OCR A-level PE: Centre of Mass and Stability

Summary

Highlights

• Center of mass is the point where mass is distributed evenly within an object and can shift. 🌀
• For uniform objects, the center of mass is typically at the center, but for humans, it varies. 👤
• Athletes like gymnasts utilize body position changes to manipulate their center of mass for balance. 🤸
• Greater mass in athletes like rugby players can mean more stability on the field. 💪
• A broad base of support contributes significantly to an object's stability. 🌍

Key Takeaways

• The center of mass is imaginary and can shift based on an object's position and weight distribution. 🎯
• Athletes can manipulate their center of mass by adjusting limb positions to enhance stability. ⚖️
• High center of mass equals less stability; low center equals more stability. 🏋️
• A wider base of support generally leads to better stability. 🏠
• Aligning the line of gravity with the center of mass is crucial for maintaining balance. 💫

Overview

Chapters

• 00:00 - 01:00: Introduction to Center of Mass The chapter introduces the concept of the center of mass. It defines the center of mass as the point in an object where the mass is evenly distributed in all directions. The text notes that in a uniform object, this point is easy to identify, as it is typically the center of the object.
• 01:00 - 02:30: Factors Affecting Center of Mass The chapter titled 'Factors Affecting Center of Mass' discusses the concept of the center of mass, particularly in relation to human beings. It highlights that unlike regular objects, the human body is not symmetrical, but as a general rule, the center of mass is located around the navel or belly button. The text emphasizes that the center of mass is an imaginary point that is influenced by physics and can shift depending on various factors.
• 02:30 - 05:00: Sporting Examples The chapter titled 'Sporting Examples' focuses on discussing the factors that affect the position of the center of mass in a body. The concept is explained through examples such as how a person's center of mass moves when they lean forward. The chapter emphasizes understanding how altering the distribution of mass can impact the movement of the center of mass.
• 05:00 - 09:00: Center of Mass and Stability This chapter explores the concept of center of mass and its relation to stability, particularly in moving bodies. It explains how the center of mass shifts when a person moves a part of their body, like flicking a knee or leg. These movements change the distribution of mass, which in turn affects the position of the center of mass. The text emphasizes the importance of the center of mass in understanding body movement and stability.

