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30 Cards in this Set

  • Front
  • Back

forces that are directed through an object's center mass; results in a linear change

centric forces

forces that are not directed through an object's center mass; result in change in linear position and rotation in object

eccentric forces

turning effect caused by eccentric force; force is the key component


what is torque

the rotary effect of a force about an axis of rotation; measured as the product of force and the force's moment arm-the shortest perpendicular distance between a force's line of action and an axis of rotation

a force perpendicular distance from the force's line of action to the axis of roation

moment arm


axis|---------------------------|force line of action

moment arm

torque =

force x moment arm

t = f x r

units: n x m = Nm

what is a lever

a simple machine consisting of a relatively rigid barlike body that can be made to rotate about an axis or a fulcrum

1st class lever


2nd class lever


3rd class lever


equilibrium (lever system)

relative locations of the applied force (F) the resistance (R) and the fulcrum or axis of rotation determine lever classifications

what is the mechanical advantage

the ratio of the moment arm of the force (FORCE ARM) to the moment arm of the resistance (resistance arm) for a given lever

little johnny goes to the park and gets on the seesaw. if the see saw length of 8 meters rotates on an axis located at the exact midpoint. little johnny sits 3.meters to the left of the axis, and little johnny's mass is 25 kg. how much toque does little johnny produce?

T=F x r

little johnny's best friend big berth comes to park and gets on the other side of the see-saw. if big bertha weighs 700 N and sits an equal distance from axis of rotation as little johnny, what is the net torque experienced by the seesaw?

T = T + T

a force can balance a larger resistance when the force arm is longer than the resistance arm


a force can move a resistance through a large range of motion when the force arm (fa) is shorter than the resistance arm (ra)


where do the torques occur within the human body

the product of muscle tension and muscle moment arm produces a toque at the joint crossed by the muscle


because muscles lie on top of your bones they are never directly in line with the joint axis

the contractile forces the muscles produce are eccentric forces (not eccentric muscle action)

therefore muscular contractile forces produce torques that rotate our limbs about a joint axis

go to the weight room and pick up a 90 N dumbbell in your right hand (.25 m away from your elbow joint). your forearm has a weight of 60 N and its center of gravity is 15 cm from our elbow joint. if your biceps is attached 3 cm from your elbow joint, how much force would your bicep need to produce such that the net torque on your elbow is 0 Nm

T = T + T + T

0Nm= (-90N x 0.25m)+(-60N x 0.15m)+(F x 0.03m)

0Nm=-22.5Nm+-9Nm+(F x0.03m)

31.5Nm=F x 0.03


special situations

systems where the:

1. the net torque (T )of the sysem is 0Nm

2. the net force of the system (F ) is 0N, and

3. there is no motion are in a state of static equilibrium

means that there are no forces or torques, only that any forces or torques present balance each other

axis of rotation

point around which something rotates

center of mass

that point, which for certain purposes, the entire mass of the body may be assumed to be concentrated

center of gravity

that point around which mass is evenly distributed and through which the force of gravity acts

is the point where the torques created by the elemental weights sum to a total of zero

the only point that creates a static equilibrium

why is the center of gravity of interest in the study of human biomechanics

it serves as an index of total body motion

the body responds to external forces as though all mass were concentrated at the center of gravity; this is consequently the point at which the weight vector is shown to act in a free body diagram

center of gravity in humans

anitomical position, center of gravity lies at the lumbar 2/3 vertebrae

55-57% of your height

moves based on the position of the head, trunk and limbs

defines the stability and mobility of an object

point that follows Newton's Laws of Motion

what is stability

resistance to disruption of equilibrium

ability of the body to return to its original position(equilibrium)after being displaced

affected by height of the center of gravity, base of support, and weight of the body

what is balance

the ability to control equilibrium

involves stability

important part of agility

ability to change direction at high speeds

what is the base support

area bound by the out most regions of contact between the body and support surfaces

what can increase a body's stability

increase body mass

increase friction between the body and the surfaces of contact

increasing the size of the base of support in the direction of an external force

horizontally positioning the center of gravity near the edge of the base of support on the side of the external force

vertically positioning the center of gravity as low as possible

decreasing the center of gravity of a body increases its stability

reduces the moment arm

increasing the base of support of a body increases it's stability

provides a greater area for the force of gravity to act through