Lecture 13, Balance and Stability - Keeping it All in Check Flashcards
What’s the best way to walk to tightrope? Should you use a pole?
- a long pole will increase rotational inertia by increasing both mass (m) and the distribution of mass (k) - by carrying long pole and the disruption you increase your radius of gyration - you increase your moment of inertia (you become wider and heavier) - more resistance to change
- a curved pole will lower the position of the central of gravity therefore increasing stability
The Center of Gravity
the center of gravity is the point on a system around which its weight is evenly distributed or balanced
- the center of gravity is not a physical entity (its location is not marked on an object)
- it is a useful concept, though, as the center of gravity is the point where the mass in centered
the center of gravity is the point through which the force of gravity acts
my notes:
- center of gravity is a point on a system where all the stuff is equal (same amount of stuff to the left or right, above or below, in front or behind) - is not real
- center of mass then all the masses are equal / center of gravity is where the forces are all equal - still the same point on the system
- where the center of gravity is, determines how the system responds to other forces when it is applied on it and when other things act upon it - will it hold its position or change - not a physical thing but it is something we can measure (put someone on a balance board and figure out their center of gravity)
The Position of the Center of Gravity (shift)
imagine a ruler stacked with 6 quarters. if the quarters were were all stacked in the middle or if 3 were placed on either end, the center of gravity would be in the middle of the ruler. if the quarters were all placed on one side, the position center of gravity would shift towards the heavier end
my notes:
- the position of the center of gravity is not a fixed thing and thus can change - can shift the position
* when you add all the coins to one side you have added mass but disproportionally and thus the center of gravity shifts towards the heavier end
- it shifts towards the heavier thing - same amount of mass and/or forces on left and right, above and below (we can get everything equal by allowing it to shift)
- can manipulate where your position of the center of gravity is which changes how you respond to forces - can be beneficial
- as the environment and task changes you can make it easier or harder to move based on the position of the center of gravity
The Position of Center of Gravity (How it changes when you lean or tilt?)
- when standing upright with both feet on the ground, the center of gravity is positioned in the middle of the body
- when an individual leans to the side, the position of the center of gravity shifts to the side where the weight is concentrated
my notes:
- you are very stable when you are standing upright but as you lean to one side your centre of gravity shifts as you lean and if you lean too much it can shift too much which makes you become less stable more comprised and harder to hold the position
- when it shifts too much the center of gravity can become unstable where if it shifts outside of the body you can fall over
- too far left or right, too far in front or back (when tipping in front) can compromise our ability to deal with forces
The Position of the Center of Gravity (3)
the vertical position of the center of gravity is influenced by the distribution of forces (mass)
as weight is added on top of the system the center of gravity shift upwards towards the added force (heavier end)
- if you take an object and hold it above your head the center of gravity is going to travel upwards - can be problematic as you become unstable making it harder to deal with forces
- goal for human movement to control our movement is to have center of gravity closer towards midline and feet (down) - allows us to respond to external forces better
- when you get down on all fours you center of gravity drops outside of your body which can make you much more stable and be able to better withstand forces
The Position of the Center of Gravity (can be found outside)
the position of the center of gravity can be found outside of the body
- by getting down low and have their center of gravity closer to the ground they are able to withstand any external influence much better - when footballers go to contact they get down nice and low and bring their center of gravity lower which makes it easier to make the opponents topple over as their center of gravity is going to be higher - lift someone up and bring their center of gravity higher
The Position of the Center of Gravity (3 dimensions)
the position of the center of gravity can shift
how much it moves depends on the weight of the segments moved and the distance moved
- the center of gravity moves in 3 different directions (3 dimensions): up/down (y-axis), left/right (x-axis) and forward/back
- can measure its position and see how stable, in control or how vulnerable we are
- we can control force by having more stable center of gravity
Center of Gravity and Performance (translation and rotation)
- when a force acts on a system knowing where the center of gravity is and where the force acts on a system will determine how that system will move
- centric forces are when it acts through the middle and center of gravity - produces translation (change in position) specifically rectilinear motion - system moves in a straight line
- if something acts off centre (does not matter how much) - eccentric force (different from eccentric muscle actions) - produces some translation and rotation which is curvilinear motion
