Lecture 2, Defining Force: Pushing + Pulling Influences Flashcards
Defining Force
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
What are 3 things that force can do in regards to movement
force can create, change or prevent movement (if they do any of these is dependent on 3 factors)
What are 3 factors that force is dependent on?
- 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)
Force (2)
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)
Reaction Forces and Forces acting Together
- 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
Quantifying Force
- 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
Quantifying Force (2)
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)
Depicting Force
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
Internal Forces
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
External Forces
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
Gravitational Attraction
- 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 -)
Dynamography
the measurement and recording of forces and pressure
strain gauge and load cell
Strain Gauge
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
Load Cell
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)
Force Platforms
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)
