Biomechanics Exam #3
Center of Mass
Motion of the COM represents the "average" motion of the body as a whole
Kinematics
The description of motion as a function of space and time
Kinematics
The forces causing the motion are not considered
linear kinetics
The kinetics of particles, objects, or systems undergoing linear motion
radius of gyration
could lump all of an object's mass at distance k from the axis of rotation and I would be the same
moment of inertia
depends on the body position and axis of rotation
free body diagraph
diagram of the external forces and external torques acting on a system; the first step in solving a problem in kinetics
moment of inertia
difficult to get rotating, and once its has started rotating, difficult to stop
dynamic stability
difficult to quantify stability during dynamic tasks
friction
direction is opposite of motion or motion tendency
radius of gyration
distance from the axis of rotation to a point where the body's mass could be concentrated without altering its rotational characteristics
potential energy
energy by virtue of a body's position or configuration
kinetic energy
energy of motion
static balance
for equilibrium, the center of pressure must be directly below the body COM
impact
force-related deformation has two effect: energy absorbed and lost, and energy stored and released to change direction
resultant torque
found by adding the torques about an axis, taking the sing (direction) of each torque into account; same effect on rotation as the individual torques acting together
first class lever
fulcrum is in between the applied force and resistance. example = see-saw
second class lever
fulcrum or axis is at the end of the lever, resistance is in the middle, applied force at the opposite end. example = wheelbarrow
center of mass
geometric point about which every particle of a body's mass is equally distributed; the average location of the mass of a body
decrease
horizontally positioning the center of gravity away from the edge of the base of support on the side of the external force will _________ body's stability
increase
horizontally positioning the center of gravity near the edge of the base of support on the side of the external force will _________ body's stability
impulse
increases with an Increased force magnitude or Increased duration of application
center of gravity
it serves as an index of total body motion
dynamic stability
many dynamic movements, including walking and running, are unstable
coefficient of restitution
measures the elasticity of a head-on impact; determined by the properties of the materials making up both objects; important consideration where bouncing occurs
force
mechanical interaction between an object and its surroundings; the push or pull of one object on another; vector quantity
stability
more stable = able to resist a larger force
perfectly elastic
no energy lost, magnitude of each object's velocity is the same after impact as before, but direction changes
coefficient of friction
no units: indicates the relative ease of sliding or the amount of mechanical or molecular interaction between the 2 surfaces in contact
perfectly plastic
objects deform and stick together
angular momentum
quantity of angular motion possessed by a body; measured as the product of moment of inertia and angular velocity
momentum
quantity of motion possessed by a body; measured as the product of a body's mass and its velocity
center of pressure
reaction force between the body and support surface distributed over the entire contact area
radius of gyration
represents the objects mass distribution with respect to a given axis of rotation
net force
resultant force derived from the vector addition of two or more forces; reflects the effect of all the forces acting together
moment arm
shortest distane from a force's line of action to the axis of rotation
balance
the ability to control equilibrium
center of gravity
the body responds to external forces as though all mass were concentrated at the _______________-
center of pressure
the force can be summed into a single net force acting at a single point
weight
the force due to gravity; always acts at the center of mass, points towards the center of the earth
line of action
the imaginary line that extends from the force vector in both directions of a force
center of pressure
the point about which the ground reaction force is balanced
mechanical work
the product of a force applied against a resistance and the displacement of the resistance in the direction of the force
mechanical power
the rate of work production; calculated as work divided by the time over which the work was done
static balance
thus, to be in equilibrium, the body COM must be within the boundaries of the base of support
negative
torque that acts clockwise about axis of rotation
positive
torque that acts counterclockwise about axis of rotation
decrease
vertically positioning the center of gravity as high as possible will ________ body's stability
increase
vertically positioning the center of gravity as low as possible will ________ body's stability
dynamic stability
walking and running essentially consist of a series of interrupted falls
center of gravity
weight vector is shown to act in a free body diagram on ___________________
Kinematics and Kinetics
what are the Subdivisions of Mechanics
