Download Torque - Site Prof. Bertolo

Chapter 8
Torques and Moments of
Force:
Maintaining Equilibrium or
Changing Angular Motion
To this juncture

Analysis focused on linear motion
(translation)
– kinematics: position, change of position, rate of
change of position, acceleration
– kinetics: Newton’s 3 laws of linear motion
» F = ma
» Ft =  mv
What about rotation?

All points on a body or object move in
circles (or parts of circles) about the same
fixed central line or axis
– body spin around an axis (real or imaginary)
Force ==> linear motion
 ????? ==> angular motion

What about rotation?

All points on a body or object move in
circles (or parts of circles) about the same
fixed central line or axis
– body spin around an axis (real or imaginary)
Force ==> linear motion
 Torque ==> angular motion

– moment of force
– moment
Characteristics of a Force
Characteristics of a Force
Magnitude
Characteristics of a Force
Magnitude
Direction
Characteristics of a Force
Magnitude
Direction
Point
of Application
Line of Action
Line of action
relative to axis of rotation

Centric force
– line of action passes through the axis of rotation
– tends to cause translation
Line of action
relative to axis of rotation
Centric force
 Eccentric force

– line of action does not pass through the axis of
rotation
– tends to cause translation and rotation
Line of action
relative to axis of rotation
Centric force
 Eccentric force
 Force couple

– two eccentric forces
– tends to cause rotation
Torque
http://www.tri-c.cc.oh.us/metro/faculty/gram/web/torque.htm

Definition
– the turning effect of a force
– the tendency of a force to cause rotation
What factors affect
the tendency
of the force to
cause rotation???
Torque
http://www.tri-c.cc.oh.us/metro/faculty/gram/web/torque.htm

Definition
– the turning effect of a force
– the tendency of a force to cause rotation
What factors affect
the tendency
of the force to
cause rotation???
Why my pinky
can whip
your
football team.
Torque

Definition
– the turning effect of a force
– the tendency of a force to cause rotation
T=Fxr
T = torque
F = force (magnitude & direction)
r = moment arm (MA)
Moment Arm
(lever arm)
Perpendicular
distance
from
line of action
of a force to
a specified
axis of rotation
(fulcrum, pivot point)
Moment Arm
(lever arm)
Perpendicular
distance
from
line of action
of a force to
a specified
axis of rotation
Moment Arm
(lever arm)
Perpendicular
distance
from
line of action
of a force to
a specified
axis of rotation
Axis
Moment Arm
(lever arm)
Perpendicular
distance
from
line of action
of a force to
a specified
axis of rotation
Moment
Arm (r)
Characteristics of a Force
Note how changing
the line of action
of a force
may change
the sense
of the torque
Moment
Arm (r)
Effect of
changing
the line of
action of
force
To describe a torque
Specified axis of rotation
 Torque magnitude (F x r)

– units: Newtons x meters (Nm)
–
foot x pound (ft-lb)

Direction (sense) of the torque
– clockwise (-)
– counterclockwise (+)
Examples
of
Torque
Examples
of
Torque
Show the
moment arm and
identify the sense
for each force
on each figure
Examples
of
Torque
Calculate torque
created in
each figure
Sculling: offsetting torques to
create translation
Are all
three
designs
equal
in net
torque
created
at the
stern??
ADLs must consider torque
for safety and convenience
Medicine caps must
be removable by the
disadvantaged, but
inoperable by kids.
Click on the picture to
go to a website with more
information.
Muscle Names
Muscle Torque
 Muscle
applies force by ?????
Muscle Torque
 Muscle
applies force by pulling
Muscle Torque
 Muscle
applies force by pulling
 Muscle crosses a joint or joints
– brachialis and biceps brachii
– soleus and gastrocnemius
– other examples????
Muscle Torque
 Muscle
applies force by pulling
 Muscle crosses a joint or joints
 Muscle has a moment arm
– eg palpate the biceps brachii and flex elbow
» next three slides
Elbow at 90o of flexion
Elbow at 135o of flexion
Elbow at 45o of flexion
Question posed in a Physical
Therapist Assistant Class
Is it still a moment arm
if the muscle inserts
on the leg?
Reported by Kris Green, BS ‘99
Muscle Torque
 Muscle
applies force by pulling
 Muscle crosses a joint or joints
 Muscle has a moment arm
 Muscle creates torque at the joint
– muscle is a torque generator
»Bob Norman (1975)
Game Day
Draw the
Resistance
Torque
Strength training and torque
Biceps Curl

Muscle as torque generator
– moment arm changes through ROM
– muscle’s ability to create force changes through
ROM

External forces as torque generators
– segment weight
– handheld weight (dumbbell)
» moment arms change through ROM
Strength training and torque
Strength Training Machines
So our answer is:
Why could I control the
two big guys with
only my pinky?
Because, ….
Calculate Extensor Muscle
Force
Forces:
P (hand held load) = 90 N
W (HAT weight) = 80 N
Moment Arms
Lw = 25 cm
Lp = 60 cm
Lm = 5 cm
Moment arm at the knee joint
 Go
Click for
knee
website
to and read
the Web site to
see how the
moment arm at
the knee is
affected by knee
joint angle.
Ankle moment and standing
 Go
Interesting
Web
Site
on
Balance
to and read
the Web site to
see how the
moment arm of
the ground
reaction force
acts at the ankle
joint.
Forces and Torques
in Equilibrium
 Static
Equilibrium
–sum of forces on the body = 0
» F = 0
–sum of torques on the body = 0
» T = 0
Demonstration of
Net Torque
Balancing
Pennies
Solving for Net Torque
Mathematically
Wt = -p (weight of a penny)
r = distance (inches) from axis
What is force on ruler from fulcrum???
Solving for Net Torque
Mathematically
Wt = -p (weight of a penny)
r = distance (inches) from axis
What is force on ruler from fulcrum???
 T = 0 (static equilibrium)