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Newton’s Laws of Motion. Linear Kinetics. Aim. The aim of these slides is to introduce Newton’s Laws of Motion These slides include an introduction to: Newton’s Law of Gravitation Newton’s 1 st , 2 nd and 3 rd Laws of Motion - PowerPoint PPT Presentation
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B i o L a b - B i o m e c h a n i c s T e a c h i n g & L e a r n i n g T o o l B o xB i o L a b - B i o m e c h a n i c s T e a c h i n g & L e a r n i n g T o o l B o x
Newton’s Laws of MotionNewton’s Laws of Motion
Linear KineticsLinear Kinetics
AimAim
• The aim of these slides is to introduce Newton’s Laws of Motion
• These slides include an introduction to:
– Newton’s Law of Gravitation
– Newton’s 1st, 2nd and 3rd Laws of Motion
– The relationship between forces applied to bodies and the motion that those bodies experience
Newton’s Law of GravitationNewton’s Law of Gravitation
• All bodies are attracted to one another with a force which is proportional to the product of their masses (m), and inversely proportional to the square of the distance (d) between them
1 22
m × mF = G
d
Implications of Newton’s Law of Gravitation
Implications of Newton’s Law of Gravitation
• MassMass– Greater mass =
greater gravitational force
– Smaller mass =lower gravitational force
• DistanceDistance– Greater distance =
smaller gravitational force
– Smaller distance =greater gravitational force
• Most bodies in sport have relatively small mass– Attractive force between
them can be considered negligible
rrpolespoles
WeightWeight• Weight (W) is the attractive force
between the earth and any body in contact with it or close to its surface
• Product of the mass (m) of the body and the acceleration caused by the attractive force between it and the earth(g = 9.81 m·s-2)
i.e. W = m × g
• Gravity is based on:
– Mass of bodies
– Distance between bodies
r = radius of earth
requator > rpoles
gequator < gpoles
Wequator < Wpoles
rrequatorequator
Newton’s First Law of MotionNewton’s First Law of Motion
• Law of Inertia
Every body will remain in a state of rest or constant motion (velocity) in a straight line unless acted on by an external force that changes that state
• A body cannot be made to change its speed or direction unless acted upon by a force(s)
• Difficult to prove on earth due to the presence of friction and air resistance
Examples of Newton’s First Law?Examples of Newton’s First Law?
Friction & Air ResistanceFriction & Air Resistance
Air ResistanceAir Resistance
Newton’s Second Law of MotionNewton’s Second Law of Motion
• Law of AccelerationA force (F) applied to a body causes an acceleration (a) of that body which has a magnitude proportional to the force, and takes place in the direction in which the force acts
• Vitally important in sport as it forms the link between force and motion:
• Force = mass × acceleration
• F = m × a
• Assuming mass remains constant, the greater the force the greater the acceleration
• Acceleration is inversely proportional to mass– if force remains the same
and mass is halved, then acceleration is doubled
– if force remains the same and mass is doubled, then acceleration is halved
Applications of Newton’s 2nd LawApplications of Newton’s 2nd Law
F = m × a
a = 333 m·s-2
Fa = =
m
500
1.5
F = 500 N a = ?
Newton’s Third Law of MotionNewton’s Third Law of Motion
• Law of Reaction
For every force that is exerted by one body on a second body there is an equal (magnitude) and opposite (direction) simultaneous force exerted by the second body on the first
• Therefore every force which is applied by a body is accompanied by a reaction force on that body
• Difficult to visualise but can be felt:– e.g. In boxing the force applied by a punch is
experienced by the opponent’s chin and the puncher’s hand
Examples of Newton’s 3rd LawExamples of Newton’s 3rd Law• Ground Reaction Force
(GRF) is a special type of force explained by Newton’s 3rd Law of Motion
• Equal in magnitude and opposite in direction to the force applied to the ground by the body
• Needs to be considered separately in horizontal (friction) and vertical (normal) directions
AABBDDCCDDEE
Fz
(N)
Time (s)
Explaining motion using Newton’s Laws - SVJExplaining motion using Newton’s Laws - SVJ
Weight (W) vector Vertical GRF (Fz ) vector
AA
BB
DD
CC
EE
Effects of ForcesEffects of Forces
• If only one force acting:F = m × a
• If two (or more) forces acting:∑F = m × a
• In SVJ:
Fz - W = m × az
zz
F - Wa =
m
AABBCCDDEE
FFzz = = WW
aazz = = 00
FFzz = = WW
aazz = = 00
FFzz < < WW
aazz = negative = negative
FFzz > > WW
aazz = positive = positive
FFzz < < WW
aazz = = gg
Fz
(N)
Time (s)
Effect of ForcesEffect of Forces
AA
BB
CC
EE
DD
Acc
eler
atio
n (m
·s-2)
Time (s) AABBDDCCDDEE
Explaining motion using Newton’s Laws - SVJExplaining motion using Newton’s Laws - SVJ
Weight (W) vector Vertical GRF (Fz ) vector
AA
BB
DD
CC
EE
SummarySummary• Newton’s Law of Gravitation
– Attractive forces exist between bodies (e.g. a body and the Earth) that are proportional to the product of their masses and inversely proportional to the distance between them
• Newton’s First Law (Inertia)– A force is required to accelerate (i.e. change the velocity of) a body
• Newton’s Second Law (Acceleration)– The acceleration of a body is proportional to the sum of the forces
acting on it
• Newton’s Third Law (Reaction)– Any body that applies a force to another body experiences a
simultaneous reaction force that is equal in magnitude and opposite in direction to the applied force
Recommended ReadingRecommended Reading• Enoka, R.M. (2002). Neuromechanics of Human Movement
(3rd edition). Champaign, IL.: Human Kinetics. Pages 57-59 & 64-66.
• Grimshaw, P., Lees, A., Fowler, N. & Burden, A. (2006). Sport and Exercise Biomechanics. New York: Taylor & Francis. Pages 69-80 & 97-101.
• Hamill, J. & Knutzen, K.M. (2003). Biomechanical Basis of Human Movement (2nd edition). Philadelphia: Lippincott Williams & Wilkins. Pages 341 & 351-356.
• McGinnis, P.M. (2005). Biomechanics of Sport and Exercise (2nd edition). Champaign, IL.: Human Kinetics. Pages 77-99.