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9/22/2017 12:11:25 AM
Posted: 9/21/2005 6:11:46 PM EDT
[Last Edit: 9/21/2005 6:11:58 PM EDT by pattymcn]
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
Link Posted: 9/21/2005 6:13:19 PM EDT
No.

You need to do your own homework patty.



Link Posted: 9/21/2005 6:14:00 PM EDT
When my kids come to me with problems like this I tell them to think UNITS. Nothing but UNITS. Know the UNITS of the numbers in the problems. Know the UNITS of the answer and go from there.
Link Posted: 9/21/2005 6:16:22 PM EDT

Originally Posted By TheCommissioner:
When my kids come to me with problems like this I tell them to think UNITS. Nothing but UNITS. Know the UNITS of the numbers in the problems. Know the UNITS of the answer and go from there.



Yea yea just give me the damn answer and pour me a drink, scotch n soda please. Patty
Link Posted: 9/21/2005 6:19:07 PM EDT
you need physics help. I've increasing found that just because i've taken all those physics and calc classes, i dont seem to remember any of it.....
Link Posted: 9/21/2005 6:19:33 PM EDT
My hazy recollection:

F = Ma

The force pushing down on the object is (20 KG) * (9.8 m/s^2) = 196 Newtons

Pull the object at a right angle to the gravity vector, so

196 N * .023 = 4.5 N

Make a free body diagram and it's pretty clear.

Link Posted: 9/21/2005 6:22:08 PM EDT

Originally Posted By pattymcn:

Originally Posted By TheCommissioner:
When my kids come to me with problems like this I tell them to think UNITS. Nothing but UNITS. Know the UNITS of the numbers in the problems. Know the UNITS of the answer and go from there.



Yea yea just give me the damn answer and pour me a drink, scotch n soda please. Patty



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.
Link Posted: 9/21/2005 6:22:25 PM EDT

Originally Posted By mcgredo:
My hazy recollection:

F = Ma

The force pushing down on the object is (20 KG) * (9.8 m/s^2) = 196 Newtons

Pull the object at a right angle to the gravity vector, so

196 N * .023 = 4.5 N

Make a free body diagram and it's pretty clear.



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.
Link Posted: 9/21/2005 6:22:55 PM EDT

Originally Posted By pattymcn:

Originally Posted By TheCommissioner:
When my kids come to me with problems like this I tell them to think UNITS. Nothing but UNITS. Know the UNITS of the numbers in the problems. Know the UNITS of the answer and go from there.



Yea yea just give me the damn answer and pour me a drink, scotch n soda please. Patty



"Give a man a fish today and he will be hungry tomorrow. Teach a man to fish and he will never go hungry."
Link Posted: 9/21/2005 6:23:16 PM EDT
I like fig newtons.


And Pie.

But I'll settle for Oreos and milk ... mmmmmm double stuffed oreos......
Link Posted: 9/21/2005 6:23:54 PM EDT
[Last Edit: 9/21/2005 6:33:07 PM EDT by Dance]

Originally Posted By pattymcn:
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



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".
Link Posted: 9/21/2005 6:26:20 PM EDT
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

Link Posted: 9/21/2005 6:35:17 PM EDT

Originally Posted By CSM:
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
Link Posted: 9/21/2005 6:38:21 PM EDT

Originally Posted By Makarov_Mami:

Originally Posted By mcgredo:
My hazy recollection:

F = Ma

The force pushing down on the object is (20 KG) * (9.8 m/s^2) = 196 Newtons

Pull the object at a right angle to the gravity vector, so

196 N * .023 = 4.5 N

Make a free body diagram and it's pretty clear.



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.



M_M you're too funny!

OKay let me see if I can figure this out. Hang on - let me get the daughter. Patty
Link Posted: 9/21/2005 6:39:35 PM EDT

Originally Posted By mrphelps:

Originally Posted By CSM:




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.



