Intro Quiz: Moments and balance (Part 2)

Video: Moments and balance (Part 2)

﻿Hello everyone, this is Mrs. Wolstenholme. Welcome to this lesson on forces. This is our third lesson in our forces topic. Before we start, we need to make sure we have a piece of paper, a pen, and a calculator. So please pause the video now to go and get those. And hopefully you have all of those items and you're ready to go. We also need to make sure that we can concentrate so please go somewhere that is quiet and that you won't get distracted. And please either turn your mobile phone off or put it on silent. There are two main parts of our lesson in the first one we're just going to be discussing how I can move a force closer or further to my pivot to make an object balance. And then in the second one, we are going to be adding some maths in and using calculations to figure out the force and distance that we need. Then after that, you can use the exit quiz to show how much you've learnt. Before we get into new content we need to be reminded of what we learned in our first lesson. And that was the two options of making a moment bigger. The first option was I could make my force larger. So a larger force has a larger moment. The second option is I could make the perpendicular distance from my force to the pivot larger. If it's further away from the pivot, then the moment is larger and if it's closer to the pivots, then the moment is smaller. Let's have a look at a question. The seesaw is unbalanced and is rotating in a clockwise direction. What should this animal do to balance the seesaw? Well, the seesaw is unbalanced. That means that one of the moments is bigger so either the clockwise moment is bigger than the anticlockwise moment, or anticlockwise moment is bigger than the clockwise moment. It says it's rotating in a clockwise direction so that means that the clockwise moment is too big it's bigger than the anti-clockwise moment. So we need to have a look, in the diagram it's pointing to this animal on the right, the sort of cheetah looking animals so I'm going to call it a cheetah. So the weight of the cheetah is shown with the green arrow. And the moment of that force is the clockwise moment. So that force's moment is too big so I need to make that moment smaller. So how could I make the moment smaller? I can't change the cheetah so I can't change the force. So, but what I could do is move the cheetah closer to the pivot. So if I move the cheetah closer to the pivot, the perpendicular distance will be smaller. So the moment will be smaller. So if I make the clockwise moment smaller, maybe it will become equal to the anti-clockwise moment and then it will be balanced. Here is another example. The crane is unbalanced and is rotating in an anti-clockwise direction. What can we do to the load? So the anti-clockwise moment is bigger than the clockwise moment. The anti-clockwise moment is caused by the weight on the left, where it says a weight and that's because of the counterweight of the crane. So the anticlockwise moment is too big, but they want me to do something with the load. The load causes the clockwise moment. So if the anti-clockwise moment is larger than the clockwise movement, I need to make the clockwise moment bigger to balance it out. How could I make the clockwise moment bigger? I could move the load further from the pivot further to the right and if I did that, I could balance out the crane. So what can I do to increase the moment? Have a read I'll give you 10 seconds to choose the correct options. The correct answer is move the force further away from the pivot. If we want to make the movement bigger than we move the force away. What about decreasing the moment? I'm only going to give you five seconds this time, the answer is option one, move the force closer to the pivot. That would mean that the perpendicular distance was smaller and so the moment would also be smaller. If I push further away from the pivot, the moment is? The answer is larger because with further away so our distance is larger. If I push closer to the pivot, the moment is? Smaller, so really well done if you've got any of those right, or if you didn't, but now you understand what you did wrong. Here's an example of a question that comes up in an exam. So we've got a crane on our diagram and we can see that it's got a counterweight on it, a pivot and a load. Let's read the question. The diagram shows a crane lifting a load the counterweight and the load are balanced. That's good, so at the moment, the counterweight and the load are balanced, and that means that the clockwise moment from the load is equal to the anti-clockwise moment from the counterweight. Let's carry on reading the load is moved away from the pivot to the right, what happens to the turning moment produced by the load? So what I'm going to do is draw this. So it's saying that the load is moving away from the pivot. So those are my weights and my load has moved away from the pivot and that means that its perpendicular distance has increased so what happens to the turning moment produced by the load? If the perpendicular distance has increased, that means the moment is going to increase. Let's look at part two, what should happen to the counterweight to keep the arm balanced? So let's have a thing before they were both balanced the anti-clockwise one and the clockwise one were balanced. Now I've made the one from the load, which was the clockwise one I've made that one bigger. Someone's gotten bigger. So now I need to make the anticlockwise one bigger as well. So what can I do to the counterweights to make them balanced again? How could I make the counterweight moment larger? Well, I could move the counterweights further away from the pivots. I could make the perpendicular distance from the counterweight to the pivot larger because that would make the moment from the counterweight larger as well and then they could balance each other out. So I would