Definitions Flashcards
Kinetics
concerning the analysis of the forces acting on the body. (BASES)
“The study of the action of forces” Hall, 2012
“The branch of dynamics concerned with the forces that cause or tend to cause motion.” McGinnis, 2013
Kinematics:
concerning the analysis of the movements of the body. (BASES)
“The study of the description of motion including consideration of space and time.” (Hall, 2012)
“The branch of dynamics concerned with the description of motion.” (McGinnis, 2013)
A force is a push or a pull. A force accelerates or deforms (not in rigid-body mechanics) an object. Forces come in pairs: action and reaction. A force is something that can cause an object to accelerate (start, stop, speed up, slow down, or change direction). A force is known as a vector quantity (size and direction).
Characteristics: point of application, direction and its sense. (McGinnis, 2013)
Internal forces
forces that act within the object or system whose motion is being investigated. Internal forces are important if concerned with the nature and causes of injury, but they cannot produce any changes in the motion of the body’s centre of mass. Muscles can only produce internal forces, even though muscle forces can produce motion on the body’s limbs, but these motions will not change the motion of the body’s centre of mass unless external forces are acting on the system. (McGinnis, 2013)
External forces:
forces that act on an object as a result of its interaction with the environment surrounding it:
Non-contact forces (gravity).
Contact forces (air resistance, water resistance, ground…):
• Perpendicular component (normal reaction force): acting perpendicularly to the surface of contact
• Parallel component (friction): acting parallel to the surface of contact and opposes motion or sliding between the surfaces. (McGinnis, Chpt 1, 2013)
FRICTION
Friction arises when molecules of the surface are in contact and are interacting:
Friction force is proportional to the normal contact force and acts perpendicular to it e.g. pushing a book then adding another book on top. Adding weight (not mass!) would increase the normal contact force acting between the 2 surfaces but it would also increase the interactions of the molecules of the contacting surfaces, because they would be pushed together harder.
Friction is not affected by the size of the surface area e.g. if you stand the book on the table. The increase in the surface area increases the number of molecular interactions, but the decrease in pressure (force divided by area) decreases the magnitude of these interactions. Thus the net effect of increasing surface area is zero, and friction is unchanged.
The nature of the materials in contact affect the friction force between them e.g. placing a shoe compared to a book. The weight and mass of the objects are the same but the surface area changed. However that doesn’t affect the friction. So it is the difference in the type of material (soft and rough).
Static friction is greater than dynamic friction. It is harder to start something to get moving than to keep the object moving. (McGinnis, Chpt 1, 2013)
Colinear forces
are forces that have the same line of action. E.g. tug-of-war
Concurrent forces:
are forces that do not act along the same line but do act through the same point.
Static equilibrium
when an object is at rest and the forces are in equilibrium
A free-body diagram:
it is a mechanical representation and a useful tool for analyses. Only the object is drawn with all the external forces that act on it. (McGinnis, Chpt 1, 2013)
Compressive forces:
pushing forces act on the ends of an internal structure and the structure is under compression. (McGinnis, 2013)
Tensile forces:
pulling forces act on the ends of an internal structure and the structure is under tension (McGinnis, 2013)
Inertia:
the property of an object that resists changes in motion. Linear inertia is quantified as an object’s mass. Thus it is more difficult to speed up, slow down, or change the direction of a more massive object because it has more linear inertia.
Angular inertia
(rotary inertia): the property of an object that resists changes in its angular motion. It is more difficult to speed up, slow down, or change the direction of an object with more angular inertia (mass). Angular inertia is affected by mass and how the mass is distributed relative to the axis of rotation.
Angular momentum:
is the product of mass and velocity. Mathematically, angular momentum is a vector quantity, it has a size and direction
Scalar
A quantity that has only a magnitude
Speed
The rate of change of displacement with respect to time (vector quantity)
Velocity
The rate of change of distance with respect to time (scalar quantity)
vector
a quantity that has both direction and magnitude
acceleration
the rate of change of velocity with respect to time
displacement
change in position during a time interval (vector quantity)
Distance
length along a path an object has travelled (scalar quantity)
mass
the quantity of matter in an object
weight
the force that results from the action of a gravitational field on a mass
Force
A force is a push or a pull
A force accelerates an object.
Equation: F = m x a
Units: Newtons (N)
One Newton of force is defined as the force required to accelerate a 1 kg mass, 1 m/s2.
A force is a vector – it has a size (magnitude) and a direction.
Length of arrow indicates size.
Shaft/arrow indicates the direction and point of application.
