Lectures 5-11 Flashcards
What does a force do? And what does a force have?
It acts on an object to cause or resist a motion
Forces have a magnitude and a direction
What is Newton’s first law? What is the fancy name for it?
A body continues in a state of rest or uniform velocity unless acted upon by an external or unbalanced force
Law of inertia
What is Newton’s second law of motion? What is the fancy name for it?
A body’s rate of change of momentum is proportional to the size of the force applied and acts in the same direction as the force applied
Law of acceleration
What is the equation for force?
Force = mass x acceleration
1N = 1kg x 1m/s/s
Definition of weight
The gravitational pull that the earth exerts on a body
What is the equation for weight
Weight (N) = Mass (kg) x Acceleration due to gravity
W=mg
Definition of momentum
The quantity of motion that an object has
Equation for momentum?
Momentum = Mass x Velocity
Conservation of momentum (Check this, slide 14, lecture 5)
The momentum before will equal the momentum after unless the object is acted upon by an external force
Look at question, slide 18, lecture 5
Important question this
Look at lecture 5, slide 19
For the link between force & velocity
What is Newton’s third law of motion? What is the fancy name for it?
For every action there is an equal and opposite reaction
Law of reaction
Look at pages 86 + 87 in PE book
For free body diagrams
Definition of friction
A force that opposes the motion of two surfaces in contact
Look at slide 13 lecture 6 definition, bit weird though
Equation for the coefficient of friction
Coefficient of friction = frictional Force / normal force
Look at slide 16, lecture 6 for the symbols
Recap example
Lecture 6, slide 17
Recap
Lecture 6, slide 17-20
Definition of stress
Stress is the force distributed over a given area
Equation for stress? What is stress measured in?
Stress (Pa) = Force (N) / Area (metres squared)
Stress is measured in pascals
Look at slide 4, lecture 7 for symbols
Definition of strain?
Strain is the deformation of a tissue
Equation of strain?
Strain = change in length (m) / original length (m)
Strain = deformation / original shape
What are the units for strain? (Check this on recap/google, slide 5, lecture 7)
Unitless / %
Look at the stress ~ strain relationship slide 6, lecture 7
Recap it as well
Recap
Slide 8 - 21, lecture 7
Definition of centre of mass
The point at which an object or a body is balanced in all directions, the point at which weight appears to act
Definition of (static) equilibrium
A state where net force acting on an object is zero
Definition of stability
Stability is the ability of an object to resist a disruption to equilibrium and to return to its normal state
Tests of balance
Timed single leg stand
Functional reach test
Definition of centre of pressure (CoP)
The point where force acts across an area
Recap
Slide 7, lecture 9
Equation for Work
Work (joules) = Force x Displacement
Units of Work
Joules
What are the directions of Work?
Positive
Negative
Isometric contractions
In terms of directions of Work, what does positive Work mean? What type of muscular contraction is positive Work?
Force and displacement in the same direction
Concentric contraction
In terms of direction of Work, what does negative Work mean? What type of muscular contraction is negative Work?
Force and displacement in the opposite direction
Eccentric contraction
In terms of direction of work, what type of Work is an isometric contraction?
No mechanical Work
Equation for potential energy
Potential energy (Joules) = Mass x Gravity x Height
Equation for potential strain energy
Potential strain energy (Joules) = 1/2 proportionality constant x distance deformed squared
What are the units for Potential Strain Energy?
Joules
Equation for Kinetic Energy
Kinetic Energy (Joules) = 1/2 Mass x Velocity squared
What are the units for kinetic energy?
Joules
What is the conservation of energy?
Energy can neither be created nor destroyed, it can only be changed from one form to another
What is the equation for Total Energy?
Total energy = Kinetic energy + Potential energy
Equation for power
Power (Watts) = Force x Velocity
What is the unit of Power?
Watt
What are the different methods that can be used to calculate power? What is the choice of method very dependent upon?
Linear position transducer
Kinetic
Kinematic ~ Kinetic methods
The choice of method is very dependent upon the context
Recap slide 9, lecture 9
Weird
Equation for impulse
Impulse = Force x Time
Or
Impulse = Ft = /\M = m(v2-v1) = m/\v
Equation for momentum?
Momentum = Mass x Velocity
What are the applications for impulse?
Measure final velocity
Predict how much force you’ll need
How long do you need to apply a force for
How much less force is needed if I reduce mass
Example questions
Slide 8 + 10
Lecture 10
In terms of Impact, what is deformation?
Deformation occurs when impact occurs, for example the tennis/golf ball example, where it moves when hit
In terms of restitution, what is deformation?
After deformation has occurred and it has returned to its normal shape
Recap inelastic and elastic stuff
Slide 13, lecture 10
Equation for coefficient of restitution
Coefficient of restitution (e) = square root of (bounce height/drop height)
What is the centre of percussion?
The point about which a force can be applied without causing an acceleration at the centre of rotation
What are the phases on the stress strain graph?
Elastic -> yield point -> plastic
What does the elastic phase mean of the stress strain graph?
It is where the object is deformed but it will still return to its original shape
What is the yield point on the stress strain graph?
This is the point where if stress/strain continues past this point the object will deform and not return to its original shape
What is the plastic phase on the stress strain graph?
It is the point where the object has become so deformed it will not return to its original shape
Definitions of stress and strain
Strain - is the relative change in shape or size of an object due to externally-applied forces
Stress - is the internal force (per unit area, etc) associated with a strain
What does more stress result in?
More deformation
Factors affecting stability
Mass
Height of centre of mass
Size of the base of support
Line of gravity
How does the functional reach test work?
Get someone to reach out as far as they can before they feel they are going to fall over
Factors affecting balance
Posture
Vision
Environment
Muscle strength
What is Force proportional too?
Force is proportional to change in momentum over change in time
How do Force and velocity link?
Write it out, remember from slide 19, lecture 5
Hint: what is the equation for force?
What is a coefficient of friction value between?
0-1
Except in certain cases
On the coefficient of friction graph what are the stages?
Static
then
Kinetic
