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Khalid Alhumaidhi

6415067

Energy of a System

We recognize energy through the concepts of force and motion. When work is done on an object, it gains energy. Energy is made of two types, potential energy which is stored energy and kinetic energy, which consists of moving energy. Food is stored energy. It is stored as a chemical with potential energy. When your body uses that stored energy to do work, it then becomes kinetic energy.

Energy in a system may be transformed so that it resides in a different state. Energy in many states may be used to do many varieties of physical work. Energy may be used in natural processes or machines, or else to provide some service to society (such as heat, light, or motion). For example, an internal combustion engine converts the potential chemical energy in gasoline and oxygen into heat, which is then transformed into the propulsive energy (kinetic energy that moves a vehicle) (Ross, 2011). A solar cell converts solar radiation into electrical energy that can then be used to light a bulb or power a computer.

Any form of energy can be converted to another form. Most technological devices that we use are recognized as energy converters. Energy cannot be created, nor destroyed. This is the reason why it exists in many forms. For example, a light bulb converts electrical energy to radiant energy. It can come in various forms, mechanical, chemical, radiant, electrical, and nuclear.

Thermodynamics is the study of energy being converted from one form to another. There are three laws of thermodynamics. The first law states that energy cannot be created or destroyed, but it can be converted from one form to another. The second law states that heat energy can be transferred only from body at high temperature to the body at lower temperature. Heat can only be moved from high to low without external work being performed. If you want to move the heat energy from low temperature reservoir to high temperature reservoir, then something external must intercept in order for that to work. For instance, an air conditioner or a refrigerator heat moved to low temperature to high temperature, needing electricity to work or perform properly (Ross, 2011). Finally, the last law of thermodynamics states that all molecular movement stops at the temperature of absolute zero or 0 Kelvin.

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Conservation of energy

Energy is defined as the capacity to do work. The English physicist Thomas Young (1773-1829) used the term energy (Ross, 2011). It is found in many forms like light, heat, atomic and subatomic behavior, etc. Energy can be converted from one form to another and the total energy in any closed system remains constant. In classical physics, this principle was known as conservation of energy; in modern physics, it is termed the conservation of mass and energy. Conservation of energy also has another meaning and that is to ‘save’ energy. As the natural energy sources become scarcer it is important to learn how to save energy. One way of doing this is to create an energy efficient home. To create an energy efficient home there are many measures that can be taken to save energy, they include: solar energy, as the sun is a constant natural energy resource we can store that energy and use it to heat out water. ‘Sleep mode’ on computers, computers are definitely becoming more and more popular in homes, if and only if the computer needs to be on at all times you should use the sleep mode which lets the computer run on the minimum amount if energy needed (Ross, 2011). Last but not least, energy can be conserved by doing the simple things around the home to lessen the usage of the machines that do work: example, turning applications off at the power point instead of just at the switch on the appliance and shutting all windows and doors and closing all curtains to keep the heat in the house instead of relying on the heater to do all the work, and vise-versa in the summer, to open all the windows and doors to let the cool air in instead of relying on the air conditioner. It is important for all people to realize the need for conserving energy. It is impossible for all the natural resources to keep up with our forever advancing civilization, and supply us with the energy that we need.

Khalid Alhumaidhi

6415067

Chapter 9-Linear Momentum and Collisions

Linear momentum is defined as the product of a system’s mass multiplied by its velocity. In symbols, linear momentum is expressed as p = mv. Momentum is directly proportional to the object’s mass and also its velocity (Resnick, 2007). Thus the greater an object’s mass or the greater its velocity, the greater its momentum. Momentum p is a vector having the same direction as the velocity v.

