Last question - Friction forces!

[ElectroNewby]

Hello questions, so this is my last question on physics for the moment.
I do have a lot of questions to find out but I need some guidelines sot hat's why I wrote 5-6 of them on this forum. Physics are pretty cool but the maths are a little far and we need both I know heh

Okay here's a question:

A block of 10.2 Kg lays on a honrizontal surface. The static friction coeficcient between the bloc and the surface equals 0,4 while the cinetic coefficient = 0.2

What are the values of the force of friction if we progresivly add an external force of 10, 30, 50 and 70 newtons ?

much thanks for all the help!

 

[3v0]

Two things.

First please post homework questions to this forum.

Second we do not just give students the answer. We may choose to help you if you are having problems but you have to give it a go and we will help make further progress.

Posting the problems and asking for answers is a bad idea.

3v0 Moderator

 

[ElectroNewby]

I don't care about the answers but I have to validate the formulas
Cmoe on

 

[squishy36]

You should be able to find the relevant formula in any beginning physics text. Thousands of them have been published and many are available free from google books. And nobody will do your homework for you -- why should we? We already did ours when we went through school.

 

[MrAl]

Hi,

You guys dont want to give him at least a few hints? What is 'Homework Help' for then?

 

[squishy36]

Mr. Al, I certainly don't mind helping students when they post their work and highlight exactly where they're having problems -- it's usually a small misunderstanding of some concept or an algebra mistake and easy to straighten out. And, what's always interesting, those of us giving the answers often learn something new too -- even though we were in school 40 or 50 years ago or more and should know this elementary stuff. This particular situation is also conjoined with a few other threads from ElectroNewby where he/she is asking for similar help in an elementary physics class. His/her reluctance to provide any partial work appears to be independent of the forceful suggestions of others on this forum to show some work -- indeed, somewhat analogously to frictional forces, if you get my drift.

 

[Grossel]

Hi.

First of all, where are you stuck?

In general, the steps to solve it is as follows:

• Draw a figure with forces
• Find the vertical force of the floor against the box
• Find the force necessary to pull the box (static friction coefficient)
• Given the list of pull force above, any of those numbers that is lower than the force neccesary to keep the box moving should stay the same as the numbers. Any numbers that is higher will automatically floor upon the friction force neccesary to pull the box.

 

[bountyhunter]

Here are hints: gravity pulls down on a mass with a specific force. As you drag it, friction causes a retarding force. The coefficient of friction tells how much retarding force. A "static" coefficient is how much it takes to get a stopped object moving from rest, the Kinetic coefficient is how much drag it has while it's moving.

 

[crutschow]

Another hint. The kinetic friction is independent of the speed.

 

[fermat7]

Hey ...
you question still not clear , any way: you should know:

1. These two coefficients you mentionned depend only on the surface of contact.
2 . The static friction depends on the horizontal external force acting on that mass, it acts in the opposite direction of this external force.
3. The maximum value of static friction force equals to the static coefficient times the Normal force acting on that mass. this value is the limit of mouvement.
4. applying an horizontal external force which is greater than this limit, then the mass will be in mouvement and the force of friction will be constant during the mouvement whatever the speed is, and its value equals cinetic coefficient times Normal force.

Thanks