Forces

=Energy=

**What Is Energy?**
Energy makes change possible. We use it to do things for us. It moves cars along the road and boats over the water. It bakes a cake in the oven and keeps ice frozen in the freezer. It plays our favorite songs on the radio and lights our homes. Energy is needed for our bodies to grow and it allows our minds to think. Scientists define energy as the ability to do work. Modern civilization is possible because we have learned how to change energy from one form to another and use it to do work for us and to live more comfortably.

Forms of Energy
Energy is found in different forms including light, heat, chemical, and motion. There are many forms of energy, but they can all be put into two categories: potential and kinetic. Potential energy is stored energy and the energy of position — gravitational energy. There are several forms of potential energy. ||~ ===Kinetic Energy=== Kinetic energy is motion — of waves, molecules, objects, substances, and objects. ||
 * ~ ===Potential Energy===
 * **Chemical Energy** is energy stored in the bonds of atoms and molecules. Biomass, petroleum, natural gas, and coal are examples of stored chemical energy. Chemical energy is converted to thermal energy when we burn wood in a fireplace or burn gasoline in a car's engine.


 * Mechanical Energy** is energy stored in objects by tension. Compressed springs and stretched rubber bands are examples of stored mechanical energy.


 * Nuclear Energy** is energy stored in the nucleus of an atom — the energy that holds the nucleus together. Very large amounts of energy can be released when the nuclei are combined or split apart. Nuclear power plants split the nuclei of uranium atoms in a process called **fission**. The sun combines the nuclei of hydrogen atoms in a process called **fusion**.


 * Gravitational Energy** is energy stored in an object's height. The higher and heavier the object, the more gravitational energy is stored. When you ride a bicycle down a steep hill and pick up speed, the gravitational energy is being converted to motion energy. Hydropower is another example of gravitational energy, where the dam "piles" up water from a river into a reservoir.


 * Electrical Energy** is what is stored in a battery, and can be used to power a cell phone or start a car. Electrical energy is delivered by tiny charged particles called electrons, typically moving through a wire. Lightning is an example of electrical energy in nature, so powerful that it is not confined to a wire. || **Radiant Energy** is electromagnetic energy that travels in transverse waves. Radiant energy includes visible light, x-rays, gamma rays and radio waves. Light is one type of radiant energy. Sunshine is radiant energy, which provides the fuel and warmth that make life on Earth possible.


 * Thermal Energy**, or heat, is the vibration and movement of the atoms and molecules within substances. As an object is heated up, its atoms and molecules move and collide faster. Geothermal energy is the thermal energy in the Earth.


 * Motion Energy** is energy stored in the movement of objects. The faster they move, the more energy is stored. It takes energy to get an object moving and energy is released when an object slows down. Wind is an example of motion energy. A dramatic example of motion is a car crash, when the car comes to a total stop and releases all its motion energy at once in an uncontrolled instant.


 * Sound** is the movement of energy through substances in longitudinal (compression/rarefaction) waves. Sound is produced when a force causes an object or substance to vibrate — the energy is transferred through the substance in a wave. Typically, the energy in sound is far less than other forms of energy. ||

= Motion = Motion is all around us. One of the earliest goals of scientists was to explain motion. In this chapter, you will learn to describe simple forms of motion. You will also learn some causes of this motion.

Position
How do you know where you are? Right now you are sitting in front of a computer in your classroom. But, where is your classroom? The word science uses to describe something's location is position. Position is just another way of saying where something is.

Position is relative. This means that position is "related" to some other position. Another way to look at it is that position is defined by some reference point. You could say that the position of your desk is in the third row from the front of the class. In this case, your reference point is the front of the class.

Once a reference point is chosen, reference directions are needed. We usually use north, west, south, and east as reference directions. North is the direction to the north pole, south is the opposite direction, and east and west are perpendicular to north-south. Other reference directions include up and down. To describe one's position, one needs to have a reference point or position and directions from that point.

= Speed = In your mind right now, the idea for SPEED might just be "fast" and "slow". We use descriptions like "this train is slower then that train, therefore, this train takes us from here to there slower". These words are used to compare speed, but they are not precise enough. We need to define speed carefully.

Imagine, a man driving a car steadily. When he passed a traffic light, we start timing it and record the progress each second. It is shown on the picture above. The driver is going at a steady speed. We define speed as the distance traveled each given time unit, such as a second. This driver is going 10 meters per second.

WORKING WITH SPEED

Trying to find the speed might not be as hard as you think. It's fairly simple, you will only need two numbers. The time that it was taken, and the distance it has traveled. After you've got those two number, calculating speed is a cinch. You can then just divided distance by time to get the speed.