Inertia
the property of an object to resist changes in its motion
Linear motion
movement along a straight or curved line where body parts move in the same distance and direction at the same time (a.k.a translation) e.g. skating
angular motion
motion around an imaginary axis with all body parts moving through the same angle at the same time (a.k.a rotation) e.g. gymnast swinging on bar
Angular velocity
the rate of change of angular displacement with respect to time
Curvilinear motion
the body moves along a curved path and still satisfies the condition of linear motion e.g. sky diving
General motion
very common in sport, usually rotation of some body parts resulting in translation of other body parts
Magnitude
The relative size of an object. The term for the size of a vector.
Stability
Resistance to both linear and angular acceleration to disruption of equilibrium.
Centre of Mass (CoM)
the point in a body or a system of bodies about which the weight is evenly distributed or balanced and through which the force of gravity acts.
Momentum (P)
The product of mass and velocity of an object.
Perspective error (2D Video Analysis)
The distance between the performer and the camera.
Errors:
If the object is behind the Plane of Motion (PoM) then the dimensions will be smaller.
If the object is in front of the PoM then the dimensions will be larger.
If the object is not parallel to the PoM then the angle and length of object will be wrong
To reduce error:
Increase the distance between the camera and the PoM.
Place the object parallel to the PoM.
Newton’s first law
the law of INERTIA: an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force
Newton’s second law
the law of ACCELERATION: the sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = m x a
Newton’s third law
the law of REACTION: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Running speed
step rate (Hz) x step length (m)
Projectile
a projectile is an object that is subject to no external forces other than gravity.
If there is no air resistance, the projectile follows a parabolic path and the path is symmetrical by its APEX.
Factors affecting long jump performance
Distance –> flight distance –> HEIGHT, SPEED and ANGLE of takeoff
Resultant VELOCITY of a Projectile (i.e. hypotenuse in trigonometry)
Remember that RESULTANT means the sum of the vectors and that velocity is a vector. A projectile has a HORIZONTAL AND VERTICAL velocity. This means 2 VECTORS… so this can be calculated by using basic trigonometry: you can either use the Pythagorean theorem wich is a quicker and easier than using the SOHCAHTOA equation
Take off ANGLE of a projectile
This is the ANGLE at which the Centre of Gravity (CG) is projected:
Using basic trigonometry, this can be calculated by using the equation: Arctan (vertical Take off velocity/Horizontal Take Off Velocity)
What is the vertical acceleration of a projectile?
The vertical acceleration of a projectile is CONSTANT due to gravity (i.e. 9.8m/s2)
What is the vertical velocity of a projectile?
The vertical velocity of a projectile is CONSTANTLY CHANGING during its flight. Indeed, velocity = displacement x time
What is the horizontal acceleration of a projectile?
There is NO horizontal acceleration of a projectile during flight.
What is the horizontal velocity of a projectile?
Air resistance can affect horizontal velocity. In most cases, the horizontal velocity of a projectile remains CONSTANT
Vertical Motion is motion with constant……….., while horizontal motion is motion with constant………..
acceleration ; velocity
Projectile motion occurs when objects are only under the influence of………. and……….
gravity ; air resistance
The height of a projectile depends on……….
Initial Vertical Velocity
The time of flight of a projectile depends on……….
Initial Vertical Velocity
Horizontal motion of a projectile depends on……….
horizontal velocity and time of flight
The Trajectory of a projectile is influenced by……..
- projectile speed
- Height of release (i.e. projectile height)
- projectile angle
Apex
Point of Maximum Height of a projectile.
Vertical Velocity at Apex = 0
SUMMARY of the Kinematics of Projectiles
Projectile motion
A projectile ignores air resistance, therefore only gravity affects it (i.e. vertical acceleration -9.81m/s2), the object follows a symmetrical (by its Apex) parabolic path. The projectile motion describes the path of the centre of mass (CoM) of the object.
- Horizontal velocity = the same
- Horizontal acceleration = 0
- Vertical velocity before the Apex = positive and constantly changing
- Vertical velocity after the Apex = negative and constantly changing
- Vertical velocity at the Apex = 0
*Horizontal and vertical motion are independent of each other
What does EAUM stand for?
In the three equations… what will always be the same?
Equation of Uniformly Accelerated Motion Only apply it to projectiles In sport such as throwing events, jumping, gymnastics, etc... In the Equations: vertical acceleration (g) = -9.81 m/s2 horizontal acceleration = zero Vertical acceleration = constantly changing vertical velocity at the apex = 0 horizontal velocity remains constant
Range (R) of a projectile = ………(2 ways)
R = horizontal velocity x time of flight
This is from EAUM 2nd equation:
displacement of body = initial velocity of body x time involved
or
R = initial velocity x cos Alpha (angle of projectile) x time of flight
If an object takes off and lands from the same height and time of flight….. what will be the equation?
