# Physics-Energy and Motion (Science)

"A scientist in his laboratory is not a mere technician: he is also a child confronting natural phenomena that impress him as though t hey were fairy tales." -Marie Curie (1867-1934)

We use motion and energy every moment of every day without thinking about the details. For example, throwing a baseball is a complicated process—the motion of the ball is affected by its size and shape, by gravity, by the temperature and movement of the air. The pitcher may not realize how much physics is involved in the almost instantaneous calculations needed to throw the ball to a precise spot 60 feet away.

Physics is a science that uses observation to find the rules that govern why things work as they do. It’s possible to describe these rules in the complicated equations that make many people think of physics as incomprehensibly complex. It isn’t necessary to use these equations, however, to understand the basic ideas behind how things move and what powers their motion.

## Mow do spacecraft reach the moon if they use most of their fuel leaving Earth?

During the late 1960s and early 1970s, NASA’s Apollo program carried astronauts to the moon 10 times (although only 6 of the missions actually landed on its surface). Each of these missions was carried into space by a Saturn V rocket, the biggest engine ever built. About two minutes into the flight, the first stage of the rocket, the largest part, dropped away after burning all its fuel. Six minutes later, the second stage of the rocket was done, having burned most of the rest of the fuel. How could the astronauts continue on a 200,000-mile journey to the moon, having burned almost all their fuel in the first hundred miles or so?

The astronauts relied on inertia, the tendency of a moving object to keep moving. Sir Isaac Newton first described inertia in his Laws of Motion. If an object is not subjected to any force, it will continue to move in the same direction forever. Inertia explains why a car will keep rolling on a flat parking lot, even if you place it in neutral gear so the engine is not affecting the wheels. Eventually the car stops rolling because of friction with the ground and the air. A spacecraft that is above the atmosphere, however, does not experience the force of friction.

### Definition

Inertia is the tendency of a moving object to continue moving in the same direction at the same speed until a force acts on it, and of a stationary object to remain stationary until a force acts on it.

A rocket does experience a force as it moves away from Earth—gravity. This is the same force that keeps a baseball thrown upward from traveling forever in a line directly away from your hand. The Saturn rocket accelerated the Apollo craft to a speed of about 7 miles per second, using a lot of energy in the process. At this speed, the forward motion of the craft due to inertia was greater than the change due to the force exerted by Earth’s gravity. It continued to move away from Earth even though there was no longer any force pushing it away.

At some point in its travel, the force of gravity that pulled the Apollo craft toward the moon exceeded the force pulling it toward Earth. Then it began to actually accelerate toward the moon. On the return trip, the rocket used its remaining fuel to accelerate the craft against the force of the moon’s gravity. Fortunately, because the moon is much smaller than Earth, the amount of fuel needed was much smaller.

Spacecraft can use gravity to help them move toward their destination in other ways. When the Cassini-Huygens spacecraft was launched on a course to Saturn in 1998, it did not travel directly to its destination. Instead, it was launched around the sun and close to the planet Venus. As it approached the planet, it gained speed due to the planet’s gravitational pull. The craft then passed close to Earth, picking up more speed. Finally, in the year 2000, Cassini-Huygens made a pass near the giant planet, Jupiter. As it approached Jupiter, the 6-ton spacecraft’s speed increased by about 36,000 mph and its course bent to send it to Saturn. The energy used to increase the speed was balanced by an equal energy change that affected the speed of Jupiter’s orbit around the sun. The difference in mass means that the change in speed of the planet was quite a bit smaller than the change in speed of the spacecraft. As Cassini-Huygens sped up, Jupiter slowed by a fraction of an inch every billion years.

The Saturn V rocket is the biggest engine ever built. During the first 8 minutes of an Apollo launch, the Saturn V generated enough power to supply New York City with all its electricity needs for 75 minutes.

## Mow do airbags protect automobile passengers?

Automobile accidents illustrate the concept of inertia, the tendency of a moving object to keep moving in the same speed and direction and a stationary object to remain stationary. As you drive down the road at 65 mph, you move at the same speed as your car. What happens if the car stops moving suddenly—for example, by striking a bridge abutment?

A moving object also has momentum, which is the product of its mass and its velocity. During a collision, momentum is conserved, which means that the total momentum of the objects before the collision is the same as the total momentum of the objects after a collision. The inertia of the car would tend to keep it moving, but the concrete abutment has much more mass than the car. Its inertia tends to keep it from moving. Because of the high mass of the abutment, its velocity does not change much but the velocity of the car changes drastically.

