Chapter 5

You increase stability by...

-Lowering COG (bending our knees) -Widening base of support (move our feet apart) -Placing COG in the middle of the base of support -Increasing weight or mass -Increasing the moment arm of the object's weight -Decreasing potential energy

Relationship between moment arm and force

-large force and small moment arm -small force and large moment arm

Most tendons insert very close to the _____, and thus most muscles have very _____ moment arms, but the length of the moment arm can change with limb position.

1. joint 2.small

Women's center of gravity is around ___ of their height, or around the S-2 vertebra.

55%

Men's center of gravity is around ___ of their height from the ground.

57%

What creates the turning effects that cause our limbs to rotate about our joints?

A muscle creates a force that pulls on its points of attachment to the skeletal system when it contracts.

At what position is it easiest to do knee extensions on a machine?

As the knee reaches full extension. It is harder when the knee at flexion.

Think about if you were lifting a heavy bag. What elbow position would it feel easiest in?

At 90 degrees flexion because that is where moment arm is maximized.

When is it the easiest to do a bicep(s) curl with heavy dumbbells?

At Midrange. At midrange, r is at its maximum

When is it the hardest to do a bicep(s) curl with heavy dumbbells?

At the beginning and end. At those points, r is at its minimum.

What force causes a change in linear motion?

Centric force

What force causes a change in linear and angular motions?

Eccentric force

True or False: Torque is not a vector quantity.

False. Torque is a vector quantity. Units are Nm

What force causes only changes in angular motion?

Force couple

The equation for torque is

T=Fxr or Torque equals Force (F) times r (moment arm)

Say you have an object placed on each end of a teeter totter. the moment arms are the same for each object, that is their distance from the axis of rotation is the same. However, the mass of one object is much larger than another. What is this increased mass going to do to the force created and thus the torque?

The net torque would be in the direction of the larger mass object, and the teeter totter would tilt down in that direction.

Center of Gravity (COG)

The point in a body or system around which its mass or weight is evenly distributed or balanced, point through which force of gravity acts.

True or false: Center of gravity (COG)= Center of mass

True

True or False: In infants and small children the center of gravity is much higher because their heads are larger and their legs are shorter.

True.

True or False: Torques that act around the same axis can be added and subtracted, just as forces acting in the same direction can.

True.

True or False: Strength training exercises with free weights or weightlifting machines provide many examples of torques that are exerted on our limbs by nonmuscular forces.

True. For dumbbells or weights, r decreases because of the changes in length as we move our joint, so our force must increase to maintain the torque and continue the movement.

True or false: for humans, the center of gravity is typically located around the pelvis.

True. For this class.

Stability

capacity of an object to return to equilibrium or to its original position after it has been displaced.

Centric force

external force directed through the center of gravity of an object

Eccentric force

external force not directed through the center of gravity of an object

Stability is important in sports for we can manipulate our center of gravity to

increase or decrease stability.

Base of support

is the area within the lines connecting the outer perimeter of each of the points of support.

Direction of rotation: Clockwise rotation

negative

The center of gravity can move based...

on how the limbs are arranged around the body.

Force couple

pair of forces equal in size but opposite in direction and not collinear

Center of gravity

point in body or system where mass or weight is evenly distributed

Direction of rotation: Couterclockwise

positive

Torque is directly proportional to

size of force and distance between line of action of force and point about which object tends to rotate (axis of rotation)

If you had to hold a bag with only 10 to 15 degree flexion, would your moment arm be smaller or greater compare to 90 degree flexion?

smaller

Moment of force

torque produced by a force

Torque

turning effect produced by a force

If an object is in static equilibrium, the external forces must sum to ____ and all the external torques, around any axis, must also sum to ____.

zero