Physics Fundamentals: Force, Vectors, Weight, and Pressure

Fundamental Concepts of Force and Measurement

Defining Force and Its Units

Force: Every action capable of changing the state of rest or motion of a body, or producing strain. Forces are not seen, but their effects can be observed.

Unit of Force: The Newton (N).

• A Newton is the force that, when applied to a body with a mass of one kilogram, communicates an acceleration of one meter per second squared (1 m/s²).

Forces are measured using a dynamometer, typically consisting of a calibrated cylindrical tube.

Hooke's Law and Kilogram-Force

Hooke's Law: The force applied to an elastic body and the resulting elongation produced are directly proportional.

Kilogram-Force (kgf): In everyday life, force values are often expressed in pounds or kilograms-force. This unit is no longer standard in science.

• Relationship: 1 kgf ≈ 9.8 N

Representing Forces: The Force Vector

Forces are represented by arrows. This arrow is called a force vector.

The elements of a force vector are:

1. Intensity: The numerical value assigned to the force, which represents the length of the vector.
2. Point of Application: Where the force is applied, corresponding to the origin of the vector.
3. Direction: It can be vertical, horizontal, or inclined, corresponding to the line of the vector.
4. Sense (or Orientation): In the same direction, there are always two opposite senses, indicated by the tip of the arrow.

Composition and Line of Action of Forces

• Line of Action of Forces: The straight line along which the force vector lies.
• Composition of Forces: A body can have two or more forces acting upon it. The resulting total force (resultant) is the single force that produces the same effect as all of them combined.

Weight and Gravity

Weight (P): The force that attracts a body toward the Earth.

Gravity: The force of attraction that the Earth exerts on bodies, always directed towards the center of the Earth.

Center of Gravity: The point within a body where the resulting weight of all the particles that form it is concentrated.

Pressure and Atmospheric Measurement

Defining Pressure

Pressure (P): The ratio between the force exerted on a surface and the area of that surface ($P = F/A$).

Unit of Pressure: The Pascal (Pa).

Atmospheric Pressure

Atmospheric Pressure: The pressure exerted by the atmosphere on the surface of the Earth and all bodies.

The Italian physicist Torricelli conducted an experiment in 1643 to determine the value of atmospheric pressure. There are other units of pressure, and relationships exist between them.

Barometers: Measuring Atmospheric Pressure

Barometers are apparatus used to measure atmospheric pressure. There are two main types:

1. Mercury Barometers: Have a calibrated scale where the height of the mercury column is read directly in the tube.
2. Aneroid Barometers: Use a sealed metal box that undergoes deformation when the external pressure varies. This deformation is indicated by a needle.