move the counterweight away from the pivots. So really well done if you follow that along, or if you were thinking that yourself before I even said the answer. We're going to move on now to adding in some maths. So make sure you have a pen and paper ready for this part. This is the type of question you might be given. The seesaw is balanced what is the weight of the animal on the left? So if you have a look at my diagram, I can see that I've been given the distances one metre and two metre, and I've been given one of the forces 500 Newtons, but I haven't been given the force or the weight of the animal on the left. So what this question is asking me to do is to find that weight on the left, to do this I need to remember a couple of things from previous lessons. The first thing we need to remember is the equation for moment, moment equals force times perpendicular distance, or just force times distance. You might want to pause the video to write that down because we're going to need it later on. The second piece of information we need to know is that when an object is balanced, the clockwise moment is equal to the anti-clockwise moment. So when an object is balanced, the clockwise moment is equal to the anti-clockwise moment. So if I go back to this question, I've written underneath the question that clockwise moment is equal to anticlockwise moment and that's because they told me that it is balanced. So underneath both of these now I'm going to write my equation for moment force times distance. They're both moments so they have the same equation, but obviously the numbers I'm going to put in are going to be different because they each have a different force and a different distance. The clockwise moment is caused by the force on the right, from the cheetah. So underneath force there, I would write 500 and underneath distance I would write two, but on the anti-clockwise side, I don't know the force so I would just write down force. Let's have a look at that. So on the left, I've got my clockwise force and distance to give me my clockwise moment, force times distance, but on the right, I don't have my force. So I've just written down force, but I do have the distance. It's one metre so I've written down one under distance. Then I'm going to put that in my calculator and I get that a 1000 equals force I'm just going to flip those rounds and I get the force is equal to 1000 Newtons so remember that the unit of force is Newtons and that there we have it I have found my force. He is another example. The crane is balanced. The weight of the counterweight is 250 Newtons. What is the weight of the load? So they told me that the weight of the counterweight is 250 Newtons. So I'm going to add that to my diagram. Then it says, what is the weight of the load? That's the weight I don't know, but I can use the fact that it's balanced. If it's balanced that means that clockwise moment is equal to anticlockwise moment so underneath both now I can write the equation force times distance, this time it's the clockwise moment or the force on the clockwise moment side that I don't know so I would just copy that down as weight, the distance from the weight to the pivot is five, but on the anti-clockwise moment, I know the force and the distance. I can see that in my diagram. I can see that it says 250 Newtons and two metres. So I would put that into my calculator. Now I have a problem and this is something you might do in maths, where you do the same thing to both sides. So in this case, I'm going to divide both sides by five and the reason for that is that I can now have weight on its own 'cause I don't want weight to be multiplied by five. I want to know weight on its own. So if I divide both by five, I put that into my calculator and I get the weight is a 100 Newtons. So I only have to put the part on the right into my calculator because the part on the left, the fives cancel. Let's do a bit more practise on this in case we haven't seen something like this before, what is the next step in this calculation? I'll give you 10 seconds. The answer is option two. So it's weight times four then I divide it by four and I divide the other side by four as well I have to do the same thing on both sites. Let's have a look at another one. What is the next step? 10 seconds. The answer is option one so I've divided both sides by two and one more, what is the next step? 10 seconds. The answer is option two I divide both sides by 10. If you chose option three, what we've gotten confused about is we always divide by the number next to the word. So in this case, 10 is next to the word. So we're going to divide by 10, not by 450, but really well done if you've got any of those right, because they're really tricky so well done. We're going to do this one together, but first I'm just going to draw out the diagram for us. The apple has a weight of 30 Newtons and is 40 centimetres away from the pivot. What force would I have to push the ball with 30 centimetres from the pivot to balance the bar? That sounds very confusing. So let's see if we can draw a diagram to make it more understandable. First of all, it says that the apple has a weight of 30 Newtons and is 40 centimetres away from the pivot. So I can draw the force on my apple and the distance from the apple to the pivot. So that's what I've done on my diagram. Next, it says that I am going to get a force that is 30 centimetres away from the pivot. So even if I don't know what magnitude or what size the force is going to be, I can still draw my distance and my arrow, but I've just put the word push because I'm pushing down. So it's told me that it's balanced. Here's my diagram and because it's balanced, I know that I can write clockwise moment is equal to anti-clockwise movement. And then I can put in my equations for moment, force times distance and force times distance. But now we need to put the numbers in. Why don't you pause now and see if you can do the first line all