Biomechanics 9
Kinetics 2
Projekt: Cizí jazyky v kinantropologii - CZ.1.07/2.2.00/15.0199
Daniel Jandačka, PhD.

Impulse of Force
Impulse of force is the product of the resultant force ΣF and the duration of this force Δt, if the
force is constant.
I = ΣF Δt
Unit of Impulse of Force is Ns

Impulse of Force and Momentum
Resultant external force acting on a human body for certain time causes changes to its momentum.
To cause greater change to momentum we have to either use greater force for the same period of
time, or the same force for a longer period of time.
C:\Jandys\KTV\Biomechanika\Prezentace_Brno\Biomechanics
Presentation\book-2\book-2\images\25\eq01.png

How to use impulse of force to increase momentum in sport?
The greater the impulse of force, the greater the change to momentum of a body (projectile, human
body, tennis racket, ball, etc.)
The important thing is that to change momentum we have to either use greater force or increase the
duration of the same force.

When throwing light objects,
the technique used (duration of force) is much more important for the longest possible throw than
the magnitude of the force.
When throwing heavy objects,
the magnitude of the force is more important. Shot-putters are usually stronger and bigger than
javelin throwers. Their preparation is focused on enhancing their ability to exert great force (ΣF
in the impulse of force).
Top javelin throwers do not have the same maximum strength as shot-putters but they are successful
because their technique maximizes the duration of the force they exert (Δt in the impulse of
force).

How to use impulse of force to decrease momentum in sport?
Certain sport activities, on the other hand, necessitate decreasing a high initial velocity of
a human body to zero velocity. In other words we need to decrease momentum of given bodies.
As examples we can easily imagine all kinds of landing, catching of balls, pucks, etc.
For example pole vaulters cannot land on their feet, due to the technique they use, but they land
on their backs. Landing on the hard ground means a very short time of deceleration of human body,
so the forces acting on human body in such a situation would be massive and devastating. Landing
pads are made of soft and floppy material which prolongs the time of breaking the fall. The impulse
of force is the same as in landing on the ground but its magnitude is decided rather by the
duration of landing than by the magnitude of the force.
http://www.kidcyber.com.au/IMAGES/highjump.jpg

Ground reaction force
GRF= Δp/ Δt
Longer time= lower force

Newton's Third Law of Motion
For every action, there is an equal and opposite reaction.
When one body exerts a force on another body, the second body exerts a force on the first body that
is equal in magnitude but opposite in direction.
https://encrypted-tbn1.google.com/images?q=tbn:ANd9GcTAUPualYosdjT8DGzR7JRpXk45L07ujADMal8dZRx33zyr
4AJs
In such a collision we can see that ramming into an opponent with less mass has more movement
effect (assumption of same velocity).

Newton's Law of Universal Gravitation
Bodies attract other bodies with a force that is directly proportional to the product of their
masses and inversely proportional to the square of the distance between them
C:\Jandys\KTV\Biomechanika\Prezentace_Brno\Biomechanics
Presentation\book-2\book-2\images\27\eq01.png
Gravitational forces between bodies are insignificant in sport activities and we can neglect their
existence.
Only one body has a significant effect on other bodies in sporting activities (and everyday life) –
planet Earth.

Tíhová síla
Tíhová síla působící na těleso může být matematicky vyjádřena takto:
Fg = mg

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