1. Linear Kinetics of Human Movement
Chapter 12
Linear Kinetics of Human Movement
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
© 2012 The McGraw-Hill Companies, Inc. All rights reserved.
McGraw-Hill/Irwin

2. Laws of motion Newton’s first law (law of inertia)
SirIsaac Newton ( )
Body in motion tends to stay in motion at the same speed in the same direction; body at rest tends to stay at rest unless acted upon by an outside force.
What effect does gravity have on this?

3. Basic Biomechanics, 6th edition
Newton’s Laws
What is the law of inertia?
A body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

4. Basic Biomechanics, 6th edition
Newton’s Laws
A skater has a tendency to continue gliding with constant speed and direction because of inertia.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

5. Second law of motion
Law of acceleration: change in acceleration of a body occurs in the same direction as the force that caused it, and is directly proportional to the force causing, and inversely proportional to the mass of the body.
Acceleration: change in rate of motion.
Can be + or -.
Mass: quantity of matter in a body

6. Basic Biomechanics, 6th edition
Newton’s Laws
What is the law of acceleration?
A force applied to a body causes 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
F = ma
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

7. Basic Biomechanics, 6th edition
Newton’s Laws
What is the law of reaction?
For every action, there is an equal and opposite reaction.
When one body exerts a force on a second, the second body exerts a reaction force that is equal in magnitude and opposite in direction on the first body.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

8. Third law of motion
Law of reaction: for every action, there is an equal and opposite reaction.
This allows propulsion.
Friction: force resulting from resistance between two objects.
Sometimes we want to increase this to enhance performance; sometimes we want to decrease it.

9. Basic Biomechanics, 6th edition
Newton’s Laws
In accordance with the law of reaction, the weight of a box sitting on a table generates a reaction force by the table that is equal in magnitude and opposite in direction to the weight. wt R
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

10. Basic Biomechanics, 6th edition
Newton’s Laws
In accordance with Newton’s third law of motion, ground reaction forces are sustained with every footfall during running.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

11. Mechanical Behavior of Bodies in Contact
What is friction?
A force acting over the area of contact between two surfaces
direction is opposite of motion or motion tendency
magnitude is the product of the coefficient of friction () and the normal reaction force (R)
F = R
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

12. Mechanical Behavior of Bodies in Contact
What is momentum?
quantity of motion possessed by a body
measured as the product of a body’s mass and its velocity;
M = mv
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

13. Mechanical Behavior of Bodies in Contact
What is the principle of conservation of momentum?
In the absence of external forces, the total momentum of a given system remains constant.
M1 = M2
(mv)1 = (mv)2
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

14. Mechanical Behavior of Bodies in Contact
What causes momentum?
impulse: the product of a force and the time interval over which the force acts Ft
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

15. Mechanical Behavior of Bodies in Contact
What is impact?
a collision characterized by:
the exchange of a large force
during a small time interval
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

16. Mechanical Behavior of Bodies in Contact
What happens following an impact?
This depends on:
the momentum present in the system
the nature of the impact
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

17. Mechanical Behavior of Bodies in Contact
What happens during impact?
This is described by the coefficient of restitution, a number that serves as an index of elasticity for colliding bodies; represented as e.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

18. Work, Power, and Energy Relationships
What is mechanical work?
the product of a force applied against a resistance and the displacement of the resistance in the direction of the force
W = Fd
units of work are Joules (J)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

19. Work, Power, and Energy Relationships
What is mechanical power?
the rate of work production
calculated as work divided by the time over which the work was done W
P = t
units of work are Watts (W)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

20. Work, Power, and Energy Relationships
What is mechanical energy?
the capacity to do work
units of energy are Joules (J)
there are three forms energy:
kinetic energy
potential energy
thermal energy
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

21. Work, Power, and Energy Relationships
What is kinetic energy?
energy of motion
KE = ½mv2
What is potential energy?
energy by virtue of a body’s position or configuration
PE = (wt)(ht)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

22. Work, Power, and Energy Relationships
During the pole vault, the bent pole stores potential energy for subsequent release as kinetic energy and thermal energy.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

23. Work, Power, and Energy Relationships
What is the law of conservation of mechanical energy?
When gravity is the only acting external force, a body’s mechanical energy remains constant.
KE + PE = C
(where C is a constant - a number that remains unchanged)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.