Friction equation confusing me

[Drizzy]

Homework Statement

Let's say I am pushing a couch with 30 N, the velocity is constant, therefore, the friction is also 30 N but in the opposite direction. But if a push it with 50 N then the friction force is going to change to 50 N as long as it is not accelerating. So how am I supposed to used this equation: Friction force=mu * Normal force. The normal force is the same whether I have a high speed or not.

Homework Equations

The Attempt at a Solution

 

[NascentOxygen]

Normally, sliding friction would stay approximately constant at 30N here, (F = μN), and an increase in applied force would produce acceleration until opposing forces (such as increased air resistance or other resistance to motion) exactly canceled the applied increase.

 

[CWatters]

+1

You have things the wrong way around. The friction isn't 30N _because_ the velocity is constant. The velocity is constant because you are pushing with a force equal to friction. If you push harder the couch will accelerate. Friction won't magically increase unless something else changes such as mu (μ) or the normal force.
[Mod note: Fixed spelling checker travesty for CW - gneill]

 

[CWatters]

Sorry my spelling checker changed mu to my.

 

[Drizzy]

but I can drive my car at a constant speed of 40km/h and 60km/h what about the friction there?
The way I think of it is like this: I have a little box next to my computer and I am pushing it slowly, but I am pushing it with the same force. And then I pushed it again but with a higher speed. And the speed was constant too. So what happened with the friction? I know that the velocity was constant because I could see that the box didnt move slower of faster.

 

[NascentOxygen]

It is sliding friction that 's approximately constant. When revolving, pneumatic tyres exhibit rolling resistance, and this increases with speed.

When moving through a fluid (air or water), bodies encounter drag or fluid resistance, and this increases markedly as speed increases, it's at least proportional to (velocity)².

So your car needs extra power (more fuel) to push its way through air at a higher speed.

 

[Mister T]

Let's say I am pushing a couch with 30 N, the velocity is constant, therefore, the friction is also 30 N but in the opposite direction. But if a push it with 50 N then the friction force is going to change to 50 N as long as it is not accelerating. So how am I supposed to used this equation: Friction force=mu * Normal force. The normal force is the same whether I have a high speed or not.

This is an approximation. That the friction force is independent of the speed. At higher speeds air drag becomes significant and the approximation no longer works. That's the reason your car behaves the way it does. At the speeds you cited, air drag is the dominant force retarding the motion of your car. The friction force exerted on the tires by the road is the force that propels the motion of your car.

 

[CWatters]

but I can drive my car at a constant speed of 40km/h and 60km/h what about the friction there?

That is a very different situation. Air resistance isn't the same at both speeds, nor is engine power/force.

The way I think of it is like this: I have a little box next to my computer and I am pushing it slowly, but I am pushing it with the same force. And then I pushed it again but with a higher speed. And the speed was constant too. So what happened with the friction? I know that the velocity was constant because I could see that the box didnt move slower of faster.

Kinetic friction force is normally assumed and modeled to be constant independent of velocity. So its the same at both speeds. In this case air resistance is negligible.

Once moving at constant speed the net force on the box is zero. Under those conditions the force you apply is equal and opposite to the friction force.

 

[Chestermiller]

The way I think of it is like this: I have a little box next to my computer and I am pushing it slowly, but I am pushing it with the same force. And then I pushed it again but with a higher speed. And the speed was constant too. So what happened with the friction? I know that the velocity was constant because I could see that the box didnt move slower of faster.

Assuming ideal sliding friction, the friction force was the same in both cases. And the force you had to apply (equal to the friction force) to move the box at constant speed was the same in both cases. So, what's the problem?

 

[osuboone]

Homework Statement

Let's say I am pushing a couch with 30 N, the velocity is constant, therefore, the friction is also 30 N but in the opposite direction. But if a push it with 50 N then the friction force is going to change to 50 N as long as it is not accelerating. So how am I supposed to used this equation: Friction force=mu * Normal force. The normal force is the same whether I have a high speed or not.

Homework Equations

The Attempt at a Solution

The 50N force on that same object will cause it to accelerate. Draw the force diagram. The 30N friction force will be the same as long as the object and surface remain the same. Fk= N×(Coef of kinetic friction). In order to have constant velocity your applied force must be exactly 30N no more no less. Any more will result in acceleration of the object.