1. Bio-Mechanics
Force in and on the
BODY
Medical Physics

2. Force of gravity Medical Physics
G is the gravitational constant = 6.67 x Nm2/Kg2
Medical Physics

3. Biomechanics and Force
Medical application of gravitational force:
* Following of blood against the gravitational force in varicose veins (vein near the surface of the skin that has become stretched and swollen with blood) in human legs toward heart.
* Deposition of calcium and other minerals on bones that enhance the bone health
* The body weight is the force exerted by the gravitational force downward
Biomechanics and Force
Medical Physics

4. Biomechanics and Force
Force on the body
Body can be in two force states
Static : when the body is in equilibrium
Dynamic (متحرك) : when the body is in acceleration
(ساكن) W
STATIC
in equilibrium all forces vector sum has to be ZERO
SO,
F1 + F2 – W = zero
F1 + F = W
Torque
(τ) = F x d
= force x perpendicular distance F1 F2
Biomechanics and Force
Medical Physics

5. Biomechanics and Force
Levers
Lever is “the simple machine consisting of a relatively rigid bar-like body that may be made to rotate about an axis”.
Biomechanics and Force
Medical Physics

6. Biomechanics and Force
Levers
Fulcrum
Force
Resistance
1st
2nd
3rd
Biomechanics and Force
Medical Physics

7. First Class Lever Biomechanics and Force Medical Physics
In second class lever fulcrum is closed to the weight (load)
In first class lever fulcrum (pivot point) is between force (effort) and load
A third class lever has the effort (force) between fulcrum and the load.
Biomechanics and Force
Medical Physics

8. Biomechanics and Force
How can you know the type of the lever
Imagine how the lever works
Arrange the force, resistance and fulcrum
If fulcrum is in the middle First class
If force of resistance is in the middle Second class
If force of effort is in the middle Third class
Lever functions :
Increasing force using of small force to move heavy load
Increasing distance using force to move an object to longer distance
Increasing speed using force to increase the speed of an object
Moving the force from one place to another
Performance accuracy as we use tweezers to pick up a very small object
Avoiding danger ex. Heat, cold and poisons
Biomechanics and Force
Medical Physics

9. Biomechanics and Force
The lever law
The force x its arm = the resistance x its arm
The left side of the equation has to equal the right side
SO,
When force and resistance are equal, force and resistance arms are equal
When the force arm is longer than the resistance arm, force will be smaller than resistance
When the force arm is shorter than the resistance arm, force will be larger than the resistance
Example : calculate the resistance arm when a 10 N force push a lever resistance of 6 N,
knowing that the force arm is 3 cm ?
10 x 3 = 6 x ?
? = 5 cm
Biomechanics and Force
Medical Physics

10. Biomechanics and Force
Example : Give the missing parts of the table
Force
Force arm
Resistance
Resistance arm 5 10 25 ? 40 2 8 3 6 9
1 cm
2 cm
3 cm
Example :
where you can put one square to equalize each color ?
…………………………………………………………………………………
………………………………………………………………………………...
Biomechanics and Force
Medical Physics

11. Biomechanics and Force
What is the most lever type conserve effort and has a mechanical benefit ?
First Class Lever :
If the force arm is longer than the resistance arm : force will be smaller than resistance SO, it has a mechanical benefit (conserve effort)
If the force arm is equal the resistance arm : force will be equal resistance SO, it has no mechanical benefit.
If the force arm is shorter than the resistance arm : force will be larger than resistance SO, it has no mechanical benefit
Second Class Lever
Force arm is always longer than resistance arm : force always smaller than resistance SO it has a mechanical benefit (conserve effort)
Third Class Lever
Force arm is always shorter than resistance arm : force always larger than resistance SO it has no mechanical benefit
(does not conserve effort)
Biomechanics and Force
Medical Physics

12. Biomechanics and Force
Lever application in the human body
Third Class
First Class
Second Class
Biomechanics and Force
Medical Physics

13. Mechanical advantage =
Mechanical advantage of a force is its ability to do its work and it is related to the distance between the force and its arm :
Mechanical advantage =
Biomechanics and Force
Medical Physics

14. Biomechanics and Force
Forearm as a lever system
In biceps muscle :
The effort is the contraction power of the biceps muscle to bring the arm upward.
The weight (load) (W) in the palm is 30 cm apart from pivot point.
Weight of tissue and bones of the hand and arm (H) is 14 cm apart from pivot point.
The muscle effort (M) arm is 5 cm apart from the pivot point R M W
5 cm H
14 cm
30 cm
Biomechanics and Force
Unit Coordinator : Dr. Bassem M. Raafat
Medical Physics

