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