 Ok, so here's a meter stick that might be useful. I don't know what I'm going to do. But here I have 50 grams, and I have a pulley over here, and I have another pulley over here, and I have 50 grams hanging over that, 50 gram mass hanging over that end. So now I'm going to have another string, and I'm going to attach it to this, and when I hang this mass over there, just like that. And I think a lot of people would say, yes, I knew that it wasn't going to move, and it doesn't. But here is the fun part. What if I give it a push? Now I don't have my motion detector hooked up, but I'm still going to give it a push. Ok, so here goes a push that way. I'm going to push it that way. I don't have too much room to go. I'm going to push it back this way. That wasn't very far, but if you somehow measured the acceleration of this, what do you think the acceleration of this car would be? It would be traveling at a constant velocity. Ok, now here's the fun part. Now let's change the mass. One side. This is 20 grams I'm going to put on that side. Let me use these two blocks. Let me use this block to make sure it doesn't move. Now I'm going to add the 20 grams over here. Now I have 70 grams on this hanger, and 50 still over there. If I let go, I think it's obvious it's going to start moving that way. So let me just do that anyway. So then you can see it accelerates. What's the acceleration and what should it be? This is about 500 grams of the car. 50, 70. Now if you said it's going to move this way, you're not right. Because watch this. It was moving that way for a little bit because I pushed it. The net force tells you the direction of the acceleration. It does not tell you the direction of the motion. So don't get those confused. I'm going to push it again. If you want you can measure this acceleration from the video. That one wasn't good because it hit.