Lecture 2, Defining Force: Pushing + Pulling Influences Flashcards

1
Q

Defining Force

A

a force is a push or pull (acting on a system) - main definition
- a force attempts to create a change (in motion) - however it does not always create change

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2
Q

What are 3 things that force can do in regards to movement

A

force can create, change or prevent movement (if they do any of these is dependent on 3 factors)

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3
Q

What are 3 factors that force is dependent on?

A
  • the magnitude (or size) of the force: the larger the applied, the bigger the change (small may not even apply change)
  • point of application (where that force acts on a system): a force applied on a side will cause translation and rotation and a force applied through the centre will cause translation (some forces create linear actions and some create rotation)
  • relative position: objectives will move differently if other forces act on the system (what is the effect of one force given there are other forces acting on the system)
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4
Q

Force (2)

A

force are exerted by objects on other objects
- forces can deform objects
- in rigid-body mechanics, we assume that objects do not change shape
- forces can accelerate objects
- create movement and stop movement
- speed up objects, slow down objects, and cause objects to change direction
- forces are not only involved in creating the action but also preventing actions (forces may move objects but also prevent objects)

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5
Q

Reaction Forces and Forces acting Together

A
  • forces come in pairs
  • force exerted by one object on another is matched by an equal but oppositely directed force (reaction force)
  • the reaction force can create influence on the acting force
  • example: when the swimmer comes in contact with the wall, it pushes the wall that will go into the wall which will create a reaction of the wall fighting back with equal magnitude (give back) opposite in direction (reaction force is going to propel the swimmer off the wall) the forces that are outside the system are responsible for creating movement
  • reaction forces create movement
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6
Q

Quantifying Force

A
  • the SI Unit of measurement for force is the newton (N)
  • one newton force is defined as:
    ◦ the force required to
    accelerate a 1kg mass 1 m/s2
    ◦ 1 N = 1 kg x 1 m/s2
    ◦ force (F) = mass (m) x
    acceleration (a)
    ◦ F = ma
  • force is measured in newtons
  • apply a force to a mass (system that we are going to measure) - take that system and we are going to do movement (push against the mass for example)
  • mass is measured in kg
  • acceleration is the rate of change of an object
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7
Q

Quantifying Force (2)

A

a force is a vector
- a quantity having direction as well as magnitude
- that quantity has a direction that tells you which way things are happening and a number of how big or small that thing that you are measuring is
- ex: + (coming from right) or - (coming from left)
to fully describe a force, you must identify its:
- orientation (direction)
‣ + or - sign
‣ angle or degrees
size (magnitude)
‣ number value (0.0)

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8
Q

Depicting Force

A

if we wanted to represent a force (or any other vector) graphically, an arrow makes a good representation
- the length of the arrow indicates the size of the force (long line might indicate a very large force and vice versa)
- the end of the arrow indicates the point of application (how that force its acting on a system)
- the shaft of the arrow indicates the line of application
- the point of application also identifies the opposing (reaction) force (going to have a reaction force create that will be equal in magnitude opposite in direction as forces come in pairs but will not be deformed as rigid-body mechanics)
- the angle of the arrow indicates the orientation of the force
- the arrow tells you a lot about quantity

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9
Q

Internal Forces

A

forces can be classified as internal or external

internal forces act within the object or system who motion is being investigated
- in the human body: muscle pull on tendons, which pull on bones / bones push on cartilage, which on other cartilage and bones
muscle forces can produce motions of the body’s limbs
- internal forces are incapable of producing changes in the motion of the entire body

  • internal forces may be important when examining the nature and causes of the injury
  • forces that come within the system are referred to as internal
  • however, to move an entire system we need an external force
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10
Q

External Forces

A

forces can be classified as external or internal

external forces act on an object as a result of its interaction with the surrounding
- the body changes its motion only if it can be push or pull against some external object
external forces can be classified as contact or non-contact forces
- contact forces occur between objects in contact with one another
- solid contact forces exist
between two physical objects
- air resistance and water are
examples of fluid contact
forces (when you are moving at
high speeds you can feel the air
resistance)
- non-contact forces can occur even if the objects are not touching
- gravity, magnetic forces, and
electrical forces are examples
(gravity plays a huge role on the
system)
- external forces are responsible for movement of the entire system as they are critical to movement and give a force back

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11
Q

Gravitational Attraction

A
  • the force of gravity is the only non-contact force considered in sport and exercise
  • the force of gravity accelerates an object downward at a rate of 9.81 m/s2 (or -9.91 m/s2)
  • this acceleration is called gravitational acceleration or the acceleration due to gravity (ag = force that pulls us down allowing us to move faster and faster)
  • the force of gravity acting on an object is defined as weight (by definition weight is a force is something that is applied to a system that creates acceleration at a very specific rate)
    ◦ weight (wt) = mass (m) *
    acceleration due to gravity (ag)
    ◦ wt = mag
    ◦ -9.81 N = 1 kg x -9.81 m/s2 (you rate of fall will increase by 9.81m every second)
  • force is a vector (not only how big or small but also direction as well)
  • gravity is a constant = -9.81 N
  • the negative indicates it is moving downwards (reflected by -)
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12
Q

Dynamography

A

the measurement and recording of forces and pressure
strain gauge and load cell

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13
Q

Strain Gauge

A

a wire attached to two pads
- resistance of the wire changes as it bends
- how much the wire bends is related to the force applied
- will take electrical currents and convert it into force dependent how much is needed to bend that fire

