A solid ball is released from rest and slides down a hillside that slopes downward at 65.0° from the horizontal. (c) In part (a), why did we use the coefficient of static friction and not the coefficient of kinetic friction?

Video transcript

Hey, everyone in this problem, a book rests on a table inclined at an angle of 30 degrees with respect to the horizontal ground level. And we are asked why should we consider static rather than kinetic friction? In this case? No, we have four answer choices. We'll go through each of them in just a minute. But the key here is that the book rests. OK. If the book rests, that means that the book is stationary. OK. So let's take a look at our answer. Choices. Option A says um kinetic friction because only kinetic friction can act on stationary objects, right? And that is not true. OK. Kinetic friction acts on moving objects. And so option A we can eliminate, OK. Option B, only static friction can act on moving objects again. That's not correct. OK. We know that static friction acts on stationary objects. And we also know that our book is stationary. OK. So we don't have a moving object. So that one's kind of wrong in two ways. Option C because the book is stationary and only static friction can act on stationary objects. So that is exactly what we have in this case. We have a stationary object because we have a stationary object, we know that only static friction can act. OK. That's gonna keep that book from sliding, that force of static friction and that, that force we have to overcome in order to get the book to start moving. And so option C is the correct answer. In this case. Thanks everyone for watching. I hope this video helped see you in the next one.