Biomechanics 8
Kinetics 1
Projekt: Cizí jazyky v kinantropologii - CZ.1.07/2.2.00/15.0199
Daniel Jandačka, PhD.

Kinetics is part of mechanics that studies motion and its causes. In combination with kinematics it
is a powerful instrument of complex biomechanical analysis.


Newton's First Law of Motion
A body at rest stays at rest and a body in motion stays in motion with the same speed and in the
same direction unless the body is acted upon by an external force.
File:Billiards balls.jpg
If resultant forces acting on a human body are zero, the body stays at rest or stays in motion with
the same speed and in the same direction.

•Let us imagine that we are holding a barbell weighing 100 kg. With what force do we have to act on
the barbell to keep it at rest?


Momentum
•In biomechanics momentum p is the product of mass of a human body m (or mass of any object) and
its velocity v
p = mv
Momentum allows us to use a single value to express the measure of both motion and inertia of the
given body

Elastic collisions
If two bodies encounter in a totally elastic collision, their resultant total momentum is
conserved.
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There are three types of elastic collisions
1.A moving body encounters a static body, acting with a central force. Generally the moving ball
gives all its momentum to the static ball.
2.Two bodies encounter with opposite velocities and exchange their momenta.
3.The first body is faster than the second body and both bodies move before encounter in the same
direction. Again, they exchange their momenta.
http://img.ehowcdn.com/article-new-intro-modal/ehow/images/a07/6f/gf/rack-pool-balls-triangle-800x8
00.jpg

Inelastic collisions
In totally inelastic collisions momentum is also conserved but after the collision both bodies move
together in the same resultant direction.
C:\Jandys\KTV\Biomechanika\Prezentace_Brno\Biomechanics
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Let us imagine a situation of a defender with the weight of 80 kg colliding with a forward weighing
120 kg. Just before the collision the defender’s velocity is 6 m/s while the forward’s velocity is
-5 m/s (opposite direction). Will the forward move ahead and score or will he be stopped?

Newton's Second Law of Motion
Every time a human body, or any object in sport, decreases its velocity, increases its velocity, or
changes the direction of its motion, it moves with non-zero acceleration. Resultant external force
is the cause of this acceleration.
C:\Jandys\KTV\Biomechanika\Prezentace_Brno\Biomechanics
Presentation\book-1\book-1\images\24\eq01.png

Let us have a look, for example, at weight training exercises. What forces must act on a 30kg
barbell during bench press exercise? There is gravitational force with downward direction and
reaction force exerted by our arms in the opposite direction. The resultant external vertical force
(black line) is the difference between these two forces.
C:\Jandys\KTV\Biomechanika\Prezentace_Brno\Biomechanics
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Please note that the resultant force is greater at the beginning of the motion when we start moving
the barbell and hence we are accelerating. Later we move the barbell with little acceleration and
therefore the resultant force is lesser. In the opposite direction, when we are stopping the
barbell, a breaking force must be acting and therefore the barbell decelerates (negative force).

Thank you for your attention
Projekt: Cizí jazyky v kinantropologii - CZ.1.07/2.2.00/15.0199