Average Speed It's like the average of your grade. You might have an A and a C, but your average is a B. Science works a lot like mathematics, you add all the numbers together, then divide by how many numbers you've added. For example, if a car is traveling 50 meters per hour, but was slowed down to 10 meters per hour while crossing the school zone.
 * **Speed =** || **__Distance__** ||
 * ^  || **Time** ||

In this case, the car is not going at a steady speed. The speed changes continually, therefore, AVERAGE SPEED is being used. We can use the formula **speed = total distance / total time** Their average speed is 30 meters per hour. (50 + 10 = 60, 60/2 = 30) Next, let's contrast average speed to ** instantaneous speed **. Well, as you might have guessed, instantaneous speed is the speed at which you are currently traveling at the moment. For instance, if you are driving along and look down at the speedometer, your instantaneous speed at that moment would be what was displayed on your speedometer.

So, how does instantaneous speed differ from average speed? Well, let's go back to the example above. One way to get an average speed of 50 mph over 2 hours would be to simply drive at 50 mph all the time. In this case, the average speed would be the same as the instantaneous speed. However, let's say your foot is not the steadiest part of your body. If this is true, then your instantaneous speed would fluctuate a lot. However, if you still manage to cover 100 miles in 2 hours, even though your speed was fluctuating, then your average speed would still be 50 mph, but your instantaneous speed during those two hours of driving would not always be 50 mph.

Distance Time Graphs
Distance-time graphs is a way to visually show a collection of data. It allows us to understand the relationships between the data. The below is a example of a distance time graph.
 * Distance (s) || Time (s) ||
 * 0 || 0 ||
 * 1 || 13 ||
 * 2 || 25 ||
 * 3 || 40 ||
 * 4 || 51 ||
 * 5 || 66 ||
 * 6 || 78 ||
 * 6 || 78 ||


 * As you can see, the data from the table is shown in a visual format in the graph. The time (s) is shown as the x axis and the distance (m) is shown on the y axis on the graph. The points on the graph do not create a perfectly straight line so a //line of best fit//must be drawn //in.// ||

= Velocity =
 * Velocity ** is just like speed, except you add a direction to it. For instance, for speed, you would say," This car is going 55 meters per hour." But if you where talking about velocity, you would add the direction of which the car is going. For example, " This car is going 55 meters per hour, north."

= Acceleration = If you are blind-folded sitting in a smooth driving quiet car. You won't be able to tell if you are traveling, unless the car speeds up, slows down, or changing direction. A change of velocity, or you can also think of a change in speed or direction of an object, is called **acceleration**. When an object gets faster, it accelerates.

**Acceleration** - the rate of change in velocity. If something is accelerating, its velocity is changing over time. You can use that definition to determine the equation needed to find an object's acceleration. Acceleration = final velocity - initial velocity / time

Acceleration tells you haw fast velocity is changing over time. What units are used to express acceleration? Well, if velocity is measured in km/h and time is measured in hours then the acceleration unit is km/h/h or km/h2. This means that if an object is acceleration at 50 km/h2, each hour its velocity increases by 50km/h. Example problem A roller coaster's velocity at the top of a hill is 10 m/s. 2 seconds later, it reaches the bottom of the hill with a velocity of 26 m/s. What is the acceleration of the roller coaster? Initial velocity = 10 m/s Final velocity = 26 m/s Time = 2 s Acceleration = final velocity - initial velocity / time __=16 m/s__ || = = = 8 m/s/s || The unit for acceleration is fairly strange. It is in this case, meter per second per second. 2 meters per second per second seems strange, but when you really think about it, it would become more clear. It increases certain "speed" per second. Since the unit for speed is meters per second, therefore, "meters per second" per second would be a correct way of saying it increases certain speed every certain time interval.In the picture below, the car is shown for each second. The speed of the car is also shown. The car's speed is increasing steadily. It is called c**onstant acceleration**.
 * A = || __26 m/s - 10 m/s__ || = =
 * || 2s || 2s ||  ||

In this case, the car accelerates at 2 meters per second (how fast the care is moving in one second). And the correct way to say it is 2 meters per second per second or 2 meters per second squared.

Objects that are slowing down are said to have negative acceleration or **deceleration**. The same equation is used to find an object's deceleration.

Example problem At the end of a race, a bicycle is decelerated from a velocity of 12 m/s to a rest position in 30 s. what is the average deceleration of this bicycle? Final velocity = 0 m/s Initial velocity = 12 m/s Time = 30 s A= v /t A = __ 0 m/s - 12 m/s __ == __ - 12m/s __= - 0.4 m/s/s 30 s 30 s

=Law of Universal Gravitation= Galileo first introduced the idea that all objects fall at the same rate. He tested this hypothesis by timing the motion of a ball down an inclined plane. The motion of the ball is caused by gravity. Gravityis defined as the attractive force between all objects in the universe. At equal intervals, Galileo marked the distance the ball traveled. He found that the speed of the ball increased as it rolled down the ramp. The distance that the ball rolled increased with each unit of time. He found this to be true with balls of different masses. All objects accelerate at the same rate, regardless of their masses. He discovered that objects near the surface of the earth fall with an acceleration rate of 9.8 meters per second.

