thanks for the help tony i just don't get why fn = ma isn't fnet = ma? or is normal force the same as net force?
EDIT: nevermind its fg = mg which is the same as normal force.
Computer Science Canada physics - friction |
| Author: | cool dude [ Wed Mar 29, 2006 7:49 pm ] |
| Post subject: | physics - friction |
hi i have 3 physics question. 1) a 50kg toy car is being pushed with a force of 60N along a flat track. what is its static coefficient of friction if the car is stationary? 2) how will the coefficient of kinetic friction between rubber and ice change after the machine that cleans ice has cleaned the ice? please explain 3) how does mu (coefficient of friction) change when friction is decreased? please explain. (my guess is that mu will decrease because mu = friction/normal force therefore if friction decrease mu will decrease too. right?) i just started physics so we didn't learn that much so please explain well and don't make it complicated!!! |
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| Author: | Andy [ Wed Mar 29, 2006 7:53 pm ] |
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ummm why does it seem like you want us to do your homework for you?? for number 1, it would be 60N since the force of friction counteracts the applied force, thus it can never exceed the applied force, otherwise the object will move the other way. 2) the coefficient of kinetic friction will decrease since the surfaces of contact is less coarse. 3) what do you mean friction is decreased? and mu does not equal normal force.. mu*normal force = force of friction |
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| Author: | Tony [ Wed Mar 29, 2006 8:03 pm ] |
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Andy - 1. was asking for the coefficient. μ * normal force = friction force μ = friction / normal μ = 60 N / (50 kg * 9.81 m/s^2) μ = 0.122 |
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| Author: | cool dude [ Wed Mar 29, 2006 8:06 pm ] |
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thanks for the help. and no i'm not asking u to do my homework because i have a test tommorow and i would have asked my teacher instead but he was too busy and left before i had a chance. also if u read wat i said u would have seen u made a terrible mistake! i said mu = friction/normal force and u said NO it doesn't friction = mu*normal force and the mistake u did was that those r two exactly the same formulas!!! just rearanged also if u read my post i know that for question 1 it would be 60N but i don't know wat the coefficient (mu) is? |
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| Author: | Cervantes [ Wed Mar 29, 2006 8:08 pm ] |
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2) Trick question: The coefficient of friction between two surfaces is a constant. 3) See 2. Friction could be reduced by choosing two materials with a lower coefficient of friction, or by reducing the normal force. |
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| Author: | cool dude [ Wed Mar 29, 2006 8:11 pm ] |
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thanks for the help tony i just don't get why fn = ma isn't fnet = ma? or is normal force the same as net force?
EDIT: nevermind its fg = mg which is the same as normal force. |
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| Author: | cool dude [ Wed Mar 29, 2006 8:14 pm ] |
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Cervantes wrote: 2) Trick question: The coefficient of friction between two surfaces is a constant.
3) See 2. Friction could be reduced by choosing two materials with a lower coefficient of friction, or by reducing the normal force. wait so your saying by adding water to ice the coefficient won't change because its a constant? i thought it has to change since the friction would change and plus as andy said there would be more interactions involved |
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| Author: | Andy [ Wed Mar 29, 2006 8:21 pm ] |
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Cervantes wrote: 2) Trick question: The coefficient of friction between two surfaces is a constant.
err no... ever went curling? if it's constant than why do the scrubbers even bother trying to sweep the ice? you're thinking about mu being independent of the surface area of contact |
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| Author: | Tony [ Wed Mar 29, 2006 8:23 pm ] |
| Post subject: | |
well 2. _is_ tricky with wording. Coefficient between rubber (material) and ice (material), not rubber and ice's surface I guess this calls for a clarify.
cool dude wrote: i thought it has to change since the friction would change the coefficient is constant and independent of the force applied. more normal force = more friction, but the ratio will remain the same. |
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| Author: | Cervantes [ Wed Mar 29, 2006 8:25 pm ] |
| Post subject: | Re: physics - friction |
The way the question is worded, it doesn't matter what we do to the ice or rubber we're talking about in the second half of this question. cool dude wrote: 2) how will the coefficient of kinetic friction between rubber and ice change after the machine that cleans ice has cleaned the ice? please explain The question is really, "How will the coefficient of kinetic friction between rubber and ice change?" It's asking about rubber and ice in general, not about the specific rubber and ice involved in an ice cleaning machine situation. Edit: Argh! Every time I post, there are two new posts ahead of me! |
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| Author: | md [ Wed Mar 29, 2006 8:25 pm ] |
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by sweeping the ice you remove any debris that might be there (it can make a huge difference), and you melt the top layer of ice forming a very thin layer of water. The coefficient changes because of the interaction of the water layer with the ice and the stone. |
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| Author: | cool dude [ Wed Mar 29, 2006 8:27 pm ] |
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Tony wrote: well 2. _is_ tricky with wording. Coefficient between rubber (material) and ice (material), not rubber and ice's surface
I guess this calls for a clarify.