Any objects inside the car, including passengers, have their own inertia and momentum. They continue moving at the same speed the car was traveling unless something interferes with their motion. Left on their own, they travel at full speed until the force of a collision with the windshield, or some other object, reduces their velocity. This is not a healthy outcome.

So how do you protect yourself? There are two ways to reduce the effects of the force of the impact—spread force over a larger area or spread it over a longer time. Seat belts spread the force over a larger area of the body. In addition, they transfer the force to the torso, which handles impacts better than the head.

When you slam on the brakes, you seem to be thrown forward. It is easy to think that there is a force pushing you. However, the force is acting on the car around you. The passengers keep moving forward inside the car because no force has yet acted to stop their forward motion. The seat belt and airbag provide force so that the windshield does not have to.

Air bags provide even better protection. In addition to spreading the force of the collision over a much larger area—your entire upper body—air bags increase the time over which the force is applied. Although the airbag deflates in a fraction of a second after it fills, that time is substantially longer than the contact time of a person against a steering wheel or dashboard. A longer impact time means the force is not as great.

## Can you really move Earth with a long enough lever?

The ancient Greek philosopher Archimedes is said to have stated, "Give me a lever and a place to stand, and I will move Earth." Was this an idle boast, or did Archimedes really see the potential of the lever? Well, perhaps it was a bit of both.

The lever is a simple machine. You exert a force at one point and it delivers a force at a different point. In the process, you change the amount of force, the direction of force, or both. However, if you increase the amount of force exerted, there is a cost. That cost is distance. Think about using a crowbar to lift a heavy rock. What happens when you push down on the bar? One end moves, maybe a foot or so. The other end lifts the rock maybe a couple of inches.

The key to operating the lever is work. In physics, the definition of work is the product of the force applied and the distance moved. The work put into the lever is the same as the work delivered at the other end.

So when you decide to move the rock with the crowbar, you trade force for distance. If you have ever suffered a bad back strain by trying to exert more force than your body was designed to deliver, you know that it is a fair trade. In order to use even less force, you substitute a longer lever. Replace the crowbar with a 6-foot pry bar and you can raise the rock with a smaller push. Or you can lift a bigger rock. That’s where Archi-medes’s boast comes in. Could he move Earth with a lever?

In physics, a machine is something that changes the direction or magnitude, or both, of an applied force. All machines are built using combinations of the six basic machines: lever, screw, inclined plane, wedge, wheel and axle, and pulley.

In principle, Archimedes is right—assuming that you could find a sufficiently long lever, and something to balance it on, and a place to stand, and you ignore the effects of gravity and Earth’s motion, and so on. You get the picture. With all those caveats, what kind of lever would move Earth? Well, Earth is pretty heavy by human standards—about 6,000,000,000,000,000,000,000,000 tons. Let’s give Archimedes a bit of help and provide him with a team of elephants capable of lifting 6 tons of mass. How long would the lever need to be to lift Earth by 1 inch? Because the total amount of work done by the elephants is the same as the work done on the planet, we can calculate the long end of the lever, if the short end is 1 inch. The tip of the lever would be about 500,000 times as far from the solar system as is the nearest star. There is another complication. If the elephants were to push at the speed of light (the fastest possible speed), it would take them more than 2 million years to move Earth 1 inch. So, in practical terms, it is fair to say that Archimedes could not move Earth even with his amazing lever.

"One of the elementary rules of nature is that, in the absence of law prohibiting an event or phenomenon, it is bound to occur with some degree of probability. To put it simply and crudely: Anything that can happen, does happen." —Paul Dirac (1902-1984)

## Why does an extension bar make a wrench work better?

When I worked in a very small chemical manufacturing plant, we used reaction vessels that held 1,000 gallons or more of chemicals. We poured solids in through a manhole-size opening on top and closed and clamped the cover. The clamps were bolted into place using a P^-inch box wrench. Unfortunately, one of our workers was a strapping young man who could tighten the bolts around the cover so securely that none of the other employees could remove them later. The only way to open the vessel was to put a 3-foot piece of iron pipe over the wrench to make it longer. Why would the extension bar make it possible to loosen a bolt that was otherwise too tight?

The extension allowed the rest of us to increase the torque that we applied to the bolt. A wrench acts like a rotating lever. As in any lever, the work done is the product of the force and the distance. As the distance from the pivot point increases, the effect of the force that you apply to the wrench also increases. The magnitude of the torque is a product of force and distance.

Those of us who could not apply as much force gained an advantage by using this rotating lever. Keep in mind, though, that there is always a cost. We applied less force but we had to push much farther to obtain the same effect.