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14
Q

Load Cell

A

spring element in a metal case
- converts forces into electrical signals
- as the force applied to the load cell increases, the electrical signal changes proportionally
- can measure deformations - convert how much bend to how much force was needed to bend that system
lot of these devices are transducers (takes one thing and turns it into something else)
two tools that measure how much force was applied to them (change in electrical current or change in metal and change that into how much force was applied)

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15
Q

Force Platforms

A

load cells are often put force platforms (or force plates)
instruments that measure the ground reaction forces generated by a body standing on or moving across the platform
- ground reaction forces are ~1 to 1.2x body weight when walking (a bit of a bounce)
- ground reaction forces ~3 to 5x body weight when running (more of a hop when running - if you form is not quite right that extra force can do wear and tear on your body)

force plate is more common (a bunch of load cells put in the plate) - can stand on the metal device and push against it and the load cells will detect where i am standing and how much i am pushing (force plates are measuring the reaction force of how much the plate pushes back)

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16
Q

Measuring Forces

A
  • force was measured over time (impulse)
  • standing still - weight of the athlete
  • bending down - preparing for the jump
  • extending - pushing on the ground to propel the body upwards (reaction force is what lifts your body in the air eventually)
  • lifting up - body quickly comes off the ground
  • airborne - no force, no contact
17
Q

Contact Forces

A

the resistance that one surface or object encounters when moving over another

18
Q

Friction Force

A
  • force that is parallel to two surfaces in contact
  • opposes motion or prevents sliding (acts in the opposite direction - very beneficial at times)
  • a resistive force (and a force that exists when two objects come in contact with one another)
  • but…necessary for movement
  • need to know what the system is and how it connects with other objects in order to measure friction
19
Q

Normal Contact Force (reaction force)

A
  • forces come in pairs - the normal force is the REACTION to the applied force
  • the reaction force is equal in magnitude but opposite in direction to the applied force
  • the reaction force is the external influence that is responsible for movement
20
Q

Friction - why does friction occur?

A

no surface is perfectly smooth
the mountains and valleys on system catch the bumps and groove on another system
this physical contact between two systems creates resistance (friction)
- sometimes that resistance can be minimal and cause slipping but sometimes slipping can be minimal if the two systems combine well with one another

21
Q

Does surface area of contact affect friction?

A

the force of friction would be the same (the interaction between one object and the next is the same)
- the area of contact does not matter when calculating friction

22
Q

Calculation of Friction

A

friction arises as a result of interaction between molecules of the surfaces in contact
force of friction (F) = coefficient of friction (μ) x reaction force (R)
-coefficient of friction (μ)
‣ reflects the interaction
between two surfaces in
contact
‣ 0 < μ < 1
‣ cannot be zero because
that indicates no
connection and less
friction (more sliding)
‣ the closer we are to 1 that
means that are objects
are more connected and
less sliding
‣ we look at the weight of an
object as it does not
change according to the
orientation
- normal contact force (R)
‣ affected by the weight of
the object
‣ not influenced by the
surface area it covers
- friction is a force

23
Q

What are the two types of Frictions?

A

static friction and dynamic friction

24
Q

Static Friction

A

the amount of resistance that is present when two systems are in contact that are not in motion - two objects in contact not in motion

25
Q

Dynamic Friction

A

two objects moving over top of one another (there is still a resistance present but something is changing and there is movement)

26
Q

Friction and Movement - Cross-Country Skiing

A
  • to decrease friction
  • wax creates a lubricant (changes how ski interacts with snow - can decrease friction but if it can also create more grip of friction if it is icy)
  • ski can glide smoothly on the snow
27
Q

Surfing

A
  • to increase friction
  • wax creates a stickiness
  • prevents slipping off the board
  • they stand on top of wax to create some wax between top surfboard and feet (want to create more friction to remain in contact)
    both athletes will apply wax to their equipment
28
Q

Cartesian Coordinate System (what are the 4 quadrants)

A

specifies data points uniquely in a plane by a set of numerical coordinates

origin is where the x and y meet
quadrants are the 4 forces which tell us where the forces are heading
- quadrant I (+x, +y) - anything that goes up (y) and to right (x) is giving a positive sign
- quadrant II (-x, +y) - left would be given negative y but up is positive
- quadrant III (-x, -y) - down and left both negative
- quadrant IV (+x, -y) - right is positive and down is negative
* the quadrants move in a counter-clockwise manner

29
Q

Adding Forces Together

A

total (net) force is derived from two or more forces (the sum of the forces) ∑F = ma
- we have to group things are similar only - the forces cannot be different when summed together
- because those forces are acting along the same plane and same directions they can be added together (left and right)
- the object would be moved up - example 2
- forces can be summed together when acting in the same plan (colinear)
- forces acting in the horizontal and vertical planes can be summed together using vector addition

30
Q

Resolution of Vectors

A

a vector may be broken down into its (x) horizontal and (y) vertical components we can use the x and y components to calculate the magnitude o a vector using the pythagorean theorem
◦ a2 + b2 = c2
vectors can also be resolved using trigonometric functions
- sin θ = y component / hypotenuse
- cos θ = x component / hypotenuse
- tan θ = y component / x component
my notes:
if you know any 2 side lengths you can calculate the third
- as well if you know any angle and one side you can calculate the third
- allows us to take into account things that are going in different directions which allows us to find out how much they are acting together, find our another side etc.