OCR A-level PE: Centre of Mass and Stability Transcription

• 00:00 - 00:30 allow me to immediately define center of mass for you and it is the point in the body not necessarily the human body by the way the point in the object the point in the body where mass where mass is distributed evenly is distributed evenly distributed evenly in all directions now in a uniform object that's a really easy point to identify because it's literally the center of the
• 00:30 - 01:00 object but what about a human being a human being isn't regular say it's got toes and arms and legs and a head and hair and all this kind of stuff now as a general rule of thumb the center of mass is not at the thumb the center of mass is roughly at here the navel the belly button this is where we find the center of mass to be now can i stress to you it is an imaginary point and it can shift okay it is an imaginary point it's effectively a result of physics okay so it's imaginary it shifts we can
• 01:00 - 01:30 manipulate it now excuse me what i want to do next is i want to now talk about the factors affecting the position of the center of mass so let's think about the factors affecting the center of mass and i want you to get sort of to grips with the idea that if we change the distribution of our mass you know for example if this person was to lean forward if we to change the distribution of mass the center of mass moves so for example if this body was to lean forward the
• 01:30 - 02:00 center of mass would actually lower and come forward if this person was to flick her knee up at the uh flick her leg up at the knee this center of mass would now move upwards and backwards because the mass is moved in that direction so the distribution of mass affects the center of mass therefore the movement of limbs affects the center of mass so if we move our center of mass oh sorry if we move our limbs the center
• 02:00 - 02:30 of mass will move in a in association with that and finally as i mentioned already we've got the notion of uniformity of shape so if we are talking about a discus for example the district has an even distribution of mass so this is negligible it has no limbs so can't move its limbs therefore where does its center of mass appear in the center of the body in the center of the object now this is all well and good but let's see if we can apply this principle to some examples and this is where your i want you to actually have a go at
• 02:30 - 03:00 these i've got five for you i would like to ask you for all these sporting examples which letter a b c or d most actually represents the likely center of mass for these performers and if you want to pause the video and do that for yourself feel free but i'm going to go over them for our surfer here she has brought her arms out she's lowered her body so her center of mass is going to move down slightly from her navel but not far she isn't lying on the ground for example she's not put her head really low she's not stuck her uh her arms sort of backwards so
• 03:00 - 03:30 here this a is likely to be her center of mass what about this performer who's doing sort of like some sprint warm-up it looks like well the knee has lifted upwards and forwards right we yes we've got an arm out but we've also got an arm in front now i would argue that this therefore would be her center of mass c this one's a really interesting one what about our performer here this performer has their legs down here the very heavy head nice and low they've got their shoulders low as well and i would argue
• 03:30 - 04:00 their center of mass and by the way it's not on their arm the arm is just sort of transparent here the center of mass would be this one or d if this person actually arced more to get a real arc of the body we could argue that that would be d but i'm going to say c or d for that one this one though will really give us the intuition of what we need here we've got a body we've got a human body we've got the body which is connected to this very very heavy bar now i'm going to assume
• 04:00 - 04:30 that the bar and the person are roughly the same weight that's my assumption here so with that in mind the whole body is the weight and the human being where would the center of mass be well it's definitely going to have moved up right because we've got this heavy thing above so would it be here would it be here would it be here now i'm going to argue the answer is a okay the center of mass is a now you might be thinking well i'm joking that's in mid-air exactly i come back to the imaginary point notice for our high jump for example if they arc enough
• 04:30 - 05:00 they can have their center of mass traveling below the bar while each each segment of their body travels over the bar that's why the fosbe flop is in fact affected now let's go a little bit further one last one very similar to the one that i've just talked about here we've got exactly the same position although upright not squatting like this one but here they've got the same thing but notice the golf club is significantly less heavy than the weighted bar so where do you think the center of mass would be and i'm going to argue it's here and point b even though the arms
• 05:00 - 05:30 seem to be in the same position here the weight of this object is so much less that the mass is still distributed relatively close to the navel so their points i'd like to get across now and i completely understand that that feels a little bit okay what we're going to do here is we're going to talk about stability what's the relationship of center of mass to stability well i'm going to give you sort of four principles so here's principle number one the higher the center of mass
• 05:30 - 06:00 the less stable okay so if the center of mass goes upwards the object the body becomes less stable so let's see if we put that into a couple of examples can you see with this person here they're a nice squatty position yes the center of mass is high they've got the weight above their head do you think there will be more or less stable once they upright themselves move up and stand on two feet they'll raise that center of mass higher they will become less stable think about a gymnast who's performing a handstand when are they
• 06:00 - 06:30 less stable when their legs are pointed straight up in the air or when they perform their legs maybe bent at the hip and at the knee of course they're gonna be more stable in the latter because they brought their central mass further downwards point two let's have a look at mass the greater greater the mass the more stable think about let's say uh something like 150 kilogram prop forward
• 06:30 - 07:00 in rugby compared to let's say an 80 kilogram of um scrum half in rugby which one is going to be more stable on their feet and harder to knock over and tackle well of course it's gonna be the prop forward they're heavier they've got a greater mass and that's a point we want to make so the more mass the more stable and of course these combine and you've got to consider them in combination thirdly is what about the base of support okay base of support so if i meant to write this into a statement sorry the broader or the larger the base
• 07:00 - 07:30 broader the base the more stable so consider for example a headstand and a handstand the handstand has the two palms of the hand as the base a headline has the two parts at hand and the forehead which of those has got the broader base which of those are going to be more stable but of course it's going to be the headstand for example think about if you were um think about if you were for example let's say think about if you were um you were going to be defending a corner
• 07:30 - 08:00 in football and he wanted to get nice and solid so you couldn't be knocked over by the forward is perhaps trying to head the ball where you get a nice solid base broaden your legs you'd make your base more larger right rather than your feet being together which would make you very easy to destabilize and fourthly folks fourthly we want to address the notion of line of gravity now this one is a really interesting one and the point i want to make here is that line of gravity should be above
• 08:00 - 08:30 above the base so let's go back to our let's say sir for example earlier can you see here that the center of mass is above the base which is the surfboard the line of gravity the log is just there right what if that surfboard kind of slipped in this direction and all of a sudden the surfboard was here and line of gravity was here they would be destabilized right that's what we're talking about equally we've got the idea here that the base for this performer is directly underneath
• 08:30 - 09:00 the center of mass the line of gravity is acting downwards to the base if they were to for example if they were to have the bar sort of move slightly backwards if they sort of like if their shoulders are hyper-extended for example that would mean that they would become much less stable and they may well fall over so those the key principles that i want to get across to you it is both the concept of the center of mass but also the relationship of that central mass to stability hope that's helpful thank you