Forces Flashcards
What is a force?
A push or pull applied to an object. Used to change the state of motion of an object or body
What is a Contact force
Motion generating forces e.g. Pushes and Pulls (friction, tension, normal)
What is a Non-contact force
Motion controlling forces e.g. Gravity or intertial, magnetic
- Relates to mass and magnitude of attraction between bodies
- Inversely proportional to the square of the distance between them
- Gravitational force is proportional to (m1 x m2) / distance2
Describe the effect of forces on kinematic variables
Why should we care about forces?
Newtons Laws
an object will remain at rest or continue to move with constant velocity as long as the net force = 0
Inertia
Describes an objects resistance to change it’s state of motion.
Inertia is proportional to mass :. greater mass, greater resistance to changing your state of motion.
e.g. A bodys inertia, is equal to its resistance
*any object with mass has inertia
How is state of motion changed?
change velocity = change state of motion e.g. magnitude of velocity or direction. high jumper changing horizontal veloity to vertial
The magnitude of change in velocity is proportional to the net force acting on the object and inversely proportional to the mass. (f=ma) :. large acceleration = large force AND lower mass
Newtons 2nd law of acceleration
A net force accelerates an object
F = ma
Large acceleration: large force
Lighter object: greater acceleration
Acceleration: lighter object requires less force
Explain Newtons 3rd law of action-reaction
how to apply a force externally when the force is internal (e.g. muscle contracting on bones)
- “for every action, there is an opposite and equal reaction”
Internal forces applied to external surface (ground) = ground reaction force to propell forwards (propultion)
How to apply Newtons 3rd law of action-reaction
1) To have the Greatest force applied to us, need to apply the greatest possible force against that object
2) Force to accelerate in a specific direction, need to produce force in a specific opposite direction
What is weight?
represents the force of attraction between the earth and object
* i.e. changes on mars as effected by gravitational pull
e.g. 491 Newtons
weight = mass x gravity
491 / 50 = 9.81 m/s squared
What is mass?
represents the quantity of matter of which a body is composed.
e.g. 50kg
Little G
Constant Gravitational Pull
G x (m earth / earths radius (distance) squared
g = 9.81 m/s squared
Net acceleration on objects from the combined effect of gravitation (distribution of mass within earth) and centrifugal force ( from the earths rotation)
Importance of understanding gravitational pull and weight vs mass
The force acting on the body in relation to it’s mass is one of the most significant forces in biomechanics.
- No horizontal movement exists unless we overcome this force
- All projectile motion is governed by gravitational forces ( always travel in parabolic trajectory, due to force of gravity and constant pull back to earth)
Force (N)
- Force is a vector quantity - Magnitude and direction
- 1 N of force will accelerate a 1kg object by 1 m/s every second
- 1 N = kg m/s^2
- free body diagram is a technique visualising and simplifying a problem by constructing a diagram showing all forces acting on object
Free body diagram
used to help understand the potential action of all forces acting on an object
1 - identify the system
2 - seperate the system of interest
3 - identify object CoM
4 - identify the external forces
5 - find resultant of forces (sum or pythag)
What is the relationship between GRF and COP in the gait cycle context?
COP’s movement path reflects the body’s adjustments to maintain balance - correspond with postural control strategies, helping to reduce sway and maintain stability.
COP trajectory, can indicate potential gait abnormalities if deviating significantly from typical paths.
Centre of pressure (COP)
Indicates the location “balance point” of the average GRF vector. it shofts as weight transfers from heel-strike to to-off
Gait objectives
propel our bodies forward efficiently and with minimal energy expenditure
Ground reaction force fluctuation during gait
GRF changes magnitude.
GRF moves horizontally as stance progresses into propulsion
Ground reaction forces during gait
Horizontal: How fast
Vertical: lifting foot up and down
Mediolateral: rotation side to side
Ground reaction force (GRF)
The measured action-reaction force of our push against the ground
Work
when force is used to move a mass
Force x displacement = work
Units: kgm2/s2 or Nm or Joules (j)
Principles of Work
- a force can only do work if it causes a DISPLACEMENT
- a force can only do work for the DURATION of the displacement
- only the force acting on the DIRECTION of displacement can do work
Rate depended, Area under force displacement curve
Work: tissue loaded injury and how much it deforms from the load - how much can tissue withstand?
i.e.
