Download Newton`s Laws

Physics 160 Biomechanics
Newton’s Laws
Questions to Think About
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Why does it take more force to cause an object to
start sliding than it does to keep it sliding?
Why is a ligament more likely to tear during rapid
ankle inversion than during slow inversion?
When spotting a bench press, why do you assist
more on the way up than on the way down?
Newton’s Laws
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Developed by Sir Isaac Newton (1642 – 1727)
Define the mechanical basis of linear kinetics
Newton’s First Law
The Law of Inertia
“A body will maintain a state of rest or constant
velocity unless acted on by a net external
force.”
Mass is lazy
Newton’s 2nd Law
The Law of Acceleration
A force applied to a body causes an acceleration of that body
of a magnitude proportional to the force, in the direction of
the force, and inversely proportional to the body’s mass.
Fnet = ma
Fnet = vector sum of all forces in [ N ]
m = mass in [kg ]
a = acceleration in [m / s ]
2
Example
Lance Berkman swings his bat at a fastball. The
impact of the bat on the ball gives it an
acceleration of 9000 m/s2. What was the force
exerted by the bat at impact if the ball has a mass
of 0.25 kg
Newton’s Third Law:
The Law of Action & Reaction
“When one body exerts a force an another, the
second body exerts a reaction force on the
first that is equal in magnitude and opposite in
direction.”
R
OR
A
“For every action there is an equal and opposite
reaction.”
Example
Tonya crashes into Nancy while they are practicing their figure
skating routines. Tonya’s mass is 60 kg, and Nancy’s mass is
50 kg. During the collision, Tonya exerts an average force of
1000 N against Nancy. How big is the average force Nancy
exerts against Tonya during the collision? If this is the only
horizontal force acting on each skater during the collision,
what average horizontal acceleration does each skater
experience during the collision?
Force of Gravity
The attractive force of the earth on an object. The
force of gravity equals the weight of the object.
Fw= mg
Fua
Fla
Contact Forces
Contact forces result from the interaction of two objects
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Ground Reaction Force
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Acts through COM
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Joint Reaction Force
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Friction
Ground
Reaction Force
Ground Reaction Force (GRF)
• The reaction force produced by individuals when
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walking, running, etc.
As foot contacts the ground it produces a reaction
force.
A
GRF
Ground Reaction Force
Vertical GRF during a landing from a jump
GRF for Walking and Running
GRF vs. Running Styles
Magnitude of Vertical GRF
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Walking = 1 to 1.2
X Body Weight
Running = 3 to 5 X
Body Weight
Squats = up to 7.6
X Body Weight at
patello-femoral
joint
GRF Components
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GRF for walking
Note the difference
in magnitude
between the vertical
component and the
shear components
If stride length increases too much, the horizontal
component (Fx) of the Ground Reaction Force (GRF) will
tend to increase, retarding forward motion.
Fx
Fx
GRF
Fx
x
Fy
GRF
Fy
Joint Forces

Joint reaction forces will
be different than the
externally applied forces
Muscle contraction also
creates force across a joint
 Joint contact forces result
from bone-on-bone contact

Fig. 3.24
 Resistance
Friction Force
to motion as a result of two bodies
being in contact
Static – force that must be overcome to initiate
motion
 Kinetic – force that must be overcome to sustain
motion
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Is Friction Good or Bad?
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Good…
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Feet on pavement while
walking, running, or standing
Bad…
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Skin against pavement while
falling
Frictional Coefficients
Ff
Friction force
µ =
Normal reaction force
R
µ k = Coefficient of Kinetic Friction
µ s = Coefficient of Static Friction
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Ice on ice μk=0.03 (unitless)
Cartilage on cartilage μk=0.002
Jogging shoes on artificial track μs=0.7-1.1
Normal Reaction Force
Force due to the surface pushing on an
object. It is always normal or perpendicular to
the surface and points away from the surface
applied force
weight
friction
R=Normal
reaction force
Friction Force
A theoretical
representation of
friction force as a
function of the
applied force. The
applied force
increases with the
friction force until
motion occurs.
Mechanical Behavior of Bodies
in Contact
P
Is it easier
to push or
pull a
desk
across a
room?
Pv
Fw
Pushing a desk
R = Fw + Pv
P
Pv
Fw
Pulling a desk
R = Fw - Pv
Example
A football player pushes a 590 N blocking sled.
The coefficient of static friction between sled and
grass is 0.73, and the coefficient of kinetic friction
between sled and grass is 0.68. How much force
must the player exert to start the sled in motion?
How much force is required to keep the sled in
motion? Repeat for a 100 kg coach on sled.
Rolling Friction
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Between rolling object and
surface
bicycle tire and surface
 bowling ball and surface
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Rolling friction is 100 to 1000
times less than sliding friction
Example
Jay is gliding north on his crosscountry skis across a flat section of
snow at 7 m/s. Jay’s mass is 100 kg.
The coefficient of kinetic friction
between the skis and the snow is
0.10. What is Jay’s horizontal
acceleration?