In classical mechanics, linear momentum, or simply momentum (SI unit kg m/s, or equivalently N s), is the product of the mass and velocity of an object. Like velocity, linear momentum is a vector quantity, possessing a direction as well as a magnitude (Resnick, 2007). Linear momentum is particularly important because it is a conserved quantity, meaning that in a closed system (without any external forces) its total linear momentum cannot change. Because momentum has a direction, it can be used to predict the resulting direction of objects after they collide, as well as their speeds. Momentum is conserved in both inelastic and elastic collisions. (Kinetic energy is not conserved in inelastic collisions but is conserved in elastic collisions.) It important to note that if the collision takes place on a surface with friction, or if there is air resistance, we would need to account for the momentum of the bodies that would be transferred to the surface and/or air. 

Looking at momentum of a system of two particles is the sum of their momenta. If two particles have masses m1 and m2, and velocities v1 and v2, the total momentum is: p= p1 + p2= m1v1+m2v2. Keeping in mind that momentum and velocity are vectors. Therefore, if two particles are moving in the same direction, v1 and v2 have the same sign. If the particles are moving in opposite directions they will have opposite signs.

Altogether, momentum, like energy, is important because it is conserved. “Newton’s cradle” shown in Figure 1 is an example of conservation of momentum. As we will discuss in the next Atom (on Momentum, Force, and Newton’s Second Law), in classical mechanics, conservation of linear momentum is implied by Newton’s laws (Resnick, 2007). Only a few physical quantities are conserved in nature. Studying these quantities yields fundamental insight into how nature works.

Khalid Alhumaidhi

6415067

Chapter 11-Angular Momentum &Chapter 12-Statatic Equilibrium and Elasticity

Objects executing motion around a point possess a quantity called angular momentum. This is an important physical quantity because all experimental evidence indicates that angular momentum is rigorously conserved in our Universe: it can be transferred, but it cannot be created or destroyed. For the simple case of a small mass executing uniform circular motion around a much larger mass (so that we can neglect the effect of the center of mass) the amount of angular momentum takes a simple form. As the adjacent figure illustrates the magnitude of the angular momentum in this case is L = mvr, where L is the angular momentum, m is the mass of the small object, v is the magnitude of its velocity, and r is the separation between the objects (Goldstein, H. (2009).On the other hand, the term equilibrium implies either that the object is at rest or that its center of mass moves with constant velocity. We deal here only with the former case, in which the object is described as being in static equilibrium. Static equilibrium represents a common situation in engineering practice, and the principles it involves are of special interest to civil engineers, architects, and mechanical engineers (Goldstein, H. (2009). the term equilibrium implies either that the object is at rest or that its center of mass moves with constant velocity. We deal here only with the former case, in which the object is described as being in static equilibrium. Static equilibrium represents a common situation in engineering practice, and the principles it involves are of special interest to civil engineers, architects, and mechanical engineers.

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6415067

Oscillatory motion

In short, oscillatory motion is a system in which a particle or set of particles moves back and forth. Whether it is a ball bouncing on a floor, a pendulum swinging back and forth, or a spring compressing and stretching, the basic principle of oscillation maintains that an oscillating particle returns to its initial state after a certain period of time. This kind of motion, characteristic of oscillations, is called periodic motion, and is encountered in all areas of physics.

We can also define an oscillating system a little more precisely, in terms of the forces acting on a particle in the system. In every oscillating system there is an equilibrium point at which no net force acts on the particle (Clark, 2007). A pendulum, for example, has its equilibrium position when it is hanging vertical, and the gravitational force is counteracted by the tension. If displaced from this point, however, the pendulum will experience a gravitational force that causes it to return to the equilibrium position. No matter which way the pendulum is displaced from equilibrium, it will experience a force returning it to the equilibrium point. If we denote our equilibrium point as x = 0, we can generalize this principle for any oscillating system (Clark, 2007).

In an oscillating system, the force always acts in a direction opposite to the displacement of the particle from the equilibrium point. This force can be constant, or it can vary with time or position, and is called a restoring force. As long as the force obeys the above principle, the resulting motion is oscillatory. Many oscillating systems can be quite complex to describe.