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# Friction.....

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I have a physics problem, that is written below, and I have a question about it. Here is the problem: A box is given a push so that it slides across the floor. How far will it go, given that the coefficient of kinetic friction is .2 and the push imparts an initial speed of 4.0 m/s. Now, I''ve been thinking about this problem for a while, and I don''t really know what to do. Wouldn''t you need to at least know the mass of the object or the time it takes for the object to come to rest? Any help is appreciated. Thanks in Advance, --BioX

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Now, I''ve been thinking about this problem for a while, and I don''t really know what to do.

Write out the equations, leaving whatever you don''t know as a symbolic parameter. Then work from that.

Wouldn''t you need to at least know the mass of the object or the time it takes for the object to come to rest?

No you don''t.

Any help is appreciated.

Homework questions don''t belong in here.

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why not?
If you could direct me to a place where I could obtain this information I'd be most grateful, until then, this is my best resource.

--BioX

[edited by - BioagentX on November 30, 2003 9:47:32 PM]

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If you''re in middle/high school, then there''s usually homework clinics at school, or at a community center near you.

If you''re a freshman taking "basic physics you should have gotten in middle school for liberal arts majors 101" then there''s SURE to be TAs who can help you with these problems.

If nothing else fails, why don''t you call the teacher who gave you the homework in the first place, and tell him that you''re having trouble with this question, and what kinds of clarity can he give? Or search him out during recess and ask the same thing.

Or, worst case, pay a tutor.

Anyway, the crazy thing is that you already GOT all the help you need! Fruny is right on the money. If you''re too lazy to do even that, then you should save yourself some time, and take a job flipping burgers or something right now. School''s harder than that, and real life''s even worse.

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I just want an answer to this one problem so I can know how to do problems like it. I don''t need to enroll in some clinic or community center you moron. I can''t believe you even suggested that. I tried google already...no luck. And I wouldn''t dare dream of calling my teacher....who the f--k does that? I''m going to get the answer tomorrow, I just wanted to see if there was anyone willing to help me out before tomorrow. I guess not.

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he told you everything already. Write down the damn equations and just solve them. Your problem is very basic. Just write down the damn equations on paper.

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bioagentX,

I''m the moderator of this forum, and I set the policy on homework, which you can review in the forum FAQ:

Forum FAQ

There are two basic reasons why homework isn''t appropriate for these forums. First, this is a game development sight, and homework usually is not about game development. Some of these questions are a misuse of our expensive bandwidth. But, of course, some homework questions are relevant and so some of them are allowed to remain open. But we ask that you follow the guidelines in the FAQ.

More importantly, though, there are a bunch of people out there who will cheat on their homework if given a chance. Now, many students really do want to learn but there have been quite a number of posts here where the poster was obviously just trying to get a complete answer so that they could pass a test or homework, without doing hard work themselves. In 10 years they will wish they had done the work. This forum does not wish to help students cheat because it just will not help those folks in their jobs or life once they graduate. It surely will not help them find and keep a job in game development---there''s twenty thousand other hungry kids and adults who are better than that scrambling for the same job. Cheating kills a person''s problem-solving skills. Those skills don''t come for free, and take time to develop. And so we try to close threads that are homework, suggest that students discuss problems with their fellow students and teachers, and educate the community about the forum policy---what it is and why.

As you can see from the comments from other forum members, folks here are mostly aware of the policy and are very good at helping me to enforce the policy.

As for your question, its tangentially relevant to game development. Games have boxes that slide across the floor. Games have physics. And you didn''t ask for a direct answer so as far as I''m concerned your post can remain open. BUT, I hope that forum members will not just solve the problem for you and give the answer. That would disturb me.

The forum FAQ does provide you with links to more homework-oriented sites (e.g., Dr. Math).

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

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quote:
Original post by bioagentX
And I wouldn''t dare dream of calling my teacher....who the f--k does that?

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

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The mass of the box is needed to solve this problem. Coefficient of friction is force of friction over normal force. Since there is no vertical acceleration, normal force = mass times g (9.80 N/kg). Using the coefficient of friction, you then find the net force on the block which is the force of friction, and use the equation a = delta v over delta t to find delta t, or whatever you need to find. This is basic grade 12 physics.

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quote:
Original post by grhodes_at_work
...First, this is a game development sight, ...

[SpellingNazi]
You mean site, right? :-P
[/SpellingNazi]

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OK, gamechampionx, you have just proven yourself to be a moron. Mass cancels out, so you don''t need it.

--------------------------------------
I am the master of stories.....
If only I could just write them down...

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Nathaniel Hammen, you''re not that much better yourself, you still need MASS, BUT it may be a semi-null variable for the given question so you don''t need a VALUE of MASS, but mass itself is still needed.

I must say I hate questions like the first one of the topic,... but they are not that hard if you lay out all the variables you have values for, and then lay out all your given equations. If you don''t have the equations given to you in a text book or something, then why are you doing a question like that, cause you likely are not schooled enought to be needing something like it.

