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### Course: Mechanics (Essentials) - Class 11th>Unit 6

Lesson 1: How is the Voyager-I still travelling at 61,500 km/h with no fuel?

# Balanced & unbalanced forces

Lets use real life examples and study balanced and unbalanced forces. Created by Mahesh Shenoy.

## Want to join the conversation?

• I cannot understand how balanced force called motion
since force is vector quantity 5N+ (-5N) where minus stands for direction then the total net force is 0 then motion cannot occur. If the net force is non zero then it's unbalanced force can you explain about this please anyone
(6 votes)
• yes, you are right! forces cancel out each other. but force does not keep things in motion.it is the natural tendency of the object to stay in motion/ at rest when the net force acting on it is zero.in the example shown in the video where the narrator pulls the bean bag the force of friction which keeps the bag from moving, is canceled out by the force of the narrator. therefore no force is now stoping the bag from moving and due to its tendency to stay in motion, it moves.hope that helps!
(3 votes)
• if in the end anis and yours force was same how come you didnt move towards him?
(2 votes)
• Mr.Shenoy has greater mass and hence larger inertia(tendency of a object to stay in uniform motion/rest) than Ani. Basically, more force is required to move him than Ani.That's why Ani moves towards him and not vice versa.

Guess I'm quite late, hopefully this helps!
(1 vote)
• If the rope was switched with a rubber band, it would stretch when pulled with an unbalanced force, in the direction of the unbalanced force, and wouldn't change its position. So would this be considered as acceleration or not?
(1 vote)
• Well its a good question.
The rubber band has the property of stretching itself. But if we pull hard enough, it will break . So no one Knows what will happen
(2 votes)
• Earlier when the person was pulling the bean bag it didn't move. But then when the second time when he pulled it , it moved with constant speed. Why didn't it move with constant speed in the first case and if it didn't move then the the forces must be unbalanced with the friction being more than the person's force?
(2 votes)
• Good Question!
You see, Unbalanced force is required to accelerate motion of a mass. If an object is at rest, and stays in rest, is there really unbalanced force here(there's no acceleration)? If friction was more then the bean bag would move in the direction of friction!

remember---inertia--natural tendency of an object to stay in rest or uniform motion in a straight line----emphasis on the 'or' as well!
Hope this helps!
Onward!
(0 votes)
• Wouldn't your force be equal to Ani's force + friction?
(1 vote)
• the forces are unbalanced
(1 vote)