Another law credited to Sir Isaac is the Law of Universal Gravitation. One of the primary aims of science is to find general rules governing disparate phenomena. Newton 's law of gravitation is one of the most sweeping generalizations ever made.

In Newton 's day, the motion of the planets was well known, thanks to Kepler. And of course it was known that the Earth exerted a downward force on all material. What Newton realized was that a single equation could describe both the motion of planets and the motion of an apple falling from a tree. The law of universal gravitation depends on the mass of the objects and the distance between them.

What we call "weight" is the force of the Earth's gravitational attraction on a mass near it's surface. It's worthwhile noting that this force doesn't go away for astronauts in orbit, so it's not really correct to say that they're "weightless". In fact, according to the first law of motion, if the earth wasn't exerting a force on them the astronauts would fly off in a straight line rather than move in a circular orbit around the planet. The astronauts (and the shuttle or space station) are in free-fall; they're being drawn towards the earth, but their forward motion is such that the earth curves away at the same rate that they are falling, and so they stay in orbit.

Any object meets air resistance. Air resistance is the upward force of air against a falling object. When the upward force of air against the object is equal to the downward force of gravity, terminal velocity is reached. Terminal velocity is the highest velocity reached by a falling object.

=Friction= The early Greek philosopher Aristotle believed that in order to put an object in motion and keep it moving at a constant speed, a constant force had to be applied. If the force were removed, the object would come to rest. In other words, the natural state of an object was to be at rest. For example, a horse had to pull a cart continuously to keep the cart moving. If the horse stopped pulling, the cart came to rest. Based on many of your everyday experiences, you would probably agree with Aristotle. A ball rolled along the ground comes to rest. A sled glided along the snow eventually ends the ride. And a book pushed along a table soon stops. So it is not surprising that Aristotle’s idea of constant force for constant motion lasted for almost 2000 years. Galileo experimented with motion and concluded that all objects have some built-in tendency to resist changes in motion. The more mass the object had the harder it was to change its motion. He also suggested that the only thing that would keep an object from staying in motion was friction. In the seventeenth century, Isaac Newton suggested an explanation for motion that supported Galileo’s ideas. He proposed that an object in motion should move at constant velocity. No force is necessary to keep it moving in a straight line at a constant speed. If a book sliding across a table comes to rest, there must be a force acting on the book that opposes its motion. Objects do not come to rest on their own. The force that opposes motion of an object is called ** friction **. Friction is the force that brings an object to rest. When objects are in contact with each other, friction acts in a direction opposite to the motion of the moving object. The moving object slows down and finally stops. There are three basic types of frictional force:

If you try to slide two objects past each other, a small amount of force will result in no motion. The force of friction is greater than the applied force. This is static friction. If you apply a little more force, the object "breaks free" and slides, although you still need to apply force to keep the object sliding. This is kinetic friction. You do not need to apply quite as much force to keep the object sliding as you needed to originally break free of static friction.
 * Static Friction **

When two solid surfaces slide over each other, sliding friction acts between the surfaces. When you push a chair across the floor, a sliding friction opposes your motion. The amount of sliding friction present depends on two factors: the weight of the object that is moving and the types of surfaces that the object slides across. There is more friction when a stack of cartons is pushed than when just one carton is pushed. But there is less friction opposing the motion if the cartons are pushed across a smooth floor than across a carpeted one.
 * Sliding friction **

When an object rolls over a surface, the friction produced is called rolling friction. Rolling friction tends to oppose motion less than sliding friction, therefore wheels are often placed on objects to make them easier to move.
 * Rolling friction **

All liquids and gases are fluids. When an object moves through a fluid, fluid friction results. Air resistance is a common example of fluid friction. When you dive from a diving board, you encounter air resistance. It is a relatively small amount of air resistance, so your motion is slowed down only a little. But the fluid resistance of the water is great enough to stop your motion before you reach the bottom of the pool. Fluid friction usually opposes motion less than sliding motion. Substances called ** lubricants ** change sliding friction to fluid friction. Oil, grease, and wax are examples of lubricants. These substances reduce the friction and makes motion easier. Although friction opposes motion, friction can sometimes be helpful – if not necessary. Tires have treads to increase the friction of the wheels on the road. Brakes use friction to stop motion. The friction from your shoe treads allows you to walk. Why do you think sand is placed on icy sidewalks? The sand provides additional friction to the ice, thereby reducing your chances of falling. As you write, the friction rubs graphite from the pencil tip to the paper. Friction between the eraser and the paper causes the pencil mark to be erased.
 * Fluid friction **