cool dude wrote: i thought it has to change since the friction would change the coefficient is constant and independent of the force applied. more normal force = more friction, but the ratio will remain the same. just wondering is it possible to increase friction but keep the normal force the same because than the coeficient will change wouldn't it? |
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| Author: | Cervantes [ Wed Mar 29, 2006 8:33 pm ] |
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You'd have to change the surfaces involved. |
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| Author: | cool dude [ Wed Mar 29, 2006 8:37 pm ] |
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wat i mean is tony said more normal force = more friction, but the ratio will remain the same. so if i were to change the surface to add more friction does automaically the normal force change so they're always a ratio. so u can't change one without the other? therefore the coefficient will stay the same? also i still don't get the answer to question 2 because u guys r sort of saying different things. |
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| Author: | Tony [ Wed Mar 29, 2006 8:46 pm ] |
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cool dude wrote: so if i were to change the surface to add more friction does automaically the normal force change so they're always a ratio.
No, normal force = opposite to force of gravity, you can only change it by making the object's weight change. If you change the surface to modify the amount of friction generated, you'll be changing the coefficient since it's now an interaction between something and a modified surface. |
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| Author: | cool dude [ Wed Mar 29, 2006 8:50 pm ] |
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k i'm a little confused because u said Quote: the coefficient is constant and independent of the force applied. more normal force = more friction, but the ratio will remain the same. according to this the normal force and friction are a ratio and the coefficient is constant and not dependent on these forces |
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| Author: | Cervantes [ Wed Mar 29, 2006 8:53 pm ] |
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Tony wrote: No, normal force = opposite to force of gravity, you can only change it by making the object's weight change. O_o I'm sitting on a chair right now. When I stand up, the normal force for the chair will be greatly reduced. 
cool dude: You could probably ignore what I said about #2. Chances are it wasn't meant to be a trick question |
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| Author: | cool dude [ Wed Mar 29, 2006 9:11 pm ] |
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k so the coefficient will increase right? since there will be more friction because rubber will be interacting with ice as well as water (i guess) from the cleaning machine. |
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| Author: | Tony [ Wed Mar 29, 2006 9:11 pm ] |
| Post subject: | |
Tony meant to say: weight on an object 
you're right Cervantes |
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| Author: | Andy [ Thu Mar 30, 2006 8:32 am ] |
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cool dude wrote: k so the coefficient will increase right? since there will be more friction because rubber will be interacting with ice as well as water (i guess) from the cleaning machine.
err no.. it would be less.. read our posts! |
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| Author: | Cervantes [ Thu Mar 30, 2006 5:20 pm ] |
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I think the moral of the story here is that too many cooks spoil the broth. Shakespeare wrote: Confusion now hath made his masterpiece. Physics, you're no match for Cervantes!  |
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| Author: | cool dude [ Thu Mar 30, 2006 7:46 pm ] |
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i don't understand why it will be less? if there will be more interactions between rubber and the ice and now the water the friction will increase right? and when friction increases i'm pretty sure mu increases because mu = kinetic friction/normal force. please explain why it decreases? |
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| Author: | Tony [ Fri Mar 31, 2006 2:02 am ] |
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cool dude wrote: please explain why it decreases?
Water acts as a lubricant mmm... lube |
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| Author: | Andy [ Fri Mar 31, 2006 9:24 am ] |
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cool dude wrote: i don't understand why it will be less? if there will be more interactions between rubber and the ice and now the water the friction will increase right? and when friction increases i'm pretty sure mu increases because mu = kinetic friction/normal force. please explain why it decreases?
didnt you read my earlier post? it's independent of the surface area of contact.. by cleaning the ice you remove debris and other materials, also water would have a lesser coefficient of friction |
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| Author: | cool dude [ Fri Mar 31, 2006 5:12 pm ] |
| Post subject: | |
thanks i get it now!  |
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