15. Biomechanics and Force
30 W + 14 H – 5 M = Zero
30 W + 14 H = 5 M
M = (30W +14H)/5 = 6W + 2.8H (Dyne)
In biceps movement the power M remain constant but the power arm is changed (elongated or shortened) to keep the power constant
Biomechanics and Force
Medical Physics

16. Forearm with plain position
Total torque = Στ = (18Tsinα) - (36H) - (72W) = zero
18Tsinα - 36H - 72W = zero 18Tsinα = 36H + 72W Tension in the deltoid muscle (T) = (36H + 72W)/18sinα
(T) = (2H + 4W)/sinα
(H) is the weight of the arm,
(W) is the weight in the palm
(T) is tension in deltoid muscle
(T) = (2H + 4W)/sinα
α is the angle by which shoulder move or pivot
T sinα
Tcosα
Biomechanics and Force
Medical Physics

17. Biomechanics and Force
Spinal Pressure
Spinal lumber disc pressure is minimum while a person lying on his back. It becomes maximum when he sitting forward.
125 Kg/Sq. cm
250 Kg/Sq. cm 1 2 3
Pressure
Position of the body
Biomechanics and Force
Medical Physics

18. Biomechanics and Force
Friction force
Direction of motion
f = µN = μmg f F N
W = mg = N
Vertical reaction force, supplied by surface
where N is the force and µ is the friction coefficient
Medical application of friction force :
Bones of joints are separated by anti-lubricants (synovial fluid) to minimize bone friction.
Saliva also act as anti-lubricants during food chew
Lungs movement in chest
Slippery mucus materials covering intestine during rhythmic (regular) motion to minimize the effect of friction force.
Exerted force
Friction force
Surface
Biomechanics and Force
Medical Physics

19. Biomechanics and Force
Dynamics
We have two cases of dynamic motion :
With constant acceleration or declaration
F = ma
Where F is the motion force, m is the mass of moving object and a is the acceleration.
With change in time and velocity
F =
= m (vf - vi)/Δt
where v is the velocity and t is the motion time
Biomechanics and Force
Medical Physics

20. Biomechanics and Force
Problems
A 60 Kg person (mass) walking at 1 m/sec (velocity) bumps into a wall and stops in a distance of 2.5 cm in about 0.05 sec (time) what is the force developed on impact ?
F =
= (Kg m/sec2)
A person walking at 1 m/sec (velocity) hits his head on a steel beam. Assume his head stops in 0.5 in about 0.01 sec. If the mass of his head is 4 kg, what is the force developed ?
F =
= (Kg m/sec2)
Biomechanics and Force
Medical Physics

21. Biomechanics and Force
Viscosity
Gravitational force Fg= Buoyant force Fb + Retarding (friction) force F
R is the sphere’s radius,  is the density of the particle, o is the fluid density, g is the acceleration due to gravity,  is the fluid viscosity coefficient.
V = 2R2 g
V is the sedimentation rate by which particles of a solution is precipitated
Medical application of sedimentation and centrifugation force :
When red blood cells shape and size are changed in case of some diseases, blood sedimentation and centrifugation forces are changed, e.g. rheumatoid fever, rheumatic heart and gout in which RBCs are clump together increasing their effective radius and sedimentation rate.
In hemolytic anemia and sickle cell anemia RBCs break and sedimentation rate is decreased.
Biomechanics and Force
Medical Physics

22. Biomechanics and Force
Types of Fluid motion
Fluid flow is generally broken down into two different types of flows, laminar flow and turbulent flow. 
Laminar flow : is fluid motion in which all the particles in the fluid are moving in a straight line.  For example, the thin layer of fluid in contact with the wall of a pipe travels very slowly due to the friction at the wall.
Turbulent flow is an irregular flow of particles. For example, the fluid layers of increasing speed, reaching the maximum speed at the center of the pipe.
Both types of flow occur inside an object or outside an object, for example, fluid flow inside a pipe or fluid flow around a baseball.
Biomechanics and Force
Medical Physics

23. Biomechanics and Force
Reynolds Number (RN)
The Reynolds number is dimensionless quantity defined as the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.
laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion; turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces
Laminar Flow
Turbulent Flow
Reynolds Number
Low
High
Biomechanics and Force
Medical Physics

24. Biomechanics and Force
Bernoulli's principle
Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.
Where :
P is the pressure
 is the density
g is the free acceleration constant (9.8)
V is the speed
H is the elevation
Biomechanics and Force
Medical Physics