# Friction Questions: Find Force, Mass Given

• rofljohn
In summary, The conversation discusses how to find the friction force of an object moving at a constant speed over a level surface with only the force being put on the object and the mass of the object given. The equation F_friction = mu N is used to solve this problem, and the conversation also dives into the concepts of static and kinetic friction. Additionally, the conversation touches on how to find the force needed for an object to go up an incline, using Newton's second law and a free body diagram.
rofljohn
I'm having trouble with friction so bear with me, i might have a lot of questions...

If i have an object moving at a constant speed over a level surface, how do i find the friction force? I'm only given the force being put on the object (470 N) and the mass of the object (23 kgs)?

Dorothy

Dorothy Weglend said:

Dorothy
No not really :| I can't find a formula that gives me kinetic friction

Also on another problem i think i have the answer but I'm not sure. A mass of 37 kg slides down a 17 degree incline at a constant speed. Is the mu equal to ~.31?

rofljohn said:
No not really :| I can't find a formula that gives me kinetic friction

Also on another problem i think i have the answer but I'm not sure. A mass of 37 kg slides down a 17 degree incline at a constant speed. Is the mu equal to ~.31?

Yes, that's the right answer. If you can solve this one, you can solve your first one. And you must know the equation F_friction = mu N, or you couldn't have solved this one

Dorothy

Umm, actually, you don't even need F = muN to solve the first problem. Sorry about that.

I still have no clue where to begin on that problem.

I did another one. A mass of 45 kg is sliding down a 34 degree incline with a mu of .32

Yes. That is correct also. These problems are harder than the one you can't solve.

You have a force pushing on an object. You have a frictional force opposing that motion.

Consider what would happen without the friction. Since the force is 470 N, with no friction, the object would accelerate at 470 N/23 kg = 20.4 m/s^2.

But what is it's acceleration now? If you realize what that is, then you can write the force equation using the second law and solve for the friction force.

Dorothy Weglend said:
Yes. That is correct also. These problems are harder than the one you can't solve.

You have a force pushing on an object. You have a frictional force opposing that motion.

Consider what would happen without the friction. Since the force is 470 N, with no friction, the object would accelerate at 470 N/23 kg = 20.4 m/s^2.

But what is it's acceleration now? If you realize what that is, then you can write the force equation using the second law and solve for the friction force.
Since it's moving at a constant speed the acceleration is 0. So does that mean the friction force is 20.4?

So the mu is .09?

20.4 is the acceleration without any friction. F = ma, so a = F/m is how I got that.

You are on the right track, though, with a = 0.

Consider:

Sum of forces = ma, a = 0, so Sum of forces = 0.

What are the two forces that add to zero?

Dorothy Weglend said:
20.4 is the acceleration without any friction. F = ma, so a = F/m is how I got that.

You are on the right track, though, with a = 0.

Consider:

Sum of forces = ma, a = 0, so Sum of forces = 0.

What are the two forces that add to zero?
Friction force and the force being pushed on the object?

Bingo...

Sooo does that mean the friction force is equal to 470? That doesn't make any sense though because wouldn't it have to be less in order for the object to move?

rofljohn said:
Sooo does that mean the friction force is equal to 470? That doesn't make any sense though because wouldn't it have to be less in order for the object to move?

That's right. 470 N is the force of friction.

There's two kinds of friction involved in these types of problems. One is called static friction. This is the friction that keeps an object from moving. Then there is kinetic friction, which operates when an object is moving. This is an example of kinetic friction.

They work very similarly. With static friction, you apply a force, and the frictional force opposes that with an equal amount of force. As you increase the applied force, the static friction increases to match it. Eventually, you apply enough force that static friction can't increase, and the object begins to move.

At that point, it's all about kinetic friction. Since the mass doesn't change, the kinetic friction reduces the acceleration of the object. If there is enough kinetic friction (or not enough applied force), then the acceleration is reduced to zero, but that doesn't mean the object stops moving.

But I have to say you are doing very well with these friction problems.

Good luck,
Dorothy

Well for most of them it's a matter of finding the formula and plugging in the numbers. The ones in which I don't do that I'm for the most part lost.

On this next problem, I am supposed to find what force would be needed for a 35 kg mass to go up a 25 degree incline at 3 m/s/s with a mu of .11.

The net force is 105 N. What do I do with this? I've found the normal force to be 95 and the friction force to be 10.

I hope you are drawing free body diagrams. Even if these problems seems easy, later ones won't. The practice of drawing the diagrams, and translating them into mathematics will help later, even if it seems silly now.

In any case, in the current problem, you are stuck at the same place you were in the first one. You just apply the Newton's second law, as you did when you succesfully solved the first problem, and solve for the unknown force.

I drew a free body diagram for this one.

Here's what I got.

...|
...| <- Fn = 95
...o---o
?---|35 |---- <- Ff = 10
...o---o
...|
...9.8-> |

I am trying to find the ?... Do I take the net force minus the Ff or what? If I'm doing something wrong or have wrong calculations please tell me or else I'll get nowhere.

Isn't this on an incline? Maybe you just left that out because it's hard.

I think your value for the Normal force is wrong, probably because you don't quite have your FBD right yet.

Draw a block on an incline. Then attach all the forces. You should have four forces (so four arrows). Assuming you have the X axis running parallel to the incline (tilted at 25 degrees, in other words), one of these will have to be decomposed into components parallel to your tilted axis.

Phew. Well, I hope that helps. I am getting sleepy, but will try to stay up a little longer.

I don't know, it's still confusing to me but I think I'm going to go to bed and talk to the teacher about it in the morning. Thanks for all the help though.

## 1. What is friction?

Friction is a force that resists the motion of an object across a surface. It occurs when two surfaces are in contact with each other and is caused by the microscopic roughness of the surfaces.

## 2. How is the force of friction calculated?

The force of friction can be calculated using the equation F = μN, where F is the force of friction, μ is the coefficient of friction, and N is the normal force between the two surfaces.

## 3. What factors affect the force of friction?

The force of friction is affected by the type of surfaces in contact, the normal force between the surfaces, and the coefficient of friction. Other factors that can affect friction include the roughness of the surfaces, the temperature, and the presence of lubricants.

## 4. How does mass affect friction?

The mass of an object does not directly affect the force of friction. However, a heavier object may have a greater normal force, which can increase the force of friction.

## 5. How is the coefficient of friction determined?

The coefficient of friction is determined experimentally by measuring the force of friction and the normal force between two surfaces. It can also be found in tables for common materials.

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