Fundamental Concepts of Force Flashcards
Kinetics
study of forces acting on body
displacement
motion of a body/segment that occurs when force is applied (i.e., change in angular position)
force
push or pull that causes displacement
equilibrium
equal and opposite forces
if forces are unbalanced, movement occurs in the direction of the ______________.
stronger force
when a force acts on an object, it can:
deform it
change its state of motion (move it, stop it, slow it down)
2 properties of force =
magnitude
direction
normal force/ground reaction force
If a person is standing on the floor, the floor/surface applies a normal force on the person so that the magnitudes of the two forces are equal. The reactionary force exerted by the ground is the ground reaction force.
- In reality, GRF is hardly ever comprised of just a normal (vertical/perpendicular) force
compression
a force, applied perpendicularly to the contact surface that pushes or pulls one object against the other
tension
application of one or more forces that pulls apart or separates a material (aka distraction force)
shear force
where an object exerts a force parallel to something else
friction force
when one surface slides over another. Depends on surface properties involved.
Magnitude is directly proportional to the normal force of the surface on the object
F(friction) = U x N
where U = coefficient of friction
coplanar force
all forces are acting on an object or body in the same 2D plane or surface
collinear force
if all forces have common line of action (i.e., tug of war)
concurrent forces
if multiple forces (system of forces) have lines of action that have a common point of intersection
external force
a force from outside the object/body, acting upon that object/body
internal force
a force from within the object/body; internal force may hold the object/body together when it experiences external forces
gravitational force
force exerted by earth on an object (i.e., weight); it is always directed straight down (vertically)
-magnitude = (mass of object)*(magnitude of gravitational acceleration)
mass
a property of a body
-The amount of matter contained within an object
center of mass
a single force that represents the distribution of a load passes through this point
primary dimensions
length, time, mass
secondary dimensions
derived from primary dimensions
- Area (length*length)
- velocity (length or position/time)
- acceleration (change in velocity/time)
Newton’s third law
To every action there is an equal and opposite reaction
i.e., if you push a body, the body will push you back
action-reaction
when one body applies a force to another body, that second body provides and equal force in the exact opposite direction with equal magnitude
i.e., GRF
forces of action and reaction between the interacting bodies are:
- equal in magnitude
- opposite in direction
- have the same line of action
Newton’s first law (Law of Inertia)
body remains at rest, or constant linear velocity, except when compelled by external force to change its state
- a force is required to start/stop/slow down/speed up/alter the direction of linear or rotational motion
inertia
reluctance of a body to change its current state (has to be overcome to move/change direction of the object)
static equilibrium
when linear and or rotational velocities are 0 (not moving)
dynamic equilibrium
when linear and/or rotational velocity of the object aren’t 0, but is constant (constant velocity)
whiplash injury
clinical application of law of inertia
- body in equilibrium (car is parked or moving at constant velocity)
- the force of the car changes the state of the person’s seat (and body) so it is impelled forward, while the unsupported head remains at rest/equilibrium because it did not experience this forward-directed force to the same extent. The head “whips” back until it meets an opposing force to slow down
- The opposing forces that slow/stop the head are the anterior neck structures and the hardest
Newton’s second law
body with a net force acting on it, will accelerate in the direction of that force
- magnitude of this acceleration will be proportional to the magnitude of the net force, and inversely proportional to the mass of the body
- you can always have acceleration related to linear force, and rotational force (torque or moment)
- F = m*a
- Rotational force = mass moment of inertia (I) * angular acceleration (a (alpha))
Acceleration
change in velocity/time
Impulse
large force over small time period
F*time = impulse
m(v) = momentum
moment (M)
M = I * angular velocity
mass moment of inertia (I)
the object/body’s resistance to a change in angular velocity. It is a value derived from the mass of the object, and r is the distance from the point of rotation to the arc of motion of the object
Relative reference frame
describes position of 1 limb segment with respect to adjacent segment (i.e., forearm relative to upper arm)
- Limitation: can’t define motion with respect to a fixed point in space
Global reference frame
used to analyze motion with respect to ground/gravity/other external reference frame. We use Cartesian coordinate system
Cartesian coordinate system
Convention = align the x-axis parallel to the body segment of interest
Torque (moment)
associated w/rotational or twisting action of applied forces. Has magnitude and direction
M = d*F
Direction of moment/torque: right hand rule
when you curl the fingers of your right hand in the direction that the force tends to rotate the body, your right thumb points in the direction of the moment
2 ways to find the moment arm
- Lever Arm technique: always make the MA parallel to the segment of interest. If the force doesn’t make a right angle, find its component force that does - M = d*F. F is the force that makes a perpendicular with the moment arm
- Moment arm technique: extend the applied force’s line of action, until you can draw a perpendicular moment arm. Solve for d. You will need to know/calculate the distance from axis of rotation to the point of application of the force, and its angle of action to solve
Moment arm
perpendicular distance between an axis of rotation and the line of force
Lever arm
distance between the point of application of a force and the axis of rotation about which that force acts
How does joint angle affect the moment/torque produced by the muscle?
The longer the moment arm (d), the less force is required
Resistance due to gravity changes through joint’s ROM is maximum when
gravity acts perpendicular to the segment
Biomechanical analysis
application of various sciences + geometry, math, calculus
Why? Important to realize the quantity of forces the body must produce and withstand during both daily activities and rehab
Free body diagram
simplified drawings of the body constructed to help identify forces and moments acting on parts of a system
- all known and unknown forces and moments are shown in the diagram
- the parts of a system are isolated from their surroundings and the effects of the surroundings are replaced with forces and moments (drawn as vectors)
Joint reaction force
includes joint contact force, and the net effect of all other forces transmitted from one segment to another
- caused in reaction to other forces (muscles, gravity, tendons/ligaments, etc.)
- acts through joint center (axis of rotation)
- equal and opposite to the sum of the internal and external forces
- usually directed opposite to the major muscle force
