# B Frictional force and surface area in contact

1. Oct 3, 2016

### tyty123

I've read that the surface area of an object in contact with the ground doesn't not affect the frictional force acting on it as it is pushed forward.

I kinda understand what is explained but I find it difficult to reconcile with what happens in real life...

Don't wheels reduce the surface area in contact and hence reduce friction acting on the object, allowing it to move forward more easily? Or like a larger parachute results in more surface area in contact with air, slowing the fall even more.

2. Oct 3, 2016

### Diptangshu

Hello Brother,

Your Confusion is Genuine. But there is clear explanation of this.

I hope you have already studied the Microscopic aspect of Friction..... That it depends on Surface nature, not on the Area of Contact.

But let's put it in a Simple way,
Lets us consider a Normal Carpet Stage,

You take two Blocks a Very Heavy one but small and a Very Light one, but big. Now you Push both individually to check your efforts.

According to your Judgment it may seem The Big one has Higher Friction due to Higher Area, but the Smaller one has less Friction.
But in your instinct or if you Perform the Experiment..... Taking a Iron Block and a Large Plywood.... You will Feel Iron hard to Move.

The basics is that..... Friction is independent on Area or Mass.... Friction actually refer to The Coefficient 'mu(μ)'...... But not the Force Due to Friction.
F=μ*R___ refers that F is proportional to R.
And R is actually dependent on Mass or few other Factors of the Body (like, Area for Fluid Flow)

When you Say about Parachute... Force (Drag Force) actually depends on Area, but not the Quality of Friction.
Like if you open Parachute over Desert... You will feel slightly Lesser Friction Constant as Air is less dense compared to opening over a Sea Surface.
But if you Use a Large and a Small Parachute both over Desert .... The Larger will Feel High Force of Frictional Resistance.

I hope the Doubt is Clear.

Reply again if anything is not Understood.

3. Oct 3, 2016

### tyty123

It's interesting that you say that friction is referring to the coefficient and not the force due to friction. Does that mean that friction and frictional force area different? Or is there another name we call the force due to friction?

4. Oct 3, 2016

### tyty123

How about wheels? Do we say they reduce friction or frictional force?

Btw thank you so much for taking the time to guide me along. Much appreciated!

5. Oct 3, 2016

### Diptangshu

Well Brother, Friction Always Exists when you make two Surfaces contact.... No matter How small or Big it is. R is Force due to electromagnetic interaction between two Surfaces... And μ is due to nature of the Surfaces.

But When you Make a Fat man and a Thin boy stand on Floor with same Shoes on........ R for Fat man is Higher due to the Fact that R have to balance heavier weight.
So F=μ*R is High for Fat man... As μ is Constant due to Same Shoes on Same floor.
Area does not actually plays any Role here.

But In Aeroplane Wing or Parachute as you Said Area plays most important Role in Determining quantity similar to R.
That is why, if you Throw A Disk to your Friend... And then a Small Ball..... Disk reaches Later... Due to High Area.

There F=μ*A (although it isn't μ actually for fluids, but similar)

You can See the difference.... Force due to Friction has Dependence on Area, Weight or other Factors.

But μ depends on Surface nature only.

6. Oct 3, 2016

### tyty123

It's becoming clearer to me now. But on this other website they mention "large contact area, high friction, box is hard to move". Does that mean the website is wrong?

7. Oct 3, 2016

### Diptangshu

Welcome Bro,

For Wheels the Frictional aspects are a Bit different. It is explained much like tearing of bonds between Floor and Wheel.

Let's make it simple,
At The point of Contact of wheel, you attach some Threads from wheel surface to floor surface, before and after the Point of Contact.

Then you Start Rolling..... Imagine what happens, Threads against the Direction of Rotation feels tension and Try to Tear....... But Threads along direction of Rotation become Compressed....
Similar happens with the Bonds of Friction.

That is why it is Easier to Break bonds in Rolling than in Sliding.
Area Doesn't play a Role... Only mass does.... In determining Force of Friction.
And for μ, surface nature plays only role as always.

8. Oct 3, 2016

### Diptangshu

Yes it is Wrong.... Which website is this?

Try more Fundamental Books of Physics.

It's Leonardo da Vinci who discovered Laws of Friction.

And the Most Fundamental quality is that Friction is independent on Area.

9. Oct 3, 2016

### tyty123

10. Oct 3, 2016

### Diptangshu

Well I don't know how he states these misleading things.... But what I said is Absolutely the Fundamental Concept.

You can Search other Websites... Like Quora, Wikipedia etc

11. Oct 3, 2016

### tyty123

Ya. Just realised there are many websites with the wrong information...

Here is another from a friend. http://www.skwirk.com/p-c_s-4_u-193...nd-reducing-friction/Friction/Forces/Science/

"Reduce the contact between the surfaces.

Have you ever tried to roll a cube? Spheres are the best shape for reducing friction because very little of a spherical object is in contact with the other surface. Several types of wheels, such as skateboard wheels, contain small spheres called ball bearings to reduce the friction between the moving parts. You can witness the effect of ball bearings by comparing the friction between sliding a book on a table and then doing the same, but using marbles between the book and the surface of the table. Notice how the marbles act as ball bearings, reducing the friction."

It's scary that all this misinformation is floating around on the Internet.
But thanks for the right version of frictional force. I learned something today!

12. Oct 3, 2016

### Diptangshu

You are Most Welcome Brother,

If there were Surface area controlling Frictional Forces...then Many Problems in mechanics would be Solve very easily! But it is not so!

always Refer to popular websites and Popular Fundamental Books.

Rolling Friction is Less just for the Reason I mentioned. No Area comes into Play except Fluid Fow!

Have Good Day, tyty123.
For any other Mechanics Query, you can ask me, I will try to explain or refer to some good articles!

Only way to Reduce friction is To Alter Surface Quality.... Rub your Hand on Smooth Wall and Mirror..... Mirror will give very high μ, compared to wall.

13. Oct 3, 2016

### tyty123

Thank you, Diptangshu. You've been a great help!