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LOL, we're doing this EXACT kinda thing here in new mexico, and I think that the equation you need is Fnet = Ufriction*Fnormal. Sorry if I violated a policy, but this stuff is fresh on my mind, and I'd like to help out. In fact, can you just rearrange future questions so that they look like game programming questions? I.e. I have a box in my game world, and I'm trying to apply a physics model to it...

[edited by - bjmumblingmiles on December 2, 2003 11:09:40 PM]

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quote:
Original post by intrest86
quote:
Original post by grhodes_at_work
...First, this is a game development sight, ...

[SpellingNazi]
You mean site, right? :-P
[/SpellingNazi]

Yah, mon. My bad.

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

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Folks,

Lets try to be friendly here. You do not save face or become a champion by attacking someone else who makes a minor mistake or fails to include every tiny infinitesimal detail of an algorithm. Please do correct mistakes, but do it in a friendly manner so that people will want to actually continue reading the thread beyond your post.

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

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quote:
Original post by bjmumblingmiles
In fact, can you just rearrange future questions so that they look like game programming questions? I.e. I have a box in my game world, and I''m trying to apply a physics model to it...

Surely you can. If you do such a thing just to sneak a homework problem in so that you can cheat, well then, you might just pull the wool over our eyes and get the treasure you seek---the free answer to your immediate problem. But, in the long run, what goes around comes around and you will discover you have cheated yourself.

I''m not saying that it is never good to be given the answer. Sometimes it is. An answer can sometimes just cause something in your brain to snap, leaving you with a clear understanding of a theory or algorithm. BUT, it is best to be given the answer by an educator, a professional, who is equipped---and paid---to spend extra time trying to push you beyond the question whose answer you were given. You aren''t guaranteed of that in school, but there''s a much better chance that your teacher will push you beyond than anyone here will. Everyone here helps out for free, with no guarantees. And by my estimate most of the people here do not have formal training or professional experience. There are lots of damn smart folks here who know there stuff. Both the gamedev staff/moderators and regular members. But, know that you gets what you pays for. One way or another.

Remember that hard work is a form of payment that produces most excellent rewards.

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

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btw, the mass isn''t needed

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quote:
Original post by grhodes_at_work
Now I have a Masters degree, 75% of a Ph.D, and a stable, excellent paying job...

How do you have time to be doing a PhD and have a job at the same time? Or did you give up on the PhD at 3/4 of the way through?

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*blink* I thought you needed the mass for the slowing down factor...

OK.
Theres going to be a friction acting, right?
So you need to figure out something that makes the box slow down.
Its a really simple problem(sorry), so I can''t help more than that.

Reread your book, I''m pretty sure it has something like this in the examples(even given a crappy book).
Also hit up your classmates for answers, and talk to the teacher.

Talking to teachers might be considered gauche and sucking up to them, but trust me, in my 2.5 years of college it helps far more than it hurts.

~V''lion

Bugle4d

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Homework clinics are free (and don''t need enrollment, usually; walk-ins welcome). Calling teachers is free. Heck, asking your parents is free, as is asking your classmates, or maybe even people from the class above.

Unfortunately, all of these solutions involve actual face-to-face (or telephone) contact with real people. I know that can be scary at times, because it''s so hard to keep up your stone hard teenager attitude.

The teenagers that actually end up mattering ten years from now (that is, in the unimaginably distant future) are the ones that know how to drop the facade, and talk to people.

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quote:
Original post by Vlion
Also hit up your classmates for answers

Lol? you meant beat them up until they talk?

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I wish people would stop saying its a simple problem. Simple is so relative. And it''s considerably harder if you don''t know the constant acceleration formula (you know the one I mean), you start going into calculus to derive the damn thing. (or messy algebra).

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Y
^ V0=4 m/sec
| N k=0.2
| /\ --->V0
| |
| |
| f<----*
-------------------|------------------>X
|
\/
mg

( I dont know why the drawing doesnt shows right at the post, it does at edit mode :/ )

N is the counter force working on the body
N=mg

1st Law of Niuton on Y axis ( no acceleration )
=============================
the sum of all forces = masa * acceleration
N-mg=0
since you havent specified the masa of the body i presume its 1
and g=9.8 round it up to 10, g=10 and since the gravity working against axis Y g=-10

N=1*-10=-10 N=-10

2nd Law of Niuton on X axis
===========================
f=ma

f=k*N=0.2N=0.2*-10=-2
-2=1*a
a=-2 // the acceleration on the x axis is -2

Now you got all you need to solve this problem

X(t)=X0+V0x*t+a*t^2/2

(a) X(t)=0+4*t-2*t^2/2

the speed equation:
V(t)=V0+ax*t
when the body will stop its speed will be 0
V(t)=0
0=4-2*t
t=2sec // after 2 sec the body will stop
now we put t=2sec in equation (a)
X(t=2sec)=4*2-2*4/2=8-4=4 meters

So the bodey will stop after 4 meters

[edited by - spree on December 5, 2003 1:48:52 AM]

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