# Homework Help: Friction on top of object with frictionless surface?

1. Nov 9, 2008

### lax1113

Friction on top of object with frictionless surface????

1. The problem statement, all variables and given/known data
If an object1 has friction between it and object2, and object 1 rests on object 2, when a force is appliced to object 1 that is horizontal, what impact will it have on object 2 when the surface object 2 is on has negligible friction.

3. The attempt at a solution
This isn't the exact problem, but this is the part that i cannot understand. According to the answers in the back of the book, object 2 seems to be experiencing a force that is equal to that of object 1, minus the static friction. Whereas object 1 uses the kinetic friction to get its net force. I dont understand why this would be, if it even is.

2. Nov 9, 2008

### asleight

Re: Friction on top of object with frictionless surface????

The second object will move with the first object as long as the force applied does not exceed the force due to static friction between the objects.

3. Nov 9, 2008

### lax1113

Re: Friction on top of object with frictionless surface????

So, what about the case of one object having a different acceleration than the other object? Say the top one accelerates at 6m/s^2 and the bottom only 1m/s^2. Is this due to the fact that the bottom one has no actual force acting on it but just is being acted on by the friction of the top one?

4. Nov 9, 2008

### asleight

Re: Friction on top of object with frictionless surface????

No, the only way these can accelerate at different rates are if the force applied is greater than the force due to static friction between the blocks. If this happens, the top object will accelerate with respect to the bottom block and will end up falling off of the front.

$$\sum F_{top}=F_a-F_f=m_{top}a \sum F_{bottom}=F_f=m_{bottom}a$$

Something like that.

5. Nov 9, 2008

### lax1113

Re: Friction on top of object with frictionless surface????

I got ya slight,
Thanks,
It is like the second example you gave, the acceleration is only for that moment in time, so after a while, the top would fall off of the bottom.

How do you make the white part? where you write stuff out.....
Would help alot when i try to explain stuff haha

6. Nov 9, 2008

### asleight

Re: Friction on top of object with frictionless surface????

The coefficient of static friction that would hold the boxes together is equal to the force applied for all times except the initial push that sets it into motion and when the static force turns into kinetic friction and the top box accelerates off of the bottom. And you can put tags using the sigma icon in the create-a-thread box.

7. Nov 9, 2008

### lax1113

Re: Friction on top of object with frictionless surface????

Thank you very much!
Although I intially needed help with this problem, I think that the insert box might be even more helpful! I can never post questions here because it is so hard to understand with all the ^n and parenthesis everywhere.

Thanks!

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