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 x
Linear Kinetics

• 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
– The relationship between forces applied to bodies and the motion that those bodies experience

• 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
F = G m × m
1 2 d 2

• 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 ( 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 poles r equator r = radius of earth r equator g equator
W equator
> r poles
< g poles
< W poles

• 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

Air Resistance
Friction & Air Resistance

• 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

• 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 a = = m
500
1.5
a = 333 m·s -2 a = ?

• 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

• Ground Reaction Force
(GRF) is a special type of force explained by
Newton’s 3 rd 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 ( F z
) vector
C
A
B
Time (s)
D
E

• If only one force acting:
F = m × a
• If two (or more) forces acting:
∑ F = m × a
• In SVJ:
F z
W = m × a z
 a = z
F - W z m

A
a a z z
F < W
= 0
C
D
B
Time (s)
E

Explaining motion using Newton’s Laws - SVJ
Weight ( W ) vector Vertical GRF ( F z
) vector
C
A
B
Time (s)
D
E

• 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

• 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.