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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 6: 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.

## Want to join the conversation?

• 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
• 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.
• What if there is another of force of friction acting for the orange guy in scenario #5?
• as long as it is the same, it will act on the rock with the same net force
• So the scenarios 3, 4 and 5 have net force? And are therefore unbalanced forces? Is it right?
• If they have a non-zero net force then the are unbalanced.
• why does the friction stay at 2N in all of the diagrams?
• 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.
• At Sal 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...
• 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.
• 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?
• 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.
• At Sal 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?
• 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!
• 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?
• 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