The Venturi Tube

The operation of venturi tube provides a good illustration of Bernaoulli,s principle. Used originally for measurement of flow of water in pipes. The device consists of a conical nozzle like reducer through which air or water can enter, a narrow section called the throat, and a conical enlargement for the outlet that attains the same size as the inlet but more gradually. The quantity of air drawn through the inlet will be discharged through the same sized opening at the outlet. The velocity of the air must therefore increase as it passes through the inlet cone, must attain a maximum value in the throat, and thereafter gradually slows down to it’s initial value at the outlet. The pressure in the throat is consequently less than at the entrance. The lower drawing of the image shows a venturi tube with manometers, which are gauges for measuring pressure. Bernoulli’s principle states that the total energy of a particle in motion is constant at all points on its path in a steady flow. The pressure in the throat of the venturi tube is less than the pressure in any other part because of increased velocity. This is explained by the fact that the same amount of air passes all points in the tube in a given time. That is, the air must come out of one end of the tube as fast as it enters the other end. Since the throat is smaller, the air must travel faster through the throat for the same amount of air to pass through the throat as passes through the inlet and outlet. The total energy remains constant in any system. In the venturi tube the energy of motion has increased in the throat, hence it is necessary for the energy of pressure to decrease. The reduction of pressure in the throat as compared with the pressure in the inlet is shown by the height of mercury or other liquid in the manometers.

Newton’s Law of Motion

When a magician snatches a table cloth from table and leaves a full setting of dishes undisturbed, he is not displaying a mystic art; he is demonstrating the principle of inertia. Inertia is responsible for the discomfort felt when airplane is brought to a sudden halt in the parking area and the passengers are thrown forward in their seats. Inertia is a property of matter. This property of matter is described by Newton’s first law of motion, which states: Objects at rest tend to remain at rest; objects in motion tend to remain in motion at the same speed and in the same direction. Bodies in motion have a property called momentum. A body that has great momentum has a strong tendency to remain in motion and is therefore hard to stop. For example, a train moving at even low velocity is difficult to stop because of its large mass. Newton’s second law applied to this property. It states: When a force acts upon a body, the momentum of that body is changed. The rate of change of momentum is proportional to the applied force. The momentum of a body is defined as the product of its mass and its velocity. thus,

Momentum = mass x velocity or M = mV Now if a force is applied, the momentum changes at a rate equal to the force or:

F = rate of change of momentum    = (Mf – Mi) / t

Substituting mV for M:

F = (mfVf-miVi) / t

Since the mass does not usually change, mf = mi = m. Then

F = (mfVf-miVi) / t

F = (mVf – mVi) / t

= m(Vf-Vi) / t

From the previuos section the second term is recognized as acceleration. Then the second law becomes:

F = ma

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Continued………………….

Newton’s third law of motion is often called the law of action and reaction. It states that for every action there is an equal and opposite reaction. This means that if a force is applied to an object, the object will supply a resistive force exactly equally to and in the opposite direction of the force applied. It is easy to see how this might apply to objects at rest. For example, as a man stands on the floor, the floor exerts a force against his feet exactly equal to his weight. But this law is also applicable when a force is applied to an object in motion. When a force applied to an object is more than sufficient to produce and sustain uniform motion, inertia of the object will cause such a resistive force that the force opposing the motion of the object equals the force producing the motion. This resistance to change in velocity due to inertia is usually referred to as internal force. When several forces act upon an object to produce accelerated motion, the sum of the external forces are in a state of unbalance; however, the sum of external and the internal forces are always in a state of balance, whether motion is being either sustained or produced. Forces always occur in pairs. The term “acting force” means the force on body exerts on a second body, and reacting force means the force the second body exerts on the first. When an aircraft propeller pushes a stream of air backward with a force of 500 pounds, the air pushes the blades forward with a force of 500 pounds. This forward force causes the aircraft to move forward. In like manner, the discharge of exhaust gases from the tailpipe of a turbine engine is the action which causes the aircraft to move forward. The three laws of motion which have been discussed here are closely related. In many cases, all three laws may be operating on a body at the same time.