Lecture 9 - Forces and Movement

Question 1

how are action and reaction forces related?

Answer 1

• equal in magnitude
• opposite in direction

Question 2

what is an internal force?

Answer 2

• action forces
• originate from muscle contractions applying force on the skeleton

Question 3

what is an external force?

Answer 3

• reaction forces
• originate in the external environment and act on the body
• created by our action forces
• required for acceleration (you need something to push against)

Question 4

What are resistive forces?

Answer 4

• forces, internal or external, that act to slow or stop a motion
• play a stabilizing role in biomechanics
• ex: tension, resistance, contact and viscosity

Question 5

what are motive forces?

Answer 5

• forces, internal or external, that act to accelerate a segment

Question 6

what is triphasic motor control?

Answer 6

• using resistive and motive forces in a sequence to control motion
• has 3 phases

Question 7

what are the 3 phases of triphasic motor control?

Answer 7

1. agonist muscles
2. antagonist muscles
3. co-contraction (fine motor control)

Question 8

what is pressure?

Answer 8

• a force divided by the perpendicular area over which the force acts
• the pressure needs to change because force of gravity doesn’t change (imagine walking with boots vs snowshoes)

Question 9

what is frictional force?

Answer 9

• a force that resists the motion of one object in contact with another
• coefficient of friction x normal force = friction

Question 10

what is centripetal force?

Answer 10

• an action force that always points towards the center of rotation
• equal and opposite to centrifugal force (away from center) which is a reaction force

Question 11

what is work?

Answer 11

• force x distance moved
• no movement = no work
• if movement is opposite to force, work is negative

Question 12

what is power?

Answer 12

• work/time
• (force x distance)/time
• force x velocity
• no velocity = no power
• if movement is opposite to force, power is negative

Question 13

what is kinetic energy?

Answer 13

• the amount of energy an object has due to velocity
• is work is positive, kinetic energy (and velocity) increase
• KE = 1/2 mass x velocity^2

Question 14

what is potential energy?

Answer 14

• stored energy that can change into kinetic energy easily
• if no friction, mechanical energy is conserved
• PE(gravity) = mass x g x height
• PE(elastic) = 1/2 spring constant x elongation^2