- a couple is two forces both off centre acting in opposite directions, equal in magnitude only results in rotation ONLY as it anchored in space so no change in position (pure moment - moment dealing with rotation and only rotation)
- knowing the center of gravity does two things: it affects how the system responds to external forces or even if it responds to external forces as forces creates, changes or prevents movements
Equilibrium, Balance and Stability
- equilibrium is a physical state, in which opposing forces equal out
- balance is an even distribution of weight enabling someone or something to remain upright and steady
- balance is a static concept - motionless - means you are in control and are not moving
- stability is the capacity of an object to return to equilibrium or to its original position after it has been displaced
- stability is a dynamic concept
my notes:
- equilibrium is the desire to control or be in control
- balance and stability are two different types of control (forms of control in unique situations)
- if you want to be in a state of equilibrium you want to be in a state where you are in control of what is happening
- when you are trying to balance and there is little wobbles those do not count as you are still trying to hold your position and are not falling - sufficient for balance
- constantly correcting and moving but as long as you hold your position it counts as balance
- if you are in static equilibrium you are balanced
- reflex is where a message is automatically sent and we will respond by trying to gain stability again
- regaining control is dynamic equilibrium - controlled movement to regain balance - getting back to the static state - stability
- for us to be balanced nothing is happening and so when you add up all the forces it is equal to zero
Equilibrium, Balance and Stability (2)
- the center of gravity of an object is its balance point or the point about which the moments of force created by the weights of all the parts of the object sum to zero
- for an object to be in balance (in a state of equilibrium), the external forces acting on it or the moments of those forces must sum to zero
- when you add up all the forces, torques; nothing is going to be happening
Calculating Equilibrium (static)
static equilibrium: a motionless state
- there is no net influence acting on the system
ΣFx = 0 horizontal forces
ΣFy = 0 vertical forces
T = 0 rotary Effects
- everything is going to be zero - nothing is happening
- you add up all forces in x direction, y direction or all the torques where it is always going to be zero as we are in static equilibrium
Calculating Equilibrium (dynamic)
dynamic equilibrium - a moving state
system is accelerating but in control over any external influence
- ∑Fx–max = 0 horizontal forces
- ∑ Fy–may = 0 vertical forces
- ∑T - Iα = 0 rotary effects
- there is movement occurring - the sum of the forces are not equal to zero - you are undergoing acceleration and are trying to counter the change to try to get back into control
- there is an extra force to counteract the change so you can move with control
- the second part is the movement (max, may, Iα)
- is in a inbalance in forces and then you create an extra force
Factors Affecting Stability (base of support)
- the stability of an object is affected by the base of support, the area bound by the outermost regions of contact between a body and support surface
- as the base of support increases (area of contact), stability increases
- the more space you occupy, the more stability you have and a better ability to recover from any situation
- it is not just about keeping control but also trying to keep your body as level as possible so that your center of gravity is right in the middle of the base of the support
- however when you walk you try to lose control slightly
- the stability of an object is affected by position of the center of gravity
- walking is a combination of losing balance and then regaining it - bringing center of gravity closer to edge of the base of support becomes much easier to initiate the movement
- coming to the edge of the diving board makes it much easier to initiate the movement - can change your ability to control or create movement if you desire
Factors Affecting Stability (taller)
it would be easier to knock over a system with a higher center of gravity
assuming the system was rigid and falling around a fixed point
stability decreases as the center of mass moves up the system
the higher the center of mass, the greater the amount of torque its motion creates about the support surface
- not just about the base of support as the taller you are the easier it is to knock you over as the center of gravity is going higher and higher
- getting lower gets you more control in the situation
- mass is also important as the more resistance to change you have; when you have more mass the system is move stable and more in control
Factors Affecting Stability (mass)
the stability of an object is affected by the mass of a system
- according to Newton’s First Law, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force
- this resistance to change, or inertia, is proportional to mass
- as mass increases, stability increases
- as mass increases, the contact (reaction) force also increases, which increases friction
- so, the amount of friction present also influences the level of stability
- a system with more mass is more stable and more in control
- mass is not only more inertia but more friction which means more reaction force and more stable
