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Balanced and unbalanced forces

Primer on identifying balanced and unbalanced forces. Created by Sal Khan.
Video transcript
You'll hear the terms , Balanced Forces and Unbalanced Forces a lot when you're dealing with Physics and what I wanna do in this video is give you a bunch of scenarios and have you think about whether the forces in these scenarios are balanced or unbalanced. So let's start with the scenario where we have the ground. And then sitting on the ground we have a rock. Now the force of gravity is going to be pulling the rock downwards towards the center of the earth and it's going to be pulling it, as essentially the weight of the rock, it's going to be pulling it downwards , let's say with a force of 5 N downwards. But then the rock is being supported by the ground, the ground is keeping it from being plummeted downward. So the ground is providing an upwards force or normal force in this situation. The ground is providing a normal force, I'll do it in green. The ground is providing a normal force in this situation . So these forces have the same magnitudes but they're going in opposite directions. So that is the first scenario. Now let's think about another scenario. So I'll draw the rock again. We'll assume it's the same rock. The force of gravity is still downwards. 5 N downwards. And there's still a normal force , the ground supporting the rock of 5 N upwards. 5 N upwards. And now there's some character who's trying to push the rock. So we have some character here and he's trying to push the rock and he's applying a force of 2 N to the right. But then there's a force of friction between the rock and the ground that is going 2... I'll do friction in orange 2 N to the left So that right over there is the force of friction. It's going against that guy's pushing. Now let's do another scenario. Once again a very similar scenario. Let's draw another scenario where I have a rock now. Maybe the same rock. And here I have a 5 N force downwards. Force of gravity, like the rest of the scenarios. And I have the normal force. The rock is being supported by the ground 5 N upwards. And this guy over here has been able to push a little bit harder. He's pushing a little bit harder. He really put his back into it. And now he's pushing with a force of 3 N to the right and the force of friction is still 2 N to the left. So that right over there is the force of friction. Now let's do a couple of more scenarios. Now let's imagine that this is the ground . And I have the rock. The rock is not resting on the ground. So the only force I have acting on the rock right now is the force of gravity, acting downward. We're assuming we're not going to think too much about-- Actually let's think about it a little bit, let's also put, so let's put it...so I have the force of gravity. That's 5 N down. But I have some air resistance here. You can view it as the force of friction of the air, and let's say that is 1 N up. This is the force of friction or you can call it air resistance. As if this thing bumps into all of the air particles as it is falling to the ground. The last scenario I'll draw with the rock again as that seems to be the theme of the video. The last scenario, that's the ground. The rock is resting on the ground. The rock is resting on the ground. So I have the force of gravity, 5 N downwards. The rock is being supported by the ground, 5 N upwards. And now this guy is pushing really hard. So now he's applying 4 N in that direction. You have 2 N from the force of friction, I won't that draw just yet; and you also have another character right over here, who is trying to keep this guy from pushing the rock. So he's pushing in the other direction at 1 N. So you have 2 N of the force of friction. You have this guy pushing 1 N against this guy's motion to the left, so between this guy and the force of friction, you have 3 N going left. So now I'll let you think about which of these have an unbalanced force in them, or another way to think about it, which of these have a Net Force going on? So let's look at the first scenario, we have a 5 N force of gravity acting towards the centre of the earth. 5 N normal force of the ground supporting the rock. These have the same magnitude in the exact opposite directions so they cancel out. These forces completely balance each other out. There is zero net force going on, there are no forces the way I've drawn it, going on in the horizontal direction. So you have zero net force, these right here are balanced. We would not consider this one of the scenarios we see an unbalanced force. Let's go to scenario 2. Once again, 5 N up and 5 N down, they are balancing each other. Then in the horizontal direction, this guy is pushing of 2 N to the right is being completely balanced by the force of friction of 2 N to the left. Because they're balanced there is no net force and this rock isn't going to accelerate. So once again these are all balanced forces or there's no net force going on right over here. In scenario 3. Once again, in the vertical direction. The force of gravity is being balanced by the normal force of the ground. Keeping the rock from plummeting or accelerating towards the centre of the earth. And you do have two forces that are counteracting on the horizontal direction. This guy's pushing harder with 3 N, but the force of friction is now 2 N to the left. So you do have a net force to the right. 3 N to the right, 2 N to the left. You have a net force of 1 N to the right. Or you see it's right based on how I drew the vector. So you do have a net force or in another way of thinking about it, that this is a scenario where you have unbalanced forces, in particular, in the right left direction. Now let's look at scenario 4. We only have forces acting in this vertical direction. You have the force of gravity 5 N downwards. You have a counteracting force, the force of air resistance, I N upward. But they don't completely balance out. There is still a net force of 4 N downwards. Therefore this is an unbalanced situation. And finally in the last scenario. We're sitting on the ground in the vertical direction, normal force counteracting the force of gravity. Those are balanced. But in the horizontal direction. To the right we have more force, 4 N, than what we have going to the left. 3 N is the guy on the right plus the force of friction. So in this situation there is a net force in the rightward direction. A 1 N force to the right or 1 N net force. This is a scenario where we have unbalanced forces.