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Science
Physics
Biomechanics
Biomechanics Exam 3
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Terms in this set (47)
form
Classifying motion enables precise description and mechanical analysis
Description/appearance of a motion
2 approaches for studying the biological/mechanical aspects of human movement
- Qualitative Approach
- Quantitative Approach
Forms of Motion = Linear Motion (Translation)
- all points on the body show the same trajectory
- rectilinear: straight path
- curvilinear: curved path
Forms of Motion = Angular Motion (Rotation)
- rotation about an axis
- orientation of different points on body change relative to each other
- axis of rotation:
• within the body
• outside
Forms of Motion = General Motion
• combination of linear motion + angular motion
• Most human motion is general motion
Position
-location of object of interest in space at a given instant
- unit: m (meter)
- (x,y)
Distance
•Distance:
-length of the path traveled between positions
- scalar quantity: magnitude only
-unit: m
Speed
-Rate of motion (change in position)
-how fast an object moves
- speed = distance / elapsed time
- a scalar quantity
- unit: m/s
Acceleration
-rate of change in speed or velocity
- unit: m/s2 (= m/s/s)
•Causes of acceleration (rate of change in velocity):
-change in magnitude of velocity
-change in direction of velocity
Projectile Motion (understand the concepts that explain motion of an object under uniform acceleration
(Uniform acceleration)
-object projected into the air (only forces acting are gravity and air resistance)
-examples: balls, jumper's body, arrow....
-follows a parabolic path
What three launch variables explain projectile motion and how do they relate to maximizing horizontal or vertical distance?
•Gravity:
-pulls downward & causes parabolic trajectory
-gravitational acceleration: -9.81 m/s2 (vertical only)
* Air resistance:
- affects trajectory
- normally ignored for simplicity
- So, horizontal acceleration = 0
***Same projection angle, different speeds
***Same projection speeds, different relative heights and how optimal release angle changes
Displacement
•Displacement:
-Linear change in position (net effect of motion)
- a vector quantity: magnitude + direction
Velocity
- rate of displacement
- how fast an object moves in which direction?
- a vector quantity
-velocity = displacement / elapsed time
- unit: m/s
Speed/Velocity example #1
Q. distance = 0.9 km, Dt = 0.5 hr Þ speed?
A.s = 0.9 / 0.5 = 1.8 km/h
A runner completes 6.5 laps around a 400 m track during
a 12 minute (720 s) run test. Calculate the following quantities: part 1
a. The distance the runner covered
b. The runner's displacement at the end of 12 minutes
c. The runner's average speed
d. The runner's average velocity
A runner completes 6.5 laps around a 400 m track during
a 12 minute (720 s) run test. Calculate the following quantities: part 2
The distance (l) the runner covered
l = 6.5 X 400 m = 2600 m or 2.6 km
The runner's displacement at the end of 12 minutes
d = 160 m (see picture)
The runner's average speed
s = l/t
s = 2600 m/720 s
s = 3.6 m/s
The runner's average velocity
v = d/t
v = 160 m/720 s
v = 0.22 m/s
Kinetic definition
•study of the actions of forces or explanation of motion
Kinetic properties
•Mass:
- quantity of matter composing a body
- no mass = no mechanical identity
- symbol: m
- standard unit: kg (kilogram)
•Force:
- a push or pull acting on a body
- cause of motion
- the main mechanical quantity of interest in linear kinetics
- symbol: F
- standard unit:N (Newton)
N = Force that accelerates mass of 1Kg, 1 m/s2
Examples of forces in human motion:
- muscle, weight, friction, GRF, drag, lift....forces
•Properties of a Force:
magnitude, direction & point of application
Newton's 3rd Law of Motion and How it is applied to human motion
•Law of Inertia:
-a body remains at current state of motion unless acted upon by a net external force
inertia
-the tendency of an object to keep the current state of motion
--difficulty in changing the state of motion
-proportional to mass of the object
acceleration ( AKA Newton's second LAW linear kinetics)
•Law of Acceleration:
-a force applied to a body causes acceleration
-acceleration is proportional to force, inversely proportional to mass
action reaction
•Law of Reaction:
-for every force (action), there is an equal and opposite reaction
F1 = F2
-same magnitude
-opposite direction
i. Explain and apply the Momentum and Impulse relationship and related equations how it may to understanding and quantifying human motion.
