Physical Science Study Guide for McGraw-Hill's ASVAB (page 2)
The Laws of Motion
The English scientist Sir Isaac Newton (1642–1727) formulated three laws of motion.
Newton's First Law The first law reads as follows: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Basically, this means that an object will keep doing what it is doing unless something makes it change. The something that makes an object change its state is called a force. A force is a push or pull upon an object resulting from one object's interaction with another object. An example of this is a person pushing a swing or a person pulling a suitcase.
An object has inertia when it is moving. Inertia is merely the resistance to change. An example is the tendency of your body to keep moving forward when your car comes to a sudden stop. In a collision, a seatbelt can keep you in place when otherwise you might hurtle through the windshield because of inertia.
Friction is a force that results when the surface of one object touches the surface of another. Friction causes moving objects to slow down or stop. For example, if you shove a book across a table, it will soon slow to a stop because it is rubbing on the surface of the table. Friction between two objects causes heat.
Newton's Second Law According to Newton, an object will accelerate only if there is an unbalanced force acting upon it. The presence of an unbalanced force will accelerate an object, changing its speed, its direction, or both its speed and its direction. The second law states that the acceleration of an object is dependent upon two variables—the force acting upon the object and the mass of the object. The formula for acceleration is .
Newton's Third Law Newton's third law reads: For every action, there is an equal and opposite reaction. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction—a force on the chair and a force on your body. When a bird flaps its wings downward, the air pushes it upward, allowing it to fly. A gun recoils when it is fired.
Describing Motion In physics, there are various ways to describe motion. Speed is how fast an object is moving. A fast-moving object has a high speed, while a slow-moving object has a low speed. An object with no movement at all has a zero speed. The average speed during the course of a motion can be calculated using the following equation:
Velocity is defined as the rate and direction at which an object's position changes. Velocity includes both speed and direction, such as moving 55 miles/hour in a westerly direction or 6 meters/second upward.
Average velocity can be calculated using the equation:
Acceleration is defined as "the rate at which an object changes its velocity." An object is accelerating if it is changing its velocity. If an object is not changing its velocity, then the object is not accelerating. A falling object accelerates as it falls. If you could measure the motion of a falling object, you would notice that the object has an average velocity of 5 m/s in the first second, 15 m/s in the second second, 25 m/s in the third second, 35 m/s in the fourth second, and so on. By pushing down the gas pedal, you can accelerate your car.
Work, Energy, and Power
Some of the most important concepts in physics are work, energy, and power.
Work Work results from a force acting upon an object, causing it to change position or move from one place to another. There are three key aspects to work: force, movement, and cause. In order for a force to qualify as having done work on an object, there must be a change of position caused by the force. Here are some common examples of work: a person carrying a box of books upstairs, a horse pulling a plow through the fields, a weightlifter lifting barbells, a person pushing a grocery cart down the aisle of a grocery store, a shot putter launching the shot, and an ice skater lifting his partner overhead. In each case, there is a force exerted upon an object that causes the object to be displaced. Work is measured in joules.
Energy In physics, energy is defined as the ability to do work. Energy can be either potential or kinetic.
Potential Energy An object can store energy as the result of its position. A can of soup on a shelf has potential energy. When it falls from the shelf onto the floor, it releases its energy. A roller-coaster car has potential energy when it is at the top of the track. It releases the energy when it plunges downward. An arrow in a drawn bow has potential energy resulting from its position. If the bow is not drawn, or pulled back, the arrow has no potential energy. The can of soup and the roller-coaster car have potential energy that is caused by gravitation—being pulled toward Earth. The second form of potential energy is elastic potential energy. Elastic potential energy is the energy stored when an object is stretched or compressed. For example the arrow in a drawn bow has elastic potential energy. Other examples include stretched rubber bands, bungee cords, trampolines, and springs. The more stretch or compression that is exerted on the object, the more potential energy it holds.
Kinetic Energy Kinetic energy is the energy of motion. Any object that has motion has kinetic energy. The direction does not matter. A moving car has kinetic energy, as does a moving ice skater or a soup can falling off the shelf. When the soup can falls off the shelf, its potential energy is changed to kinetic energy.
Forms of Energy Energy cannot be created or destroyed; it merely changes from one form of energy to another. Forms of energy include:
Power Power is how much work is done over a given period of time. Work can be done very quickly or very slowly. For example, a rock climber scaling a sheer cliff may take a long time to raise his or her body just a few meters. But a hiker who selects an easier, less vertical path might raise his or her body the same few meters in a much shorter period of time. Even though the amount of work is the same, the hiker does the work in considerably less time than the rock climber. The hiker has a greater power than the rock climber.
The formula for power is
The standard metric unit of power is the watt. Watts are used to measure the amount of energy consumed by an electric device.
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