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BioLab - Biomechanics Teaching & Learning Tool Box
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 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 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
m1 × m2
F=G
2
d
Implications of Newton’s
Law of Gravitation
• Mass
– Greater mass =
greater gravitational force
– Smaller mass =
lower gravitational force
• Distance
– 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
Weight
• 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
rpoles
requator
r = radius of earth
requator > rpoles
gequator < gpoles
Wequator < Wpoles
Newton’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?
Air Resistance
Friction & Air Resistance
Newton’s Second Law of Motion
• Law of Acceleration
A 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
Applications of Newton’s 2nd Law
• 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
F = 500 N
F=m×a
F
500
=
a =
m
1.5
a = 333 m·s-2
a=?
Newton’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 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
Explaining motion using Newton’s Laws - SVJ
Weight (W) vector
Vertical GRF (Fz ) vector
Fz (N)
C
A
D
B
Time (s)
E
C
D
B
A
E
Effects 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
Fz - W
 az =
m
Effect of Forces
Fz <
=
>W
Fz (N)
aazz ==anegative
=0
g
zpositive
A
C
D
B
Time (s)
E
D
B
A
E
C
Explaining motion using Newton’s Laws - SVJ
Weight (W) vector
Vertical GRF (Fz ) vector
Acceleration (m·s-2)
C
A
D
B
Time (s)
E
C
D
B
A
E
Summary
• 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 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.