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You'll hear the terms , Balanced Forces and Unbalanced Forces a lot when you're dealing with Physics

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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

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are balanced or unbalanced.

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So let's start with the scenario where we have the ground.

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And then sitting on the ground we have a rock.

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Now the force of gravity is going to be pulling the rock downwards towards the center of the earth

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and it's going to be pulling it, as essentially the weight of the rock,

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it's going to be pulling it downwards , let's say with a force of 5 N downwards.

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But then the rock is being supported by the ground,

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the ground is keeping it from being plummeted downward.

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So the ground is providing an upwards force or normal force in this situation.

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The ground is providing a normal force, I'll do it in green.

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The ground is providing a normal force in this situation .

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So these forces have the same magnitudes but they're going in opposite directions.

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So that is the first scenario.

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Now let's think about another scenario.

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So I'll draw the rock again.

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We'll assume it's the same rock.
The force of gravity is still downwards.

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5 N downwards.

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And there's still a normal force , the ground supporting the rock of 5 N upwards.

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5 N upwards.

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And now there's some character who's trying to push the rock.

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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.

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But then there's a force of friction between the rock and the ground that is going 2...

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I'll do friction in orange

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2 N to the left

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So that right over there is the force of friction. It's going against that guy's pushing.

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Now let's do another scenario. Once again a very similar scenario.

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Let's draw another scenario where I have a rock now. Maybe the same rock.

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And here I have a 5 N force downwards.

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Force of gravity, like the rest of the scenarios.

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And I have the normal force. The rock is being supported by the ground 5 N upwards.

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And this guy over here has been able to push a little bit harder.

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He's pushing a little bit harder. He really put his back into it.

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And now he's pushing with a force of 3 N to the right

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and the force of friction is still 2 N to the left. So that right over there is the force of friction.

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Now let's do a couple of more scenarios.

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Now let's imagine that this is the ground . And I have the rock. The rock is not resting on the ground.

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So the only force I have acting on the rock right now

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is the force of gravity, acting downward.

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We're assuming we're not going to think too much about--

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Actually let's think about it a little bit, let's also

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put, so let's put it...so I have the force of gravity.

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That's 5 N down. But I have some air resistance here. You can view it as the

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force of friction of the air, and let's say that is 1 N up.

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This is the force of friction or you can call it air resistance.

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As if this thing bumps into all of the air particles as it

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is falling to the ground.

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The last scenario I'll draw with the rock again as that seems to be the

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theme of the video.

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The last scenario, that's the ground.

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The rock is resting on the ground.

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The rock is resting on the ground.

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So I have the force of gravity, 5 N downwards.

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The rock is being supported by the ground, 5 N upwards.

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And now this guy is pushing really hard.

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So now he's applying 4 N

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in that direction.

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You have 2 N from the force of friction, I won't that draw just yet; and you also have

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another character right over here, who is trying to keep this guy from pushing the

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rock. So he's pushing in the other direction at 1 N.

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So you have 2 N of the force of friction. You have this guy pushing 1 N against

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this guy's motion to the left, so between this guy and the force of friction, you have 3 N going left.

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So now I'll let you think about which of these have an unbalanced force in them, or another way to think about it,

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which of these have a Net Force going on?

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So let's look at the first scenario, we have a 5 N force of gravity acting towards the centre of the earth.

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5 N normal force of the ground supporting the rock.

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These have the same magnitude in the exact opposite directions

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so they cancel out. These forces completely balance each other out.

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There is zero net force going on, there are no forces the way I've drawn it, going on in the horizontal direction.

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So you have zero net force, these right here are balanced.

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We would not consider this one of the scenarios we see an

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unbalanced force.

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Let's go to scenario 2.

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Once again, 5 N up and 5 N down, they are balancing each other.

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Then in the horizontal direction, this guy is pushing of 2 N to the right

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is being completely balanced by the force of friction of 2 N to the left.

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Because they're balanced there is no net force and this rock isn't going to accelerate.

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So once again these are all balanced forces or there's no net force going on right over here.

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In scenario 3. Once again, in the vertical direction.

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The force of gravity is being balanced by the normal force of the ground.

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Keeping the rock from plummeting or accelerating towards the centre of the earth.

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And you do have two forces that are counteracting on the horizontal direction.

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This guy's pushing harder with 3 N, but the force of friction is now 2 N to the left.

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So you do have a net force to the right. 3 N to the right, 2 N to the left.

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You have a net force of 1 N to the right.

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Or you see it's right based on how I drew the vector.

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So you do have a net force or in another way of thinking about it, that this is

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a scenario where you have unbalanced forces, in particular, in the right left direction.

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Now let's look at scenario 4.

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We only have forces acting in this vertical direction.

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You have the force of gravity 5 N downwards.

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You have a counteracting force, the force of air resistance, I N upward.

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But they don't completely balance out.

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There is still a net force of 4 N downwards.

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Therefore this is an unbalanced situation.

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And finally in the last scenario.

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We're sitting on the ground in the vertical direction, normal force counteracting the force of gravity.

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Those are balanced.

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But in the horizontal direction.

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To the right we have more force, 4 N, than what we have going to the left.

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3 N is the guy on the right plus the force of friction.

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So in this situation there is a net force in the rightward direction.

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A 1 N force to the right or 1 N net force.

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This is a scenario where we have unbalanced forces.