Displacement = deformation
Force = load
W = force x displacemet COSine
Positive work
Object is moving in the same direction of applied force
e.g. Concentric contraction
Negative Work
Object moves in opposite direction of applied force
e.g. eccentric contraction
e.g. box sliding - pulling in one direction and another force of friction acting on the object/box as it slides which works against its movement. (friction force acts the entire time displacement is acting :. Meets all the conditions for performing work.
Power
measures rate at which work is performed (adds time variable)
Force x displacement = watts / time = power
Force x velocity = power
Velocity = distance / time
:. Speed/velocity of work = power
More power means more energy used within a shorter amount of time
More mechanical work = more energy required :. High impact :. Injury
Power limmitations
Time
-rate of ATP production: rate of which energy can be cycled through the muscle from actin and myosin cross-bridge cycling
- connecting cross-linkages and activating muscle fibred takes time. (heavier load, more active fibers)
- Force-velocity relationship: rate limited force production
- Max power: 1/3 of max shortening velocity
Power rules (same as work)
- Power is derived from the work equation
- can not calcuate work from isometric contractions
- Measured in the direction of movement (velocity in rotation)
- +ve (producing) or -ve (absorbing)
- time dependednt wave more
Instantaneous power
highest power value achieved during the observed measurment (moment in time)
Average power
calculated average force and average velocity for a given moment
Conventional ‘swing’
Pressure differences (Form Drag) -> spin -> magnus force
more tubulent boundary layer = delayed seperation and lower pressure -> spin towards rough side
Ball swings towards rough/turbulent side (same direction of seam angle)
shiny/smooth side facing batsmen (‘bald, helmet’)
Reverse ‘swing’
> 85mph, smooth side (laminar flow) becomes turbulent and rough side = thick and weak turbulence -> earlier seperation -> ball swing towards smooth side (opposite direction of seam angle)
Shape of curve in gait ground reaction force
- Confidence to load stance foot
- Height change in COM
- Indicates speed of movement
How is the power produced at joints determined? (Rotational Power)
Net/sum of torque and angular velocity of the joint
Power (angular) = torque x angular velocity
Power production
Both angular velocity and angular acceleration have the same sign (+/-), power = POSITIVE (w)
Power Absorbtion
Both angular velocity and angular acceleration have different signs ( +/-), Power = NEGATIVE (w)
*i.e. breaking mechanism
Joint power amputee
- No knee flexion in stance phase
- More -ve knee power
- More hip power required for amputees
- No propulsion from ankle
What are the forms of mechanical energy?
Kinetic (Linear and Rotational)
Potential (Gravitational and Strain)
Law of conservation of mechanical energy
- If the resultant force acting on a body is a conservative force, the body’s TOTAL energy will be conserved.
Conservative force = not doing work (i.e. applied force through displacement :. If doing work, NOT conservative) - Resultant forces will be conservative if all external forces are conservative
A force is conservative if it does no work around a closed path (motion cycle)
i.e. can’t make contact with anything else
Transfer of energy
Transfer between kinetic and potential
Have to be airborne (no other external forces acting on the body)
:. Trade off potential and kinetic to make the total energy constant
i.e. conservation of momentum/energy within the air and ability to transfer momentum/ energy between potential and kinetic
Total energy = kinetic + potential
1. Max velocity = max kinetic energy
2. As the ball ascends the potential energy increases while kinetic energy decreases (gravity is slowing the flight)
3. At the peak of trajectory: velocity = 0 and kinetic energy = 0. Maximum height = maximum potential energy
4. Downward flight: reverse change in energy occurs.
Work-Energy theorem
Net work done by forces = change in Kinetic energy (mass x velocity)
Increase work = increase energy (if conserved/transferred effectively by reducing Air resistance, gravity, sound, heat from friction as they all take energy AWAY from the object! (=NON-CONSERVATIVE FORCES)
Kinetic Energy
related to a bodies motion
Linear: mass x velocity x (1/2)
Rotational: mass moment of inertia x angular velocity x (1/2)
e.g. higher from ground = more Kinetic energy
Strain/Deformation Energy
(potential)
Stored energy in an object
k (tension) multiplued by X (change in distance squared) (i.e. deformation) x 1/2
E.g. bow - amount of energy stored has en effect of how far the arrow will travel and more displacement (pull further) = more energy
What are the characteristic which trigger us to move from walking to running?
