static vs. kinetic

[Pamplemousse123]

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Hi there,

I know this is the basic stuff ( 🙁 ) but I'm having trouble in realizing the difference between static and kinetic friction. I know that static friction is the amount of force that a surface exerts on an object to prevent it from moving when the object is not sliding and kinetic is the amt of force that a surface exerts on an object to prevent it from moving when the object is sliding. i also know that the max static friction is always bigger than the kinetic friction.

example 2.12b from TBR (Summer 2009 version, I believe) made me confused though:
A second back-packer grabs...yadayadaya... what is the largest magnitude of the frictional force that is acting on the pack? I thought the question is asking for the kinetic force since the backpack is sliding with constant velocity, but the answer explanation says that it's the static force that answers the question. Please let me know why it is the static force that is acting on the pack while it is sliding.

 

[FrkyBgStok]

i don't have TBR but i think you are missing an important part of the question. in audio osmosis that use an example that may work here. imagine you place your hand on the top of a book and slide it across the table. static friction is the force between your hand and the book and kinetic friction is the force between the book and the table. the static friction is ultimately causing the net force. in your question, is a backpacker dragging a pack or something of that nature, because you could use the static friction of the persons feet. maybe this helps?