physics - friction

[cool dude]

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!!!

[Andy]

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

[Tony]

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

[cool dude]

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?

[Cervantes]

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.

[cool dude]

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.

[cool dude]

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

[Andy]

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

[Tony]

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.

[Cervantes]

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!

[md]

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.

[cool dude]

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?

[Cervantes]

You'd have to change the surfaces involved.

[cool dude]

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.

[Tony]

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.