# Homework Help: Static friction that wasn't a maximum

1. Jan 17, 2008

### Khemkhajon

1. Describe a situation where you were dealing with static friction that wasn't a maximum

Im getting stuck on this question. Cause i cant think of a situation please some help would be appreciated

Last edited: Jan 17, 2008
2. Jan 17, 2008

### Khemkhajon

Also can someone not just tell answer but explain please.

3. Jan 17, 2008

### Shooting Star

Can you tell me very simply what you understand by static friction?

4. Jan 17, 2008

### Khemkhajon

I thought static friction was always at its maximum, because I know That static friction force must be overcome by an applied force before an object will move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction
so im super stuck

she says its not a trick question

5. Jan 17, 2008

### Shooting Star

Suppose there is a book on your table. You push it gently and horizontally with force F1, but it does not move. That means your force F1 is canceled by frictional force F1 acting in opp dircn. Again you push with force F2>F1, but still it does not move, meaning that the force of friction has now increased to F2 and canceled your applied force.

Like this if you keep on increasing, there will be some maximum force F_max, at which point the frictional force is not able to resist your push anymore. That is the maximum force of static friction. At that point, F_max = kN, where k is the co-eff of static friction. At all other times, when the applied force F is less than F_max, then F < kN.

Now you give some example.

6. Jan 17, 2008

### Khemkhajon

o ok so like if I pull a rock and doesnt move but keep increasing my force of pull it will overcome the maxium force of static friction. but im confused sort of because max friction must be overcome. but i that example arent we just increasing our force to overcome the max friction. Isnt the question asking when is static friction not at its maximum.

Last edited: Jan 17, 2008
7. Jan 18, 2008

### Shooting Star

You have been asked to give examples where the force of static friction is less than the maximum. As long as two surfaces in contact are stationary wrt each other, most probably the frictional force is less than maximum. The maximum represents a limiting case. As soon as the applied force exceeds the maximum, things start to slide relative to each other, and then kinetic friction takes over.

Suppose there's a ball rolling down an incline without slipping. As long as it’s not slipping, the frictional force is less than max. If a ladder rests against a wall, and you are climbing up, the friction is less than maximum up to a certain point. At the maximum, the ladder will just begin to slide.

8. Jan 18, 2008

### stewartcs

Have you ever stood on a ramp and not slid down it? How about a roof?

CS

9. Jan 18, 2008

### thharrimw

static friction is at a min. when the force applyed is less then the Fs,max. an example of this is if you try to push a object wirh a Fs,max of 5000000N and you apply a force of 9810N you don't have Fs,max you have a lesser force becouse friction can't be greater then the froce it is oposing

10. Jan 18, 2008

### Shooting Star

Let me repeat that in almost all cases where static friction is involved, it is less than the maximum.

Just think randomly of a few cases where static friction is involved. Like the examples stewartcs gave.