The wrench works best when your push is perpendicular to its handle. This is because the force that adds to torque is only that part that is perpendicular to the handle. This makes sense if you think of pedaling a bicycle. The pedal arm rotates around an axle in the same way that a wrench rotates around a bolt. You feel the greatest effect of your pedaling when the arm is parallel to the ground, that is, perpendicular to your leg. When the pedal is at its lowest point, you can push down with as much force as you can generate but it will not propel the bike. As you pedal, you alternate your force from one side to the other to obtain the maximum result from the force generated by your muscles.

A wheel-and-axle combination is a kind of rotating lever. A force applied to the outside of the wheel causes a rotation. Or, a force applied to the axle can cause the wheel to rotate. In this case, the torque is very small because the distance from the pivot point is small.

Torque is a force that causes a change in rotation. The magnitude of a torque is calculated by multiplying the force by the distance from the center of rotation to the point where the force is applied.

Gears are an application of the wheel and axle. If force is applied to the outside of the gear, it rotates around the pivot point. Increasing the size of the gear increases the torque. The same amount of horsepower can move a large dump truck or a race car. Their engines are used in different ways, however. The truck engine generates a large amount of torque, which allows it to apply enough force to move a heavy load. The race car generates much less torque with the same applied force. However, it tends to get from one place to another in a shorter time.

## Why do figure skaters spin faster when they pull their arms in?

During an ice-dancing routine, a skater starts spinning in place on the ice with her arms extended. As she draws her arms inward, raising them above her head, she spins faster and faster without exerting any force. What causes her spin rate to increase if she is not adding energy from her muscles?

When an object is moving in a straight line, it has momentum that tends to keep its motion constant. When an object is rotating around an axis, it has angular momentum, a tendency to keep rotating. Angular momentum does not change unless a force causes a torque that increases or decreases the momentum. For example, a toy top, once it is set in motion, continues to spin for a long time. Eventually it slows and stops because there is a force—friction— that acts to cause a torque that changes the rotation. The skater’s sharp skates on polished ice minimize friction so she keeps spinning.

Angular momentum is the measure of the tendency of an object rotating around an axis to keep rotating unless a torque is applied to it.

Each part of a rotating object has a moment of inertia, which is the product of its mass and the square of its distance from the axis of rotation. The angular momentum is the product of the moment of inertia and the rotational speed. The angular momentum of the spinning body is the sum of the angular moment of each of its parts. If you are talking about an object such as a sphere, whose mass is evenly distributed around the axis, the calculation of angular momentum is fairly simple. If the spinning shape is irregular, however, such as a person, with a mass that is not evenly distributed around the axis, the calculation becomes complex. Fortunately, it is not necessary to calculate angular momentum in order to use it.

Scientists have observed that angular momentum is always conserved. That means that the total angular momentum of a spinning object does not change unless a force acts on it. Decreasing the distance from the axis of rotation decreases the moment of inertia by the square of the change in distance, so cutting the distance in half decreases the moment of inertia by a factor of four. If the moment of inertia decreases but the angular momentum stays constant, the rate of rotation must increase. As the skater brings her arms in, she spins faster and faster, going from about 2 rotations per second to 10 or more rotations per second.

The conservation of angular momentum also explains why Earth spins on its axis. According to one theory, the solar system formed from a large cloud of cosmic dust which was spinning in space. Parts of the cloud were pulled together by gravity, forming pockets of greater density which pulled even more matter inward. These clumps of matter eventually formed the sun and the planets. As the rotating material pulled closer and closer together, the total angular momentum was conserved. A large, slowly rotating cloud of matter spread out across vast areas of space became a small number of rapidly spinning balls of dense matter.

A spinning object that experiences no force will continue spinning. During NASA’s Gravity Probe B experiment, precision gyroscopes, spinning at 4,300 rotations per minute in a vacuum, were sent into space. During the course of the experiment, the gyroscopes turned billions of times without any additional energy being added.

## What keeps riders in their seats on a looping roller coaster?

When you get into a roller coaster that loops upside down, the attendant makes sure that your safety belt or safety bar is in place. These devices are not what keeps you in place, though. Physics takes care of that.

Acceleration is a change in direction or speed. When something moves in a circle, it is constantly changing direction, so it is constantly accelerating, even if its speed remains the same. The force that causes the acceleration toward the center of a circle is called centripetal force. Think about what happens if you swing an object attached to a string in a circle. The string remains tight and its pull forces the object to move in a circle. If you let go of the string, the object flies away. You can feel the constant force at the other end of the string.

The same thing happens when you ride a roller coaster through a loop. Because you are constantly changing direction, you are accelerating the whole time. The track is constantly pushing the car into a new direction. (It may seem strange that a stationary object can push, but in physics, forces come in pairs. The car pushes against the track and the track pushes against the car.) As a result, the seat of the car pushes you in the same direction.

