Lecture 9: Forces and Movement

Question 1

Turning left

Answer 1

To turn left, you apply force/push on right side

Question 2

External forces

Answer 2

Reaction forces

They originate in our external environment and act on the body

Question 3

Internal forces

Answer 3

Action forces

Arise from our muscular contractions applying force to our skeleton

Question 4

Internal vs external forces

Answer 4

Related by Newton’s 3rd Law

When you don’t have something to push against, you can’t accelerate your centre of mass

With a surface to push against, internal forces create an external rxn. force at point of contact

Question 5

Resistive forces

Answer 5

Forces that act to SLOW or STOP a motion
- play a STABILIZING role in biomechanics

Examples
- tension in an eccentrically contracting muscle to slow a segment
- resistance in connective tissue during stretching
- contact btwn soft tissue in 2 adjacent segments
- viscosity in muscle or jt capsule

Question 6

Motive forces

Answer 6

Forces that act to ACCELERATE a segment

Question 7

Triphasic motor control

Answer 7

Cooperation of resistive and motive forces

Ex. reaching for a glass of water

1. Fire agonist muscles to get arm moving toward glass (motive)
2. Fire antagonists to slow hand down just before reaching glass (resistive)
3. Fire agonist WITH antagonist to stabilize jts and stop segment motion (co-contration)

Question 8

Pressure

Answer 8

Force divide by perpendicular area over which the force acts

Type of external force

Question 9

Frictional force

Answer 9

Force that resists the motion of one object in contact w/ another

Friction = force divided by the perpendicular area over which it acts

Friction = coefficient of friction x force

Type of external force

Question 10

Coefficient of friction

Answer 10

Different for different materials of floor

Represents % of bodyweight available to stop your foot

Gum would increase coefficient of friction

Question 11

Friction in joints

Answer 11

Coefficient of friction in jts is about 5%
- not a lot of friction which is good

osteoarthritis = higher coefficient of friction + loss of synovial fluid

Question 12

Centripetal force

Answer 12

Force that is needed to maintain path (ex. it pulls a gymnast towards the high bar) – reaction force

When an object is following a ROTARY path

Always points towards centre of rotation

Question 13

Centrifugal force

Answer 13

Points AWAY from centre of rotation

Equal and opposite force to centripetal force

Ex. what the bar feels – ACTION force

Question 14

Work

Answer 14

Force x distance moved

No movement = no work
Movement in opposite direction = negative work

Question 15

Power

Answer 15

Work divided by time OR Force x velocity

No velocity = no power
Movement in opposite direction = negative power

Question 16

Kinetic energy

Answer 16

Amount of energy that an object has due to its velocity

KE = 1/2 mass x (velocity)^2

Question 17

Work and kinetic energy

Answer 17

KE1 + Work = KE2

If work is positive, kinetic energy and velocity increase

Negative work = slowing down and storing PE

Question 18

Potential energy

Answer 18

‘STORED’ form of energy that can change into kinetic energy very easily

PE(gravity) = mass x g x height

PE (elastic) = 1/2 spring constant x (elongation)^2