What is impact? a collision characterized by: the exchange of a large force during a small time interval.
What happens following an impact? This depends on the nature of the impact and the momentum present in the system
E.g. Humans in contact with other humans, humans in contact with other objects
(ground, implements), other objects in contact with other objects (ball with floor or
racket/bat/club)
What is the nature of impact?
It is described by the coefficient of restitution, a number that serves as an index of elasticity for colliding bodies; represented as e
1. Explain when and how it may be important to maximize, minimize or optimize friction forces in human motion
On this and the following slides, we will discuss the biomechanical role of friction in different movement situations and whether or not friction should be minimized, maximized or optimized in the situation depicted in the pictures. On this slide, we focus on manipulating friction by changing the coefficient of friction.
kinematics
•study of the appearance or description of motion
Angular Measurement - Explain the difference between absolute and relative angular measurement and when each may apply to human motion.
What is a relative angle?
angle at a joint formed between the longitudinal axes of articulating body segments (i.e. joint angle)
What is an absolute angel?
angular orientation of a body segment with respect to a fixed line of reference
-reference lines are usually vertical or horizontal
Units - know standard units for each mechanical quantity
radians compare angular and relative
Radians: 1 rad = 57.3°
The size of the angle at the center of a circle when arc is equal in length to the radius of the circle.
Angular Distance:
Length of angular path
i. Angular Velocity
-rate of change in angular position
-angular velocity = angular displacement / time
-unit: rad/s or deg/s, or RPM's
i. Angular Acceleration
-rate of change in angular velocity
-unit: rad/s2 (rad/s/s) or deg/s2
Angular Motion & Linear Motion Relationship - List, explain and be able to apply the primary equation and situations where this relationship is important to human motion.
Since velocity is displacement over time, linear and angular velocity are related by the same factor that
relates displacement: the radius of rotation (r).
v = rw,This is the mechanical relationship (equation) between linear and angular velocity.
Angular displacement
-net effect of angular motion
-vector quantity
--change in angular position
- counterclockwise is POSITIVE
- clockwise is NEGATIVE
What is the relationship between linear
and angular acceleration?
The acceleration of a body in angular motion can be resolved into two
perpendicular linear
acceleration components
Mechanical Work
-Product of average force applied and amount of displacement in direction of that force
-Work = (force)(displacement)
-W = F d (displacement in direction of force)
- unit: J (Joule) = N·m
•Modulating work is important for performance....
Work is the means by which energy is transferred from one system to another....i.e. human to ball, ground to human in posture and gait.
Mechanical Power
-rate of work
- (work) / (elapsed time)
-(force)(velocity)
-unit: W (Watt) = J/s
•Example: stair climbing
30 steps (25 cm/step), weight = 580 N, Dt = 15 s
Average Power = ?
Displacement = (30)(25 cm) = (30)(0.25 m) = 7.5 m
Power = (580N)(7.5m) / (15s) = 4350Nm / 15s = 290 W
W = F d
Mechanical Energy
-capacity to do mechanical work
-unit: J (N m)
Kinetic energy
energy due to motion (velocity)
potential energy
energy due to position: gravitational (height) or elasticity (deformation or strain)
i. Explain Principle of Conservation of Mechanical Energy and how this principle may be applied to explain and quantify motion.
This slide shows the conservation of total ME = PE + KE = 29.4 J at all ball positions, if the only external force is gravity. The correlation between the kinetic and potential energies of the vertically tossed ball illustrates a concept that applies to all bodies when the only external force acting is gravity. The concept is know as the law of conservation of mechanical energy: When gravity is the only external force, mechanical energy stays constant.
a. Explain the Mechanical work and energy relationship
•Mechanical Work causes equal changes in the mechanical energy of the system
-W = DME = DKE + DPE
•Conservation of ME Principle:
-when gravity is the only external force acting on a system, ME remains constant
-MEinitial = MEfinal, ME = KE + PE = constant or
-No Work = no change in ME; 0 = W = DME = DKE + DPE
•if the force acts in the same direction as the body moves, the work done by the force is_______
positive work
•if the force acts in the opposite direction as the body moves, the work done by the force is ___
negative work
torque
...
internal torque
...
external torque
...
levers
...
mechanical
...
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