When you are at the bottom of the loop, you don’t notice the upward push; you are used to it because there is normally an upward push against the downward pull of gravity. As you go through the loop, your own inertia tends to keep you moving in a straight line away from the center of the circle. At the same time, centripetal force is always pushing you toward the center of the circle. As you go over the top, you feel like you should continue up into the air, but the car is pushing against you to prevent it. Even without the safety bar, you would stay in the car.

Centripetal force also affects you when you go through the bottom of a dip or over the top of a hill on the roller coaster. As the roller coaster passes through a dip, centripetal force constantly pushes it upward toward the center of the curve. At the same time, the rider is pushed downward by gravity. The combination of the upward force and gravity makes you feel heavier in the dip. As you ride over top of a hill, centripetal force pulls the car beneath you toward the center of the curvature. However, the car does not push on you this time because you are above it and the force is downward. Your inertia carries you upward until the safety bar exerts a downward force. This is when you realize how important the bar is. If the force is large enough, you will feel weightless at the point where the downward force of gravity is exactly equal to your inertia upward.

The feeling that you are being pushed outward during a circular motion is often called "centrifugal force." In reality, there is no outward force during circular motion. What is called centrifugal force is actually a result of inertia, the tendency to keep moving in a straight line. No force is pushing outward.

## How does the space station stay in orbit?

Like any other satellite, including the moon—a natural satellite—the International Space Station (ISS) orbits Earth. It follows an almost circular path around the planet almost 16 times each day. What keeps the ISS in its orbit?

The station has been in place since its assembly was begun in 1998 and has been inhabited since 2000. The space station is located in a low Earth orbit, about 210 miles above the surface. As it is built, each piece is moved into the orbit by a rocket-powered craft, such as an American space shuttle or a Russian Soyuz module. Once placed in orbit, the assembled parts continue in a course around Earth with no additional propulsion. In effect, the space station is always falling but never hitting the ground.

Remember, due to inertia, a moving object continues moving in a straight line unless a force acts on it. There is a force that acts on every object near Earth—the force of gravity. If you throw a ball, it does not continue moving indefinitely in a straight line because gravity causes it to move toward the center of the Earth. In a few seconds, the ball collides with Earth and the force of friction stops its motion.

The space station in its orbit also has inertia that causes it to tend to move in a straight line away from Earth. However, it is also subject to the force of gravity. It is pulled toward the center of the planet. This force is at a right angle to its inertial motion so it does not change the forward motion. It does pull the space station downward, just like any other falling object. While gravity is pulling downward, the tendency to move in a straight line would carry it away from Earth. As a result, the space station is always falling but always the same distance from the surface. Gravity is the centripetal force on the satellite, pulling it toward the center of its orbit.

If some force were to slow the forward motion of the space station, gravity would pull it to Earth. Satellite orbits must be located above the atmosphere. Otherwise, the force of friction with the air would slow the satellite, causing it to fall. If the force of gravity were suddenly to disappear, the space station would head away in a straight line.

Gravity is a force of attraction between any two objects. The force of gravity increases proportionally to an increase in mass of the objects and decreases proportionally to the square of the distance between the objects.

In fact, the space station does not hold a perfect orbit. There is some atmospheric friction, even though the atmosphere is very thin at its altitude, so it tends to lose about a mile and a half of altitude every month. When it reaches a lower altitude limit, the station is pushed several miles higher by a rocket or space shuttle.

Because the space station and everything in it is falling freely, people inside it experience weightlessness. This does not mean that they are unaffected by gravity. When we stand on Earth, the weight that we feel is not the pull of gravity. Instead, it is the upward force exerted by the ground or other object beneath us. This is a reaction force that pushes in the direction opposite the gravitational pull.

Think about descending in a very fast elevator. The floor of the elevator is dropping rapidly so the sensation of weight is reduced. You feel almost as if you were able to float. This is what is happening when astronauts experience weightlessness. The space station and all its contents, including people, are falling freely all the time. Because all of the walls are moving in the same free manner, astronauts do not feel any reaction forces to gravity. In effect, they float freely with their surroundings.

"A raised weight can produce work, but in doing so it must necessarily sink from its height, and, when it has fallen as deep as it can fall, its gravity remains as before, but it can no longer do work."

Because of the term "weightless," many people believe that the space station is outside Earth’s gravitational field. In fact, the strength of Earth’s gravity at the altitude of the space station is about 90 percent of its strength at the surface. Earth even exerts a gravitational force on the most distant galaxies, although that force would be far too small to measure.

