00:01
Okay, in this problem we have two blocks.
00:04
We have block a sitting down here, and then on top of it we have block b.
00:19
And block a has a mass 25 kilograms.
00:25
Block b has a mass 10 kilograms.
00:30
And they are both accelerating to the right with an acceleration of 2 meters per second squared.
00:40
And there's friction between the two of a static coefficient of .8.
00:52
So there's some friction in between these.
00:55
We want to find the static frictional force acting between the blocks.
01:00
So we're trying to solve for a force, which is a clue to use newton's second law to look at the forces acting on every block here.
01:10
So let's draw a free body diagram first off.
01:18
Okay.
01:23
So starting with block a, we have the force of gravity pushing it down, and then the normal force from block b acting on block a.
01:39
So gravity is pushing block a down, but then there's also block b sitting on top of it.
01:44
So that normal force is pushing down.
01:46
Pushing up, we have the normal force from the ground on block a.
01:54
And then to the left, we have the frictional force.
02:00
I'll do lowercase f from block b.
02:03
So as the system is moving to the right, block b is going to try to slide to the left, right? it doesn't want, it tries to oppose this change.
02:14
And because of that, it's going to exert a force on block a.
02:20
Now this is all under the assumption that block a is the block driving this.
02:27
This motion.
02:29
So it's almost like you're pushing on block a.
02:31
If you were pushing on block b, it would be a little bit different here, but we're going to assume that there is some force acting on block a...