Linear Kinetics Biomechanics Exam 3 Study Guide

LINEAR KINETICS

Newton’s 1st Law?

* Law of inertia
* A body remains at its current state of motion (constant velocity) unless acted upon by an external force

What is inertia? Relationship between inertia and mass?

* Tendency of an object to keep the current state of motion
* Difficulty in changing the state of motion
* Inertia is proportional to mass of an object
* Mass=measure of inertia in linear motion

Newton’s 2nd Law?

* Law of Acceleration
* An external force applied to a body causes acceleration

F=ma? If F=0, what happens?

* Force= Mass/ Acceleration
* Acceleration= Force/Mass
* If force is equal to zero, then acceleration is zero because they are proportional to one another

Meaning of Net Force

* Sum of all the forces acting

Resistance vs. Propulsion

* Minimize friction as resistance
* Lubrication-synovial fluid (m=0.01)
* Skating - gliding
* Tire inflation & paving
* Maximize friction as source propulsion
* Source of horizontal GRF
* Cleats: prevent slipping
* Skating - push off

Newton’s 3rd Law?

* Law of Reaction
* For every force (action), there is an equal and opposite reaction.

What is reaction?

* Same magnitude
* Opposite direction
* EX:
* Bullet vs. Gun
* Fist Fighting
* Propulsion in swimming (pushing water behind you)
* Hockey players

Ground Reaction Force (GRF)?

* Reaction supplied by the ground
* Passive force

Importance of the GRF?

* Important for human motion
* Major external force that accelerates the body
* Speeds up
* Slows down
* The only external force one can voluntarily control
* Weight is always constant & downward

How Does a high jumper utilize the GRF during takeoff?

* Slow-down of rightward (+) velocity
* Slow-down of downward (-) velocity
* Speed-up of upward (+) velocity

Law of Gravitation?

* All bodies are attracted to one another due to their masses

Properties of gravitation

* Proportional to masses
* Inversely proportional to (separation distance)^2

Weight? Relationship between weight and mass?

* Gravity exerted to a body by the earth
* Weight= (mass)(gravitational acceleration)=mg
* Direction: downward
* Passes through the COM of the body

Gravitational acceleration?

9\.81 m/s^2 in downward direction towards the earth

Friction?

* Force acting at the are of contact between two surfaces
* Source of horizontal GRF

Magnitude

* Proportional to
* Normal reaction force (N)
* Friction coefficient (WEIRD U)
* Cannot exceed your force

Direction

* Opposite that of motion or motion tendency

Friction Proporties

* Friction coefficient
* Force

Sliding friction vs. Rolling friciton

* Sliding friction
* Friction due to relative motion of the surfaces
* Rolling friction
* Friction due to temporary deformations of the surfaces
* Sliding vs Rolling
* Sliding friction >> rolling friciton
* Beneficial to convert sliding to rolling for smaller friction

Static friction vs Kinetic friction?

* Static friction
* Ff experienced with no motion (F=Ff)
* Max. static friction = force required to initiate motion
* Kinetic (dynamic) friction
* Ff experienced during motion (F>Ff)
* Force required to maintain motion once initiated

Max static friction

* Max. static friction = force required to initiate motion

Force required to initiate motion and force required to maintain the motion?

* Max. Static Friction: force required to initiate motion
* Kinetic (dynamic) Friction: Force required to maintain once initiated

Your Force vs. Friction? Net Force?

* Force is a push or pulls acting on a body that causes a motion
* Net force is the sum of all the forces acting
* Friction is the force acting at the area of contact between 2 surfaces

Lubrication vs. Friction? Role of synovial fluid in a synovial joint?

* Lubrication decreases friction. Synovial fluid reduces friction in the joints which in turn reduces injury between bones.

Resistance vs. Source of propulsion

* Minimize friction as resistance
* Lubrication- synovial fluid (m = 0.01)
* Skating - gliding
* Tire inflation & paving
* Maximize friction as source of propulsion
* Source of horizontal GRF
* Cleats: prevent slipping
* Skating- push off

Friction vs. Horizontal GRF

* Friction
* Force acting at the area of contact between 2 surfaces
* Source of horizontal GRF
* Used as a source of propulsion to MAXIMIZE friction
* Ex. Soccer Cleats, Ice skates

Momentum? Its Importance?

* Amount of motion
* M=mv
* Vector
* Direction=direction of velocity
* Important in giving & receiving impact, collision etc.
* Unit: kg\*m/s
* Important in giving & receiving impact, collision, etc

Principle of momentum conservation?

* If no external force, total momentum remains constant

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Impulse?

* The accumulated effect of force exertion over a period of time
* Vector quantity: direction= direction of force
* Unit: Ns
* Causes change in momentum

Strategies in giving/ receiving impulse?

* Giving impulse (batting, throwing, etc)
* Maximize impulse by maximizing the momentum of the apparatus (batting)
* Elongating elapsed time
* Receiving impulse (landing, catching, etc)
* Reduce impulse, if possible
* Reduced impulse force/ pressure by increasing time/area

Mechanical Work?

* Force applied against a resistance
* Accumulated effect of force exertion over a displacement
* Work = (force)(displacement)
* Scalar
* Unit: J (Joule) =Nm

Positive work vs. Negative work

* Contribution vs taking away
* Determined by the angle between f &d
* If THETA > Pi/2 rad (opposite), negative work

Muscle Contraction types vs Work?

* Concentric contraction: positive work
* Eccentric contraction: negative work
* Isometric contraction: no mechanical work

Work while overcoming the gravity (weight): mountain or stair climbing?

* Mountain
* Stair Climbing

Mechanical Power?

* Rate of work
* Intensity of work
* Rate of energy generation/consumption
* Power= (work)/(elapsed time)
* Power= (force)(velocity)
* Unit: W (Watt)= J/s
* Scalar

Mechanical Energy? Potential vs Kinetic?

* Capacity to do mechanical work
* Mechanical energy (ME) = Potential E + Kinetic E
* Potential (PE): energy due to vertical (height)
* Kinetic (KE): energy due to motion (velocity)
* Scalar
* Unit: J

Mechanical energy conservation? Diving ex?

* If gravity is the only external force acting on a system, ME remains constant
* EX: Diving (work causes a change in mechanical energy of the system
* Positive work: Increases energy of the system
* Negative work: Decrease energy of the system