You ever doubted me? tsk. tsk. tsk.
Link Posted: 9/21/2005 6:42:48 PM EDT

Originally Posted By Dance:

Originally Posted By pattymcn:
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



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".



Patty
Link Posted: 9/21/2005 6:45:28 PM EDT
Your daughter's gotta be in college because I'm in the highest level of physics offered in most American high schools and the only thing we've been taught about friction is "ignore it for now."
Link Posted: 9/21/2005 6:45:29 PM EDT

Originally Posted By mcgredo:
My hazy recollection:

F = Ma

The force pushing down on the object is (20 KG) * (9.8 m/s^2) = 196 Newtons

Pull the object at a right angle to the gravity vector, so

196 N * .023 = 4.5 N

Make a free body diagram and it's pretty clear.




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.
Link Posted: 9/21/2005 6:47:20 PM EDT

Originally Posted By Roadhawk:

Originally Posted By mcgredo:
My hazy recollection:

F = Ma

The force pushing down on the object is (20 KG) * (9.8 m/s^2) = 196 Newtons

Pull the object at a right angle to the gravity vector, so

196 N * .023 = 4.5 N

Make a free body diagram and it's pretty clear.




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.



everything is always under accelleration from gravity. yes it is the correct way.
Link Posted: 9/21/2005 6:49:08 PM EDT

Originally Posted By MagKnightX:
Your daughter's gotta be in college because I'm in the highest level of physics offered in most American high schools and the only thing we've been taught about friction is "ignore it for now."



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
Link Posted: 9/21/2005 6:53:46 PM EDT
[Last Edit: 9/21/2005 6:54:47 PM EDT by ColonelKlink]
Link Posted: 9/21/2005 6:54:45 PM EDT
[Last Edit: 9/21/2005 6:58:19 PM EDT by thebomber]
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
Link Posted: 9/21/2005 6:58:01 PM EDT
Link Posted: 9/21/2005 7:01:00 PM EDT

Originally Posted By MagKnightX:
Your daughter's gotta be in college because I'm in the highest level of physics offered in most American high schools and the only thing we've been taught about friction is "ignore it for now."



OMG!!!



Essayons
Link Posted: 9/21/2005 7:01:40 PM EDT

Originally Posted By ColonelKlink:

Originally Posted By CSM:

Originally Posted By mrphelps:

Originally Posted By CSM:




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.



You ever doubted me? tsk. tsk. tsk.




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.



It isn't required but if its not too much trouble I would like to see it. I'm pretty visual. Patty
Link Posted: 9/21/2005 7:15:21 PM EDT
[Last Edit: 9/21/2005 7:21:54 PM EDT by ColonelKlink]
Link Posted: 9/22/2005 3:01:37 AM EDT
[Last Edit: 9/22/2005 3:14:29 AM EDT by thebomber]

Originally Posted By ColonelKlink:

Originally Posted By pattymcn:
It isn't required but if its not too much trouble I would like to see it. I'm pretty visual. Patty




very well.

colonelklink.zerohold.net/diagrams/freebody.gif



That's about the size of it. I wish I had a dollar for every FBD I've drawn .


It's more interesting if its on a slope


Yea then you have to pull out some trig to determine the component of force applied

Bomber
Link Posted: 9/22/2005 3:11:11 AM EDT

Originally Posted By ColonelKlink:

Originally Posted By MagKnightX:
Your daughter's gotta be in college because I'm in the highest level of physics offered in most American high schools and the only thing we've been taught about friction is "ignore it for now."




Are you serious? Jeeze, what the hell are they teaching kids these days.



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?
Link Posted: 9/22/2005 5:31:11 AM EDT

Originally Posted By ColonelKlink:

Originally Posted By pattymcn:
It isn't required but if its not too much trouble I would like to see it. I'm pretty visual. Patty




very well.

colonelklink.zerohold.net/diagrams/freebody.gif



That's awesome. Thank you very much.

Patty
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