Lecture 5 - Biomechanics

Question 1

How do motor systems create motion? What laws apply?

Answer 1

exert force; Newton’s and Euler’s laws of motion

Question 2

Motor control depends on…

Answer 2

viscous and elastic properties of the body

Question 3

What is mechanics?

Answer 3

the study of force and motion

Question 4

Relate force with motion (what type of motion is it?)

Answer 4

F = ma (Newton’s 2nd Law) (translational motion)

Question 5

Unit for force

Answer 5

Newtons

Question 6

define 1 newton

Answer 6

1 newton = 1 kg m/s²

A force of 1 newton imparts to an object of mass 1 kg an acceleration of 1 m/s²

Question 7

Newton’s 3rd Law describes…

Answer 7

How force propagate through linkages

Question 8

Newton’s 3rd Law

Answer 8

Objects will exert equal and opposite forces on each other

Question 9

Newton’s 3rd Law in terms of linkages

Answer 9

If bone A exerts a force F on bone B in the linkage, then bone B exerts a force -F on bone A

Question 10

Law for rotational motion is discovered by…

Answer 10

Euler

Question 11

Euler’s law is expressed in terms of…

Answer 11

Torque (~force) & inertia (~mass)

Question 12

Define: inertia

Answer 12

resistance to rotary acceleration

Question 13

The same object has different inertias depending on…

Answer 13

the point used on the object

Question 14

In real life, inertia is a ___ and varies with ______

Answer 14

matrix;

the orientation of the rigid body

Question 15

Euler’s Law

Answer 15

T = Ia

(torque = inertia x rotary acceleration)

Question 16

What is rotary acceleration (α)?

Answer 16

rate of change of ω

Question 17

How to find the direction of α vector?

Answer 17

Right hand rule: point thumb in direction of α and the direction of the vector is where the fingers curl

Question 18

Formula for torque

Answer 18

T = M x F (moment arm cross product force)

Question 19

Torque depends on…

Answer 19

forces’ application points and directions

Question 20

How to find net torque?

Answer 20

For each F_i acting around the joint, find it’s moment arm M_i and compute T_i = M_i X F_i

Sum all the torque vectors (ΣT_i)

Question 21

Direct mechanics

Answer 21

computing motion from force

i.e., after finding net T, find rotary acceleration using α = T/I

Question 22

Plant is subject to ____ and ___ forces

Answer 22

intrinsic, driving

Question 23

driving forces

Answer 23

muscle forces ordered by the brain;

force from controller

Question 24

intrinsic forces

Answer 24

viscous and elastic forces of the tendons and other tissues

Question 25

Why do we need to know about intrinsic forces?

Answer 25

Newton’s Law tells us about total force influences (intrinsic + driving), but we are interested in only observing driving force

Question 26

examples of viscous forces

Answer 26

friction, air resistance

Question 27

viscous forces increase with…

Answer 27

velocity

Question 28

what is terminal velocity for skydiver in spread position? In straight position?

Answer 28

spread = 200 km/h

straight = 300 km/h

Question 29

A constant driving force acting against a viscous force yields…

Answer 29

a transient (lasting a short time) acceleration followed by a constant velocity

Question 30

elastic forces depend on ____ rather than its ____

Answer 30

position of an object rather than its velocity

Question 31

define: stiffness

Answer 31

ratio of change in force to change in length

Question 32

example of elastic force

Answer 32

springs

Question 33

A constant driving force acting on an elastic system brings it to…

Answer 33

an equilibrium position where the elastic force exactly balances the driving force

Question 34

For an elastic system, the driving force determines…

Answer 34

(steady-state) position

Question 35

Define: resonance

Answer 35

things have natural frequencies, and stimulating at that natural frequency produces a very strong response