Is There Friction Between Objects Without an Exerted Force?

[Dorothy Weglend]

Is it correct to say that there is no frictional force if there is no force exerted?

For example, two blocks resting on top of each other on the floor. If I push on the lower block, and there is no motion, then there is a frictional force between the bottom block and floor, which is stronger because of the increased mass from the upper block.

But there would not be any frictional force between the two blocks until the bottom block starts to move. Is this right?

Thanks,
Dorothy

 

[Doc Al]

But there would not be any frictional force between the two blocks until the bottom block starts to move. Is this right?

Yes, that's right. Look at it this way. Since friction is the only horizontal force available to the top block, you know that if that block is not accelerating then the friction on it must be zero.

 

[Dorothy Weglend]

Yes, that's right. Look at it this way. Since friction is the only horizontal force available to the top block, you know that if that block is not accelerating then the friction on it must be zero.

Oh, great. Thank you.

But if the lower block is pushed hard enough that it slides out between the upper block and the floor, then do I consider the static friction between the blocks?

It seems to me I have to, even though the block is sliding at that point, the force must be great enough to overcome the static friction between the blocks, so I have to use that to calculate the strenth of the push.

THanks so much!
Dorothy

 

[Doc Al]

Static friction will do as much as it can (up to its maximum value) to prevent sliding between the blocks. But if the lower block is pushed too hard, the needed acceleration for the top block to move with the bottom block (and thus not slip) may be too great for static friction to provide. And once it starts sliding, kinetic friction is what's going on.

 

[Dorothy Weglend]

Thank you, Doc Al. That's great. Wonderful.

Dorothy