that there must be an equal force of +196 Newtons acting upon the block by the surface, think of this as the normal force. It is a vector(a magnitude and direction) drawn perpendicular to the surface. Sooooooo, this 196 Newtons is what we are interested in. In order for the block to move you need to overcome the frictional force, which we dont know, but we do know both elements required to calculate it, the coefficient of friction, and the normal force on the block. So all we have to do is multiply them. Ff= mu*Fn, or (.023)*(196Newtons). Notice that the coefficient of friction is a unitless quantity leaving us with a static frictional force of 4.508 Newtons, that must be over come before the object can move. I can draw pictures or a free-body diagram if you require it. It's more interesting if its on a slope. |
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Frictional force = mu times the normal force. Mu = coeff of friction = 0.023 Note it has no units Normal force is the mass of the object assuming it's sitting flat on the surface times gravity FF = .023 x 20 kg x 9.807 m/s2 Bomber ETA "G" Been a long time since I took Statics |
Are you serious? Jeeze, what the hell are they teaching kids these days. |
It isn't required but if its not too much trouble I would like to see it. I'm pretty visual. Patty |
That's about the size of it. I wish I had a dollar for every FBD I've drawn  .
Yea then you have to pull out some trig  to determine the component of force applied Bomber |
Yeah no kidding. I had friction lessons in high school. 9th grade I think. You guys still learning about square Earths and using slide rules too? |
That's awesome. Thank you very much.  Patty |