Remember that the projectile will be symmetrical by the apex so therefore Tup = Tdown
T = Tup + Tdown
Tup = V x sin Omega (angle of projection) / gravity
Tup = Tdown ==> that makes 2x
Therefore T = 2x (V x sin Omega / gravity)
The effect of landing height from a projectile:
If the height of release is greater than the height of landing (h = +ve), the optimal angle is………..
If the height of release is lower than the height of landing (h = -ve), the optimal angle is……….
<45 degrees ; >45 degrees
Static friction
definition
Also known as Surface or Contact friction:
When 2 surfaces are not moving relative to each other.
The force which opposes the onset of movement.
Dynamic friction
definition
When dry friction acts between 2 surfaces that are moving relative to each other.
Sliding friction
definition
The force which opposes the sliding of one body over another.
Rolling friction
definition
The force which opposes the rolling of one body over another.
laws of friction
Law 1: The force of friction is directly proportional to load (i.e. force perpendicular to the surface)
Law 2: The force of friction is independent of the apparent area of contact
Mechanical work
The product of force and the amount of displacement along the line of action of that force.
There are two types of work: positive and negative work
Work = Force x Displacement
Units (N/m or Joules)
Energy
Energy is the CAPACITY to do work.
There are two forms of mechanical energy:
1. Kinetic: energy due to MOTION.
2. Potential: energy due to POSITION or DEFORMATION. It has 2 forms:
a) Gravitational Potential Energy: Energy due to the object’s POSITION relative to earth
b) Strain Potential Energy: energy due to the DEFORMATION of an object.
Kinetic energy
equation
Kinetic energy = 1/2 x Mass x Velocity²
Gravitational Potential energy
equation
Gravitational Potential energy = Mass x Gravity x Height
Units: Joules or N/m
Power
equation
Power = work / time
OR
(force x displacement) / time
Units: Watts
Power
definition
The rate of doing work
Torque
definition
Also known as Moment, is the rotational effect of an eccentric force
Torque
equation
Force x Moment Arm (perpendicular distance)
Units: Nm
Angular inertia
definition
Resistance to Angular Motion
Angular Momentum
equation
H = I• Ѡ
Mass is resistance to……….
linear acceleration
Moment of inertia is resistance to……….
angular acceleration
what are the 2 factors that affect moment of Inertia
mass
mass distribution around axis of rotation
Moment of Inertia
equation
(single mass) I = mr²
(many masses) I = Σmr²
Radius of Gyration
definition, practical example and symbol
Radius of Gyration (k): length from axis of rotation to a point where mass of the segment would be concentrated to produce an equivalent moment of inertia
Practical example: Sprinting – flexed knee redistributes mass in order to reduce distance of concentration of mass from axis of rotation which therefore reduces radius of gyration. Meaning the leg can be rotated forward faster and individual can run faster.
Spinning on chair – tuck in all body parts you will spin faster than if you are spread out. Due to smaller moment of inertia.
Body segment
equation
I = mk²
Angular momentum
symbol, equation and practical example
H = Moment of Inertia (I) x Angular Velocity (Ѡ)
Practical example: A diver performs a piked front somersault by transferring the angular momentum from the upper body to the lower body and vice-versa. Done by altering their body position which therefore alters their moment of inertia.
First class lever definition
The applied force and the resistance force are on opposite sides of the fulcrum
Second class lever definition
the resistance force is between the applied force and the fulcrum
Third class lever definition + mechanical advantage + advantages + limitations
the applied force is between the fulcrum and the resistance force mechanical advantage <1 Advantage: maximise speed distance lifted is larger Limitations: Amount of weight lifted is limited requires effort > resistance
Wheel and Axle-like Arragement definition + examples in the human body
a wheel attached to an axle, both of which rotate about a common axis
Can either magnify force or increase the speed of motion – as for levers most maximise speed
E.g. Obliques or medial rotation at shoulder
The kinetic Link Principle definition
Also known as the Summation of Speed. There is a sequence of acceleration and deceleration of links (segments) from proximal to distal, which results in a tremendous speed at the distal end.
Pushlike patterns; describe when it would be used, its characteristics and the path it follows
Used when a large force must be applied
or when maximum accuracy of projection is required
Is characterised by the body parts moving the object forward from behind the object
Takes a rectilinear path, (more in a straight line)
Define the Speed-Accuracy Trade Off + examples
In the performance of many skills, the outcome is determined by both speed and accuracy.
As the demands for accuracy increases, the speed of the movement decreases.
Examples: penalty kick in football, throwing a ball, long jump approach to take-off board
what type of movement would be used in throwing to maximise velocity?
Throwlike movement
what type of movement would be used in throwing to maximise accuracy
A pushlike motion
