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## Physics library

### Course: Physics library > Unit 3

Lesson 3: Balanced and unbalanced forces# Balanced and unbalanced forces

When forces are balanced, they cancel each other out, resulting in no change in motion for the object they are acting on. Unbalanced forces do not cancel each other out, and result in a change in motion for the object they are acting on. Explore balanced and unbalanced forces in physics through five different scenarios involving a rock and various forces acting on it. Created by Sal Khan.

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- In the final diagram, why is there a frictional force of 2N only in the left direction? Should there be one on the right too since there is a second person pushing the rock. Therefore an overall of 0N of frictional force(13 votes)
- There are two kinds of friction which are kinetic friction and static friction. Kinetic friction exists when there is relative motion between two surfaces in contact and static friction exists when two surfaces in contact are at rest(with respect to each other) and there is a net force that tries to make the surfaces perform relative motion. In this case the rock is at rest with respect to the ground therefore static friction will exist and static friction always exists in the opposite direction of the net force.

Don't include friction first. Find the direction of the net force (excluding friction). The direction of static friction will be opposite to that direction. Then if the net force is able to overcome static friction, there will be motion in the direction of the net force. Now kinetic friction will exist and that will be opposite to the direction of motion.(1 vote)

- So the scenarios 3, 4 and 5 have net force? And are therefore unbalanced forces? Is it right?(3 votes)
- If they have a non-zero net force then the are unbalanced.(4 votes)

- What if there is another of force of friction acting for the orange guy in scenario #5?(4 votes)
- as long as it is the same, it will act on the rock with the same net force(2 votes)

- At0:50Sal says that there is a 'force of 5 Newtons' What does he mean by that? I have heard him use that term several times in the past couple videos, and i am confused as to the meaning of it...(1 vote)
- Force is the acceleration of the mass, 1 Newton is the necessary force that's used to move an object with 1kg of mass at a 1m/s²:

m(kg) a(m/s²)

F=m.a

1=1.1

it's very important to remember that the force is a vector, it needs direction and orientation.(6 votes)

- why does the friction stay at 2N in all of the diagrams?(3 votes)
- Since the surfaces in contact are the same, the
**coefficients of static and kinetic friction**are the same in all cases. This can probably be given as a reason as to why the friction in motion and also, impending to move is 2N. It turns out that generally, coefficient of kinetic friction is approx. equal to coefficient of static friction. (slightly lesser but we can ignore that)

This is only my suggestion, of course. What may be the truth is that Sal wanted to keep things simple so he used the same force of friction everywhere and modified scenarios so that would be possible.

More on coefficients of friction here.

https://www.khanacademy.org/science/ap-physics-1/ap-forces-newtons-laws/friction-ap/v/intuition-on-static-and-kinetic-friction-comparisons(2 votes)

- I've heard about terminal velocity at my school, but it sounds like it has something to do with air resistance. What is terminal velocity and what does it or what does in not have to do with air resistance?(2 votes)
- Terminal velocity is the velocity at which the resistance and or drag force on an object from a fluis that it is moving through is equal and opposite to the sum of all the other forces on the object causing it to have zero net force on it.

So the only reason that I can see you may want to not use air resistance is that terminal velocity can be used for an object being moved through any fluid.(3 votes)

- At5:10Sal says that the purple character is pushing to the right with a force of 4 Newtons. But, the orange character is pushing to the left at a force of 1 Newton, and we have friction so there is a total force of 3 Newtons to the left.

But, doesn't friction have effect on the orange character as well as the purple character?(2 votes)- You are absolutely right. If this were in the real world, both characters would have to fight friction in order to push the rock.

However, this is a physics problem, so it's not that simple. :-) For the sake of the physics problem, we define rightward as the direction of motion. That means friction (a leftward force) is*opposing*motion. The guy on the right, who is pushing leftward, is also opposing motion. The guy on the right becomes part of the opposing force, so his force is*added*to the force of friction.

Were this a real-life situation, you are correct. Both characters would have to fight the force of friction.

I hope that makes sense!(3 votes)

- Why isn't there a normal force acting between the human and the rock in the horizontal direction?? There is normal force between every contact force right?(3 votes)
- can someone please help me understand the concept in this question my teacher gave me, it'd help a lot, thank you

Which combination of three concurrent forces acting on a body could not produce equilibrium? Explain.

(A) 1 N, 3 N, 5 N (C) 3 N, 4 N, 5 N

(B) 2 N, 2 N, 2 N (D) 4 N, 4 N, 5 N(2 votes)- This is an interesting problem, never seen one like it.

I know the answer but won't give it to you, but here is a hint.

Find the combination of three vectors that cannot be placed end to end so that the sum is 0 (ie. sum of forces is zero so object is in equilibrium). In other words, find the combination of side lengths which make it impossible to create a triangle.(3 votes)

- What happens if there is more force downwards the rock .Will it go down the earth(1 vote)
- Let's say there is a tiny rock, small enough to be carried in a person's hand. Then, they find some soft soil, the person pushes with a force of 10 Newtons, and the ground will try providing a normal force back upwards, but the force that the person is exerting is larger than that of which the ground can, so then the rock would go down into the ground. This would be an example of an unbalanced force. Hope this is helpful!(5 votes)

## 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.