Fundamental Concepts
Fundamental Concepts in Physics
Measurement:
Physicists make extensive and careful use of measurement. They use one of two versions of the metric system., the centimeter-gram-second system, or the meter-kilogram-second system. In the customary system, the fundamental units from the foot-pound-second system. For metric/customary conversions,see the table on the following page. It is also important to be familiar with the standard metric prefixes for expressing quantities as well as the common conversion factors used in physics.
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Mass and weight
Mass is the amount of matter in a given object. The mass of this book would be the same anywhere in the universe. Weight is the pull of gravity on a given mass. This book would weigh only one-sixth of its Earth weight on the moon because of the moon has one-sixth of the gravitational pull of Earth. In physics, this distinction between mass and weight is very carefully maintained. In ordinary life, however, mass and weight are usually treated as if they were the same thing. The basic unit of mass in the metric system is the gram, but people in ordinary life may talk about how many grams a coin weights. The basic unit of weight in the U.S. customary system of measurement is the pound, but people may say that a pound is about the same as 2.2 kilograms, a measure of mass.
Force.
Weight is one kind of force, Force causes motion to change in speed or direction and is recognized from this property. A book is acted upon by the force of gravity, that is, the book has weight. If the book is dropped, the force on the book causes it to move toward the center of Earth.
Velocity and Acceleration.
Motion can be categorized in various ways. A steady motion in a single direction, such as a car traveling along a straight highway at 55 kilometers per hour, is measured as velocity. Velocity is the rate of change of the car's position in a unit of time for such a steady motion. if the motion is changing in any way (for example, if the car is speeding up, braking, or turning), the velocity will change at every instant.
The rate of change of velocity is called acceleration. The unit of time occurs twice in a measure of acceleration; for example, if the driver of the car were to speed up gradually by 10 kilometers per hour every hour, the acceleration would be 10 kilometers per hour per hour. Physicists often write this as 10 km/h^2. While constant velocity does not produce a force, constant acceleration of a mass results in force. You can feel this force as a car picks up speed or slows down.
Vector and scalar quantities. What is the difference between a vector and a scalar? A vector is a physical quantity that has direction, while a scalar is a quantity with no direction. Thus, quantities such as displacement, velocity, and acceleration are vectors, while quantities such as time, temperature, volume, work , and energy are scalars.
Vectors are usually represented with an arrow the direction of the arrow indicates the direction of the vector, and the length of the arrow is the magnitude of the vector. Vectors can be added and subtracted, but they do not obey the same rules of mathematics as scalars.
Work
Work is done when a force moves an object in the direction of the force. The amount or quantity of work done is the product of the force and the distance the object moved. In physics, no work is done unless motion takes place. Merely applying force to an object is not considered to be work unless motion takes place.
Energy
This is the ability to do work. Energy is measured by the amount of work performed. There are many types of energy, including electrical, heat, mechanical, chemical , and nuclear. These types of energy are present in three forms.
Potential Energy: Potential energy is the energy an object has stored in it. Probably the most familiar form of potential energy is energy stored in an object owing to the object's change in position. A hammer that has been raised has the potential to do work when it falls. An increase in the height to which it is raised or an increase in the weight of the hammer will increase the amount of potential energy. A spring that has been tightly coiled also contains potential energy, in much the same way.Sometimes the energy produced in chemical reactions is thought of as potential energy.
Kinetic energy: this is the energy an object or body has because of its motion. A hammer applies the force of kinetic energy as it strikes a nail. The amount of kinetic energy of a moving body is a result of its mass and the square of its velocity. Since the velocity is squared, the kinetic energy increases rapidly where the velocity increases. Tripling the speed of an object will increase its kinetic energy nine times.
In a system, the sum of potential energy and kinetic energy is constant. For example, the potential energy that a hammer loses in falling is equal so its gain in kinetic energy.
Res-mass energy: The third form in which energy can exist is expressed by Albert Einstein's theory of relaitivity. Einstein showed that if matter could be completely annihilated, the amount of energy released would be equal to the product of the mass times the speed of light squared. It is this form of energy that is exploited in nuclear reactors and nuclear or thermonuclear bombs.
Power
The rate at which work is done is called power. Power is the amount of work done per unit of time. It is generally more important to know the power of an engine than the amount of energy the engine can generate. Consequently, automobiles are rated by horsepower and electric lights by watts, both units of power.
Momentum
Momentum is defined as the product of the mass of a moving object and its velocity. Momentum is a measure of the tendency of a body to remain in motion or the resistance of a body to being stopped.
The effect of one solid body striking another is determined largely by the momentum of the bodies. A light straw moved by a tornado to a great velocity can have enough momentum to be driven into a tree . Similarly, a very massive object, even though it is moving slowly, can crush everything in its path. In both cases, the momentum determines the result.(Of course, a massive object that is moving fast has even more momentum.)