## Why can’t you cool the kitchen by opening the refrigerator?

On a hot day, there is one cool spot in the kitchen—inside the refrigerator. If you let the cool air flow from the refrigerator into the room, the temperature of the room should drop, right? Unfortunately, it doesn’t work that way.

The problem boils down (no pun intended) to physics. Specifically, the laws of thermodynamics say that there is no way for you to get something for nothing. So, if you make one place cooler, you have to make another place warmer. As you cool your perishable foods, you heat up the kitchen. A refrigerator is a kind of heat pump. It removes heat from the inside of a box and then it has to dump the heat somewhere else, outside the box. In order to move heat, it compresses a gas. The gas gets warm and then it is cooled by a fan. The heat goes out of the back of the fridge. Place your hand behind your refrigerator when it is running and you should feel a flow of warm air. The cooled gas is then allowed to expand inside coils on the cool side of the box. When gas expands, it gets even cooler than the air around it. Heat flows from the air to the coils. Then the gas is again compressed and cooled, starting the cycle again.

Left to its own devices, heat only flows from warm places to cool places. To make it go the other way, making the cool inside of the fridge even cooler, requires work. To do that work requires a machine and it is impossible for a machine to be 100 percent efficient. That means that as it runs, the machine dumps the heat from the inside of the refrigerator and it dumps even more heat due to its inefficiency. In fact, at its best, a refrigerator has an efficiency of about 40 percent. The additional heat comes from running the motor and compressor.

So what does this all mean for kitchen cooling? Initially, you will dump cool air from the refrigerator into the kitchen, dropping the average temperature of the room by some amount. Immediately, however, the thermostat in the refrigerator detects that the temperature inside has risen. The motor kicks on, the compressor kicks on, the system starts cooling the air in the box but it is quickly warmed by air coming through the open door. The final result, thanks to the inefficiency of the heat pump and the dumping of the same heat that has been removed, is that the room heats by about 2′/2 degrees for every degree that it is cooled.

The expansion and contraction of a gas is not the only process that can be used in refrigeration. Any reversible process that involves an energy change could possibly be used. Currently many researchers are studying magnetic refrigeration based on changes in energy of a magnetic material when it is in a magnetic field compared to its energy when it is not in a magnetic field.

## What is the difference between A C and D C electric current?

An electric current is a stream of moving electrons, tiny particles with an electric charge. The motion of the electrons carries energy that can be used to do work. There are two primary ways that the energy can be tapped. When you turn on a light bulb, the filament inside the bulb resists the flow of electrons. This resistance is caused by interactions with atoms in the filament and energy is produced as heat and light. Electric heating elements work in heaters and stovetops the same way. Electrical energy can also be converted to mechanical energy in an electric motor by the interaction of the electric current and a magnetic field to produce motion. You have probably seen electrical devices labeled for AC (alternating current) or DC (direct current). What is the difference between the two types of current?

In direct current, electrons flow from one place to another in a single direction. For example, when you turn on a flashlight, electrons flow from the negative pole of the battery, through the light bulb, and then to the positive terminal. As they pass through the bulb, the electrons lose energy and light is emitted. In an alternating current, the electrons don’t flow from point to point. Instead, they move back and forth inside the wire, changing direction in a cycle of 60 times each second. When alternating current flows in a light bulb, the electrons move back and forth within the element, losing energy due to resistance to their motion.

Early in the history of electric power, both types of current were used to transmit electricity. The first power plants used direct current. However, alternating current is able to transmit energy over much greater distances without losing energy. In addition, AC voltage can be changed very easily using a transformer. It is more efficient to transport the power over long distances at very high voltage, but for use inside homes and businesses, a lower voltage is necessary for safety. A series of transformers in the transmission and delivery system reduces the voltage from production to use.

For some uses, it does not matter whether the power source is AC or DC. Electric lighting, for example, which is based on resistance to current, works with either type of power. Other applications require a specific type of electric power. Electric motors are designed to work with either AC or DC current and will not operate if the wrong source is used.

Direct current is normally used in low-voltage applications. Batteries and solar power systems can only produce direct current. Most electronic devices also require DC power. In order to operate these devices on the household AC current, an adapter is needed. AC adapters convert the alternating current into a direct current, which operates the device or charges the battery without damage.

Although the current flows instantaneously through a circuit when it is turned on, each individual electron moves slowly within the circuit. In a DC circuit, such as the one between a car’s battery and its headlights, each electron may move about 1 millimeter per second, pushing along the electron ahead of it. In an AC circuit, the electrons do not move through the circuit at all. Each electron moves back and forth along the wire.

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