Lecture one part 1: forces on and in the body

Lecture one part 1: forces on and in the body
Forces

Forces in the body Forces on the body
1- Gravitational force 1- Static force
2-Electrical force 2-Dynamic force
3-Nuclear force 3-Frictional force

The force controls all motion in the world, the important force in the body is the muscular forces that cause the blood to circulate and the lungs to take in air and other. Physicists recognize four fundamental forces. In order of their relative strength from weakest to strongest . They are :
1- Gravitational force
2- Electrical force
3- Weak nuclear force
4- Strong nuclear force

1- Gravitational force :-

From Newton law: There is a force of attraction between any two objects.
(F=mg) . Where: g = acceleration due to gravity (cm/sec2 or m/sec2) , m= the mass (g , kg) , f = the force (N, dyne) .
Our weight is due to the attraction between the earth and our bodies. The medical effects of gravitational force is the formation of varicose veins in the legs as the venous blood travels against the force of gravity on its way to the heart. Varicose veins are veins that have become enlarged and twisted. When veins become varicose, the leaflets of the valves no longer meet properly (as illustrated in figure-1), and the valves do not work. This allows blood to flow backwards and they enlarge even more. Varicose veins are most common in the superficial veins of the legs, which are subject to high pressure when standing. Besides being a cosmetic problem, varicose veins can be painful, especially when standing.

Fig.(1) shows the cause of varicose veins

2- The electrical force :
This force is more complicated than gravity since it involves attractive and repulsive forces between static electrical charges as well as magnetic produced by moving electrical charges (electric currents). The electrical forces are immense compared to gravitational force, for example the electrical force between an electron (e- ) and a proton (P+) in hydrogen atom is about 1039 times greater than the gravitational force between them , as shows in figure (2).


Fig.(2) Represents (a) Gravitational force, and (b) Electrical force.
One life process that appears to be electrically controlled is bone growth. Bone contain collagen which behave like n-type semiconductor (i.e. conduct current by negative charge), while mineral crystal of the bone (apatite) close to the collagen ( as illustrated in figure-3) behave as
p-type(i.e. conduct current by positive charge), at this junction the current flows easily from p-type to n-type that induced and control bone growth.
in Fig.3 shows bone structure in verious length scale

3- The nuclear force :-
A- strong nuclear force : is much larger then the other ??it acts as the “ glue " to hold the nucleus together against the repulsive
force produced by the protons on each other .
B- weaker nuclear force : is involved with electron ( beta ) decay from the nucleus .
There are two types of problems involving forces on body, the first is statics (where the body in equilibrium) and the second is dynamics (when the body is accelerated). While the friction is involved in both (statics and dynamics) .
a-Statics force
Objects are stationary ( static ) they are in a state of equilibrium when:-
1- Translational equilibrium: (the sum of force in any direction is zero) ( First condition of equilibrium ), as illustrated in figure (4-a).

2- Rotational equilibrium: The sum of the torques about any axis is zero(Second condition of equilibrium ), as illustrated in figure (4-b).

??= F . L
Where : ?: The torque (N.m), F: The force (N), and L: The vertical distance from the fulcrum (pivot )point to the line action of the force (m, cm).
??cw = ??ccw
sum of clock wise torque = sum of counter clock wise torque .

Fig.(4) Shows (a) Translational equilibrium , and (b) Rotational equilibrium

Many of the muscle and the bone system of the body acts as levers. Levers are classified according to the positions of the fulcrum, effort and load or resistance. There are three classes of levers, identified as first, second, and third class levers. We can tell the classes of levers apart by:
1. The force you apply (or the effort you make)… M.
2. An opposing force such as a weight, which is usually called the load… W.
3. The pivot point, or fulcrum of the action.
Bones as Levers :
Each of the three types of levers can be found in the human body. In each type of lever, notice where the fulcrum is located compared to the effort and the load. In your body, the effort is the force that your muscles apply to the lever. The load is the weight that resists the pull of your muscles.
1- First Class Lever

In a first class lever, the weight and force are on opposite sides of the fulcrum: An Examples of a first-class lever is the joint between the skull and the atlas vertebrae of the spine: the spine is the fulcrum across which muscles lift the head.

2- Second Class Lever
In the second class lever, the load is between the fulcrum and the force:

An example in the human body of a second-class lever is the Achilles tendon, pushing or pulling across the heel of the foot.

3- Third Class Lever
In the third class lever, the force is between the fulcrum and load :

An example of a third-class lever in the human body is the elbow joint: when lifting a book , the elbow joint is the fulcrum across which the biceps muscle performs the work.


Example:- The lever system in the body is the case of the biceps muscle and the radius bone acting to support a weight in the hand.
R:- The reaction force of the humerus on the ulna.
M:- The Muscle force supplies by the biceps.
W:- the weight in the hand and equal (100 N).



M=750N [If neglected the weight of the forearm and hand]
The force and dimension where the weight of the tissue and bones of the hand and arm (H) at their center of gravity. From this example find value of (M) when H=15N and W=5N??

Sol:
Tow torques:
1. due to the weight W
2. due to muscle M


The effect of the arm angle on the muscle force:-


Figure. Raising the arm.(a) The deltoid muscle and bone structure involved.(b)The forces on the arm. T is the tension in the deltoid muscle fixed at the angle , R is the reaction force on the shoulder joint,W1 is the weight of the arm located at its center of gravity, and W2 is the weight in the hand.
If the biceps pulls vertically, the angle of the forearmالساعد ) ) does not affect the force required but it does affect the length of the biceps muscle which affects the ability of the muscle to provide the needed force.
The arm can be raised and held out horizontally from the shoulder by the deltoid muscle (عضلة الكتف) (figure a); we can show the forces schematically (figure b). By taking the sum of the torques about the shoulder joint ,the tension T can be calculate from

W1(the weight of the arm),and W2(the weight in the hand) ). Figure).

If alpha =16 deg.
W1( The weight of the arm )=68N.
W2 (the weight in the hand )=100N.
Then T=1985.2 N.
I.e., the force needed to hold up the arm is surprisingly large.
Example:- What is the forces needed to held up the arm under the state of raising the arm (?=16°,W1=68N,W2=45N)?
Sol.:-
T (tension) at angle ? resolved into tow components. Tsin?, Tcos? and taking the sum of the torques about the shoulder joint.
T=1145N the force needed to hold up the arm is large.


b-Frictional force
A force which resists the motion between two surface (in contact) depended on the nature of the surface and is independent on the area of the surface. Friction force is always opposite to the direction of motion and tends to decrease net force. All materials have their own friction constant in other words friction force depends on the type of materials. Another factor affecting friction force is the normal force. When you apply a force to an object, then friction force becomes active and resists with the force of having opposite direction to your net force. Generally frictional force is divided to:
1- Static friction (Fs):
The effective force between surfaces that are rest with respect to on another.

Where:-
µs= coefficient of static friction (used to find the max. resistance force on an object can exert before it starters to move).
N= normal force.

2- Kinetic (sliding) friction (Fk):-
The effective force between surfaces that are in relative motion.

Where:-
Fk ? N
?k= coefficient of kinetic friction, the greater the coefficient values and the greater the friction force.
?s > ?k = (Force to start the motion is a greater than needed to keep it moving).