on your own. The clockwise movement is caused by the push and the distance on the right so underneath that, I would write push times 30. The anti-clockwise moment is caused by the apple. So I under force, I would write 30 and under distance I would write 40. And then I would put that in my calculator and this is what I would get. Now I need to divide both sides by a number. Why don't you pause the video now and see if what number I'm going to divide it by. I'm going to divide it by 30 because that's the number that's next to my push. So I would now put this into my calculator 1,200 divided by 30 and I would get 40. So my push is 40 Newtons so I found my force so really well done if you followed along with that. This time it says the seesaw is balanced how far away from the pivot is the animal on the left? So this time they've given me the forces, but I don't have the distances. So I need to find my distance. But it's exactly the same thing I don't need to panic and think oh no, I haven't done this before. It's exactly the same but instead of there being a force words that I have to write down, it's going to say distance. So let's have a look. So because it's balanced, I start exactly the same way clockwise moment is equal to anti-clockwise moment and then I'm going to write down my equation, force times distance, force times distance, exactly the same. And then I'm going to write down the force times distance of my clockwise moment, which is because of that cheetah on the right. So that's going to be 500 times two and underneath the anti-clockwise moment side, the force is 250 so I'll write 250, but under distance, I don't know the distance so I'll just write distance and you can see that there. Then I would put this into my calculator and I'd get a 1000 I'm going to divide both sides by 250, because that's what my distance or my word was multiplied by. So when I put that into my calculator, I will get four and I'll just swap those round and I get the distance is four metres so it's exactly the same as what we were doing before but this time it says distance in words, instead of force or push, okay. Here's another one, the apple again, the apple has a weight of 30 Newtons and is 40 centimetres away from the pivot. How far from the pivot should I push with a force of 20 Newtons to balance the bar? So again, I draw out my distance and my force from my apple the same as the last time and I'm also going to draw out the force that I'm pushing with. But this time I know the size of the force, it's 20 Newtons, what I don't know is how far away from the pivot it is so that's why I put a question mark under my pink line. And then again, because it's balanced, I write clockwise movement is equal to anti-clockwise moment and then the equation for both force times distance, force times distance. So underneath both of these, on the clockwise side, I'm going to write 20 times distance because I know the force, but I don't know the distance, and on the anti-clockwise side, I know the force and the distance so I can just put those numbers in. Then I would put that into my calculator and I get 20 times distance is equal to 1,200. So now I need to divide it by a number. What number am I going to divide both sides by. I want you to shout it out in five seconds, let's have a look. Hopefully lots of you said 20. So I divide both sides by 20, because that is what my distance was multiplied by and when I put that into my calculator, I get 60 and so my distance is 60 centimetres. So really good if you followed along. Let's do this one together. How far from the pivot does the counterweight need to be for the crane to be balanced? It's balanced, so I'm going to write out my normal equation. Clockwise moment is equal to anti-clockwise moment. Force times distance is equal to force times distance. Now I want you to do is pause the video and see if you can write out the correct force and distances under the words so pause the video now. Hopefully you figured it out and it's a 100 times eight on the clockwise side and 250 times of distance that we don't know on the anti-clockwise side and then I would put that into my calculator and I get 800 is equal to 250 times distance. Now let's see, what number am I going to divide both sides by? Five seconds. I'm going to divide both sides by 250, because that is what my distance is multiplied by. So if I put 800 divided by 250 into my calculator, I will get 3.2. So my distance is 3.2 metres so really well done if you followed along with that, now it's your turn to try two questions that are really similar to the ones we've been doing so pause the video now and have a go on your own. Hopefully you've had a go at those and now we can go through the answers together. Here's the answer to question one. Why don't you pause the video and read it carefully on your own. I hope you can see how similar it is to the other questions we've done and that you feel really confident you know what you are doing. Here's the answer to question two. Why don't you pause now and have a read through it and check your answer. Hopefully again, you can see how similar it is to other questions we've done and that you feel again, really confident in what you've learned. A huge, huge, huge well done for getting to the end of this lesson you should be really proud of yourselves, and I'm really impressed. What you can do now is you can go and do a worksheet where you can continue to practise what we've learned, and you can also do the exit quiz to show how much you've learnt. I hope I see you next time for our next lesson.

Worksheet: Moments and balance (Part 2)

Exit Quiz: Moments and balance (Part 2)

Completed: Moments and balance (Part 2)

In this lesson we discuss changing the moment of a force to balance an object. We also calculate the correct distance or force for an object to be balanced.

Science

Moments and balance (Part 2)

Introductory Quiz

Video

Worksheet

Final Quiz

Lesson Complete