Unit+7

Suppose a book is resting on your desk. You can move it by lifting it. You can push it from you. You can pull it toward you. The lift, push or pull is called a **force**. You use forces to make an object start moving. You also use forces to stop moving objects. You pull on the brake controls of a bicycle. You catch a baseball. Sometimes you use forces to change the direction of the motion of an object. When you hit a tennis ball or a baseball, it reverses direction. You change the direction of your bicycle with a force on the handlebars. Finally, forces can change the shape of objects. You can crumple a sheet of paper or an aluminum can with a force.

Forces can cause changes in the motion of objects. They can cause changes in the shape of objects. Sometimes, however, you can exert a force on an object and no change in motion or shape occurs. A force that produces no change must be balanced by another force. Forces that oppose each other with equal strength are **balanced forces**. A force has both strength and a direction. When two forces are balanced, they are equal in strength and opposite in direction. For example, one might be an upward force and the other a downward force. Suppose a book drops to the floor from the edge of your desk. A force must have caused the motion. You are well acquainted with this force. The force is gravity. Objects are pulled toward the center of the earth by gravity. In other words, gravity pulls objects downward. Suppose the book is back on top of the desk. The book is again at rest. Is the force of gravity still acting on the book? Yes, the force of gravity is always present, pulling the book downward. Then why doesn’t the book move? There must be another force that balances the force of gravity. The desk top is pushing upward on the book. Since no motion results, the upward force must be exactly as strong as the downward force. The upward push of the desk and the downward pull of gravity are balanced forces.
 * Balanced Forces**
 * The diagram to the right shows how the balanced forces on the book can be shown as arrows. The equal strength is represented by arrows of the same length. The direction is represented by the direction of the arrows. You may be seated as you read this sentence. Gravity is also pulling you downward. Since you are not moving downward, another force must be pushing you upward. Become aware of the chair seat pushing upward on you. This force must balance the force of gravity. As a result, you do not move. || [[image:http://johnson.emcs.net/Physical/images/Image18.gif width="130" height="119"]] ||

Forces that are not opposite and equal are called **unbalanced forces**. In unbalanced forces, one force is greater than the other. While balanced forces cause no change, unbalanced forces always cause a change in motion. When two unbalanced forces are exerted in opposite directions, the combined force is the //difference// between the two forces. The two forces would have a canceling effect or //"subtract"// from one anther and the object will move in the direction of the stronger force. If someone wins an arm-wrestling contest, it means there is an unbalanced fore. Both arms will move in the direction of the greater force. In either case, the combined forces acting on an object is called the **net force**. [ [|Back to the top] ]
 * Unbalanced Forces**
 * [[image:http://johnson.emcs.net/Physical/images/Image19.gif width="110" height="91"]] || When two unbalanced forces are exerted in the same direction, the combined force is the //sum// of the two forces. If you and your friend were pushing a car, the forces would be in the same direction. The two forces would //"add together."// The total force exerted by you and your friend would be greater because it would be the sum of the individual forces. ||

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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. [ [|Back to the top] ]
 * Fluid friction**

Newton's Laws of Motion
This law is best known as An object at rest tends to stay at rest. This happens to an object because there is always more that one force acting upon it at any given time. An easy way to think of this law in use would be to imagine a ping pong ball on a table. If you were to blow on it would roll away from you. If there was another person blowing on it at the same time it might roll towards you, if the other person was blowing harder. It could also stay were it was if both of you were blowing on it with the same amount of force. The ball could also travel away from you if you were blowing harder that the other person.



Newton's Second Law says that Force = Mass X Acceleration. This formula means that the acceleration of a body is directly proportional to the net force on it and inversely proportional to its mass. There are several variations of this formal that can be used to find out different things. The variations are m = F / a, a = F / m. These are very important and should be memorized. The net force on an object will cause it to accelerate. The acceleration will be in the same direction as the force that was action on it.

Newton's Third Law states that every action has an equal and opposite reaching. The easiest way to think of this is to picture yourself and a friend on a frozen pond wearing ice skates. If you were both standing still and you pushed on your friend then your friend would move one way and you would move the other way.

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=**Gravity**= 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. **Gravity** is 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.

= Weightlessness/free fall =

Newton concluded that free fall motion is the motion of an object in the y-direction (also known as the vertical direction) that is caused by gravity. In this case gravity is the only force acting on the object. Acceleration due to gravity relates to free fall motion. Every second an object falls, its speed increases by 9.8 m/s. As the house falls, its speed increases 9.8 m/s, making the ride an exciting experience for the rider. To determine the height of the house from the ground, use the formula used is x= (1/2)gt2, where x is the distance, g is the acceleration due to gravity and t is the time. The higher the house is from the ground, the more exhilarating the ride is for the rider. To determine the velocity of the object falling, in this case the house, use the formula used is v = gt, where v is the velocity, g is the acceleration due to gravity, and t is the time.

As you stand on the floor, you experience pressure on the soles of your feet. This happens because of the invisible, non-contact attractive force of gravitation that exists between your body and the earth. When you release a rock at waist height from your hand, you notice it accelerates down to the floor. This acceleration is also attributed to gravity.

Imagine you are standing in an elevator located at the top of a very tall building. Imagine, further, that the mechanism holding the elevator breaks, allowing the elevator and its contents to plunge to the ground below. The elevator and its contents would fall freely to the ground at 9.8 m/s2. If you could ignore your terror for the moment, you would notice that there is no longer a pressure between your feet and the elevator floor.

If you release a rock from your hand as you fall, the rock will continue to accelerate downwards as you do. The rock and your body will accelerate vertically down at the same rate independent of each other. So, the rock would seem to float beside you. This apparent floating is known as weightlessness. Of course, the freely falling rock and your body still have weight, but it is not experienced during free fall.

Astronauts in space appear to float around inside the space shuttle. They are in freefall as is the space shuttle itself. (The downward acceleration of the orbiting spacecraft is somewhat less than free fall gravitational acceleration nearer the earth's surface because the spacecraft is further from the earth's center.)

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