1. Newton’s Laws of Motion
Linear Kinetics

2. 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

3. 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

4. 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

5. Weight rpoles requator r = radius of earth requator > rpoles
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

6. 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

7. Examples of Newton’s First Law?
Air Resistance
Friction & Air Resistance

8. 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

9. 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
a = ?
F = m × a 
a = 333 m·s-2

10. 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

11. 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

12. Explaining motion using Newton’s Laws - SVJ
Weight (W) vector
Vertical GRF (Fz ) vector C
Fz (N)
Time (s) A D B E B D E A C D

13. 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 

14. Effect of Forces B D A C E Fz = W az = 0 Fz < W az = negative
az = g
Fz = W
az = 0
Fz > W
az = positive C
Fz (N)
Time (s) A D B E B D A C E

15. Explaining motion using Newton’s Laws - SVJ
Weight (W) vector
Vertical GRF (Fz ) vector
Acceleration (m·s-2)
Time (s) C A D B E E A B D C D

16. Summary Newton’s Law of Gravitation Newton’s First Law (Inertia)
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

17. Recommended Reading
Enoka, R.M. (2002). Neuromechanics of Human Movement (3rd edition). Champaign, IL.: Human Kinetics. Pages &
Grimshaw, P., Lees, A., Fowler, N. & Burden, A. (2006). Sport and Exercise Biomechanics. New York: Taylor & Francis. Pages &
Hamill, J. & Knutzen, K.M. (2003). Biomechanical Basis of Human Movement (2nd edition). Philadelphia: Lippincott Williams & Wilkins. Pages 341 &
McGinnis, P.M. (2005). Biomechanics of Sport and Exercise (2nd edition). Champaign, IL.: Human Kinetics. Pages