RDF = rate of force development
Ankle motion VELOCITY
Leg LENGTH
*walking is more energy efficient than running
Work
Product of force and distance W = fd
* no movement/displacement = no work
Mechanical energy
Is the primary agent of ingury
Kinetic (Linear or Angular)
Potential (Gravitational or Strain/Deformational)
Conservation of energy
The net work done on a system is converted into Kinetic and Potential energy.
Total energy of a system remains constant throughout the motion - in the absence of external forces (only gravity)
Transfer of energy
energy is transfered from one body/segment to another
The coefficent of restitution
Energy lost = Initial kinetic energy - final kinetic energy
Quantifies how much kinetic energy is lost due to work done by NON-conservative forces e.g. heat, sound, deformation) during a collision
E.g. helmets (one and done as will no longer have the same coeffect of restitution from impacts :. Have an expiry date)
Friction
A force that opposes the movement of two surfaces that are in contact with one another
The coefficient of friction
sets limit for when sliding begins and describes the tendency for surfaces NOT to slide.
- If force applied exceeds coefficient x normal force = sliding
once sliding begins - kinetic friction force is responsible for the resistance to ongoing motion (kinetic energy)
i.e. higher the less likely to slide (more horizontal/normal force required to initiate sliding and overcome friction force.
e.g. Iron plates = 1.0
or
i.e. lower the more likely to slide (less normal/horizontal force required to overcome friction and initiate sliding)
e.g. ice skating 0.003
What affects friction and how can they be manipulated to improve sporting performance?
The coefficient of friction
Angle of attack (incline) - off sets the perpendicular weight force (decreasing the horizontal/normal force) and therefore decreasing the friction force, making it easier to overcome
What is a Pedoti digram
‘art work’ looking line diagram
the interaction between the vertical and anterior-posterior force from a force plate.
- Shows the resltant of GRF made up from the 3 components (vertical, medio-lateral, anterior-posterior)
- Need to know the:
- vertical and horizontal forces
- the position of the COP in the plane of interest for each moment in time.
Weight (W)
Force due to gravity (acts vertically downward)
- components are either paralell or perpendicular
Paraelell weight force
Pulls the object down the slope
responsible for initiating motion
Parallel weight = mass x gravity x sin(deg)
Perpendicular weight force
Presses the object against the surface of the incline
Determines the normal force (Fn)
Perpendicular weight = mass x gravity x cos(deg)
What increases the RISK for non-contact injury?
Synthetic surfaces with greater translation/rotational RESISTANCE between the shoe and the surface = higher knee/ankle joint moments and rotational injuries (e.g. ACL or “cleat catch”)
Tennis: 200% increase in injury on synthetic (non-slide) vs natural (slide) surfaces.
What is happening in the cycle of gait: Second peak (P2) to toe off?
- The foot is unloaded and as the load is transfered to the opposite foot.
- The time taken to off load from the back foot will relate to the speed of transfer of the weight to the front foot
- Therefore, the LONGER the offloading period from the back foot, the LOWER the first peak (P1) when loading the front foot.
What does the posterior peak signify?
Heel strike
Breaking force
Deceleration (-ve)
energy absorption -> reducing forces on leg and preventing injury
maximum peak = max loading on single leg
When does crossover occur during gait?
55% of stance phase
‘neutral point between anterior/posterior’ Fh = 0, just have Fv GRF
Cross-over to anterior peak
Toe off
Pushing anteriorly = +ve force to propel forwards i.e. propulsion force from GRF
0.2 (20%) of BW