## Video transcript

me and my cousin Annie rode our unnie for short decided to play a tug-of-war sitting so I'm going to pull on this rope this way in this particular direction and Annie over there is gonna pull it in that way in that particular direction and what I want you to do is I want to show you a clip of what happened when we started playing and you should guess who is pulling harder on that rope okay so here's the clip here's what happened I won easily without any competition now if you pause can you tell me who is pulling harder on this rope me or Annie now you might think oh this the answer is obvious right let's look at one more time because I'm is moving towards me I must be pulling harder and he is not doing anything definitely I'm pulling harder right well if you think that's the answer then this video is gonna make you rethink about it to answer questions like these correctly we need to investigate a little bit about forces so let's start with the question what does a force do now the answer to this question also might seem pretty obvious but let's first take some examples and then think about it say you have a coin on a carom board and let's say you give it a tap that means you give a force over here you know what happens it starts moving okay let's take another one second example consider now a coin that is already moving to the right let's say then you might know due to the roughness of the surface a frictional force starts acting in the opposite direction and now it slows it down makes it stop another example Superman slows down a train by pushing it against its motion and you can think of more examples like throwing a ball or catching it etc now what is common in all these cases you're applying a force on an object but what does that force do is the question well let's think about it over here we push that coin and the coin started moving over here friction slowed down that coin here also Superman is slowing down the string when you throw a ball you speed that ball up when you catch a ball you slow it down and make it stop so in general can you see that a force either speeds up an object or slows it down of course it turns out force can also change direction of a moving object but nobody too much about that so if you put all this together in one Center then we can say a force causes acceleration that's what it force does it either speeds up an object or slows it down but do forces always cause acceleration well let's see take a look at this video of our tug of war what do you see well besides our oscar-winning acting faces you see that both of us are pulling on this rope so there are forces acting on this rope yet that rope is not accelerating it was addressed before and look it is still at rest why isn't it accelerating hmmm let's look at another one here's a beanbag kept on the floor now I start pulling on it and look the bag is not accelerating even though I'm putting a force on it the bag has no acceleration it's staying at rest why now we might be tempted to say things like maybe the bean bag is very heavy or maybe I am NOT pulling it hard enough but look at our statement there are no conditions applied it's not that forces strong enough will cause acceleration no it just said we said forces cause acceleration right so why isn't my force accelerating this bean bag so you know what think about this can you pause for a while and see if you can come up with an explanation all right let's see let's look at the first one and see if we can bring it out or here both of us are pulling on that rope in the opposite direction so maybe we are putting exactly the same amount of force maybe the two forces are equal and opposite and as a result their effects are cancelling out and that's why the rope is not getting any acceleration that kind of makes sense right okay what about here maybe even hear my pulling force is canceled by some other force acting in the opposite direction on this bag who might be putting such a force well here's a clue I put a carpet for a reason because the carpet surface is rough did you guess it it's friction you might know whenever we try to move things on a rough surface the floor or that surface opposes that motion by putting a force in the opposite direction in which you're trying to move it and that force is called friction and if you need more clarity on you know where this friction comes from we've talked about it in a little bit detail in the previous videos will be great idea to go back and watch that but anyways now we can kind of guess what's going on even here maybe the force of friction is exactly equal and opposite to my force again their effects are cancelling out and so the bag is not accelerating and so from our new findings we can say that whenever we have equal and opposite forces acting on anybody then their effects cancel out and as a result in physics we often say there is no net force acting on the body the word net can be thought of as effective no effective force so even though there are forces acting on this rope and this bag there is no net force acting on them and what we found is that if there is no net force acting on an object there is no acceleration on that object and in such situations these forces are also called balanced forces it kind of makes sense right because they are equal and opposite they are balancing each other and so balanced forces means no net force no acceleration ok now that we know that if there is no net force we will got no acceleration we can try and answer the question what is needed to get an acceleration how do we accelerate bodies imagine over here if I start pulling more than my cousin what would happen well I'm pretty sure you can guess now a little hand that rope will start speeding up towards me and so now the Rope will start accelerating now I didn't do this demo because this rope is a little rough and it could hurt his hands and he had an exam the next day and the same is the case with beanbag as well if I were to increase my pull making the force larger than friction now that beanbag will start accelerating towards me and so whenever the forces don't cancel out that's when we have acceleration so in general we will say a net force a net force causes an acceleration this means force it shouldn't cancel out in other words the forces should be unbalanced that's all so we can write an unbalanced force causes acceleration so if the forces are balanced then there is no net force no acceleration otherwise we will get an acceleration okay now let's put this knowledge to a test I'll show you one more clip and you have to tell me whether it's under balanced or unbalanced forces okay here goes so you can see over here the beanbag is moving towards me I'm pulling it towards me but what's important it's coming at a constant speed that's important okay so again there are two forces one is my pulling force so I'm pulling it and the second is the force of friction so can you tell whether these two forces are balanced or unbalanced give it a try pause the video and think about this all right let's see if you look at it one more time because the bag is moving towards me we might think that you know the Bell we are pulling harder than the force of friction that's why it's moving towards me so we might think my force is larger than the force of friction and as a result this is unbalanced situation but that's wrong and the reason is if it was unbalanced then this beanbag would have accelerated in this case it would have accelerated towards me if my force was larger it would speed up towards me you know what would that look like let's look at to the right it would look somewhat like this look over here if my force was larger then it would look somewhat like this see it would accelerate towards me but is it accelerating no we saw it is going at a constant speed so this can't be true similarly do you think this can be possible do you think friction can be larger than my force again that can't be possible because again it would accelerate this time because friction is opposing the beanbag would slow down if this was true let's look at again over here then the beanbag would slow down this what would happen if friction was larger that's not happening now because there is no acceleration there's only one possibility my force must be exactly balanced by the force of friction and so this is a situation of balanced forces so even though the bag is moving towards me it is still under balanced forces now having said this I always had one question that always disturbed me so if you go back to the original video my question was if my force is balancing the force of friction exactly and what's keeping this bag in motion why is it moving how is it moving well if you don't have the same question then remember things in motion have a natural tendency to stay in motion you don't need to push it to keep it in motion and we've talked a lot about this in a previous video so if you need more clarity great idea to go back and watch that video and then come back over here anyways now we have everything we need to answer your original question so if we come back to the initial tug-of-war clip and what is important to see is that I'm pulling an e with a constant speed the rope is moving towards me at a constant speed so who do you think is pulling harder me or Annie well now I'm pretty sure you'll agree with me that because this rope was not accelerating it is moving at a constant speed our forces must be balanced which means I am pulling with the same force as which my cousin was pulling the rope with and so what did we learn in this video we saw that if two forces are exactly equal and opposite then they are balanced and such forces do not create any acceleration and what's important is that just because forces are balanced does not mean an object has to be at rest it can be moving with a constant speed as well