kinetic energy, potential energy, and thermal energy
what are the three forms of energy
static and kinetic friction
what are the two types of friction
perfectly elastic and perfectly plastic
what are the types of impact
thermal energy
what is heat energy
kinetic energy and potential energy
what makes up mechanical energy
principle of conservation of angular momentum
when angular momentum is conserved, there is a trade off between moment of inertia and angular velocity
static friction
when objects not sliding relative to each other
kinetic friction
when objects sliding relative to each other
Newton's 2nd Law (Law of Acceleration)
Σ F = m a
Newton's 1st Law (Law of inertia)
A body will maintain a state of rest or constant velocity unless acted upon by an external force
friction
A force acting over the area of contact between two surfaces
Newton's First Law - angular law of inertia
A rotating body will maintain a state of rest or constant rotational motion unless acted on by and external torque that changes the state; this is the basis for the principle of conservation of momentum (total angular momentum of a system remains constant in the absence of external forces)
Linear Motion (Translation)
All parts of an object or system move the same distance in the same direction at the same time
torque
a force that causes rotation
impulse
a measure related to the net effect of applying a force (F) for a time (t)
base of support
a person can mover the center pressure to any point within the ______________ but cannot move it outside of it
lever
a simple machine that consists of a bar that pivots at a fixed point called a fulcrum
static equilibrium
a system is at rest and will remain at rest; no translation or rotation is occurring or will occur; Newton's 1st Law; net external force in x or y direction equal zero; net torque produced by all external forces and all external torques equals zero
stability
ability of a body to return to its original state if disturbed
stability
ability to resist a disruption of equilibrium
inertia
Concept relating to the difficulty with which an object's motion is altered
Newton's 3rd Law (Law of Reaction)
For every action, there is an equal and opposite reaction
Newton's 3rd Law (Law of Reaction)
If body 1 applies a force to body 2, then body 1 experiences a reaction force from body 2: Of the same magnitude At the same point In the opposite direction
principle of conservation of momentum
If the net external force acting on a system = zero, total momentum of the system remains constant
Newton's 2nd Law (Law of Acceleration)
If there is a net external force acting on a body, the acceleration of the body's center of mass is: Directly proportional to the net force Inversely proportional to the body's mass In the direction of the net force
Newton's 1st Law (Law of inertia)
If there is no net external force acting on a body: If the body's center of mass is not moving, it will remain motionless If the body's center of mass is in motion, it will continue to move at a constant velocity (i.e. at the same speed in the same direction)
principle of conservation of momentum
In the absence of external forces, the total momentum of a given system remains constant
Center of Mass
Point representing the "average" location of the mass of a body
Center of Mass
Position of the COM depends on body configuration
linear kinetics
Study of the action of forces in a straight line; Studies translation and its causes
kinetics
Study of the relationship between the forces acting on a system and the motion of the system
inertia
Tendency to resist change in state of motion; Proportional to mass; No units
mass
The quantity of matter composing an object; The measure of inertia for linear motion; The property giving rise to gravitational attraction; Units: kilogram (kg)
Kinetics
The relationship between the forces acting on a system and the motion of the system
law of conservation of mechanical energy
When gravity is the only acting external force, a body's mechanical energy remains constant
impact
a collision characterized by: the exchange of a large force during a small time interval
decreases
a decrease in body mass will ______________ body's stability
moment arm
always perpendicular to the line of action and passes through the axis of rotation
decrease
an decrease in friction between the body and the surfaces of contact will ______________ body's stability
decrease
an decrease in the base of support will _____________ the body's stability
increases
an increase in body mass ____________ body's stability
increase
an increase in friction between the body and the surfaces of contact will ______________ body's stability
increase
an increase in the base of support will _____________ the body's stability
equilibrium
an object is at rest and will remain at rest
free body
an object, body or body part of a mechanical system that is being focused upon for analysis
external force
applied to a system from outside the system
internal force
applied to a system from within the system; not included in mechanical analyses
base of support
area enclosed by all points at which the body contacts a supporting surface
third class lever
axis as one end, applied force in the middle and resistance at the opposite end. example = using a hammer to drive a nail into a piece of wood