00:02
All right, so i have here this smooth horizontal floor, and i've got three boxes sitting on the floor, and their masses are given in the colors, 5 kilograms, 3 kilograms, and 2 kilograms, with a 50 -newton force applied to the 5 -kilogram block that's on the left.
00:18
First, we're going to draw a 3 -body diagram for each of the boxes.
00:23
So for the red box, the 5 -kilogram box, we have the normal force on the box by the floor, pointing upward.
00:31
So that's normal.
00:32
And we're going to have a few normal forces, so i'm going to label them.
00:35
This is the normal force by the floor.
00:37
And then force of gravity pulls downward.
00:42
That's force of gravity.
00:44
We have that 50 -newton force going to the right.
00:51
It's not specified what kind of force.
00:52
I'll just call that f.
00:53
And then there's going to be a force on the 5 kilogram by the 3 kilogram, and that's to the left.
01:01
Now, this is unbalanced because the system is going to accelerate.
01:05
So this normal force by the 3 kilogram, i'll call that n3, going to the left, is going to be smaller than the applied force of 50 newtons that's going to the right.
01:17
The vertical forces, however, are balanced because our block does not accelerate upward or downward.
01:22
Okay, so that's the 5 kilogram block.
01:25
The 3 kilogram block is in the middle, and it also has the same vertical forces.
01:31
It has some upward normal force by the floor, and then a force of gravity pulling downward, and those are congruent to each other.
01:40
There's going to be a force pushing to the right, and that's the normal force on the three by the five.
01:53
On the three by the five.
01:56
And then there's a force going to the left that's on the three by the two.
02:01
And again, unbalanced, on the three by the two.
02:06
Okay.
02:09
And then finally we have the free body diagram for the two kilogram block.
02:15
And our two kilogram block also has a normal force going up, normal force by the floor, force of gravity going down, balanced, just like the others were.
02:26
And this one just has a normal force by the three kilogram block going to the right.
02:30
So that's normal force on the two by the three.
02:35
Okay.
02:36
So let's write our horizontal net force equations for each of these blocks.
02:44
So for the red block, which is five kilograms, our five kilogram block, in the horizontal direction, it's the applied force f minus the force normal on the five kilogram by the three kilogram.
03:03
Let's call that five three.
03:04
Instead of just three.
03:05
Okay.
03:08
And that equals mass times acceleration.
03:10
And that was based on the newton's second law formula, net force equals mass times acceleration.
03:18
So net force means add together the forces that are on the horizontal axis...