Category Forces in Action, Forces in Motion

In Japan, “maglev” trains run just above, not on, their tracks. Both the bottom of the train and the track itself are magnetic. The magnets repel each other, so the train hovers just above the track, enabling it to run with less friction and so reach higher speeds.

Maglev trains incorporate a basic fact about magnetic forces—like magnetic poles repel each other, and opposite magnetic poles attract each other—to lift, propel, and guide a vehicle over a track (or guideway). Maglev train propulsion and levitation may involve the use of superconducting materials, electromgnets, diamagnets, and rare-earth magnets.

Two types of maglev trains are in service. Electromagnetic suspension (EMS) uses the attractive force between magnets present on the train’s sides and underside and on the guideway to levitate the train. A variation on EMS, called Transrapid and used in Germany, employs an electromagnet to lift the train off the guide way. The attraction from magnets presents on the underside of the vehicle that wraps around the iron rails of the guideway keep the train about 1.3 cm (0.5 inch) above the guideway.

Electrodynamic suspension (EDS) systems are similar to EMS in several respects, but the magnets are used to repel the train from the guideway rather than attract them. These magnets are supercooled and superconducting and have the ability to conduct electricity for a short time after power has been cut. (In EMS systems a loss of power shuts down the electromagnets.) Also, unlike EMS, the charge of the magnetized coils of the guideway in EDS systems repels the charge of magnets on the undercarriage of the train so that it levitates higher (typically in the range of 1–10 cm [0.4–3.9 inches]) above the guideway. EDS trains are slow to lift off, so they have wheels that must be deployed below approximately 100 km (62 miles) per hour. Once levitated, however, the train is moved forward by propulsion provided by the guideway coils, which are constantly changing polarity owing to alternating electrical current that powers the system.

Maglev trains eliminate a key source of friction—that of train wheels on the rails—although they must still overcome air resistance. This lack of friction means that they can reach higher speeds than conventional trains. At present maglev technology has produced trains that can travel in excess of 500 km (310 miles) per hour. This speed is twice as fast as a conventional commuter train and comparable to the TGV (Train à Grande Vitesse) in use in France, which travels between 300 and 320 km (186 and 199 miles) per hour. Because of air resistance, however, maglev trains are only slightly more energy efficient than conventional trains.

The earth’s north and South Poles attract charged particles from the Sun. Within the atmosphere, these collide with molecules of gas to cause spectacular light shows, called the aurora borealis (northern dawn), which can be seen in the Arctic Circle.  When the weather conditions are right, the aurora borealis, also known as the northern lights, can sometimes be seen outside the Arctic Circle in the northern hemisphere.

Our sun is 93 million miles away. But its effects extend far beyond its visible surface. Great storms on the sun send gusts of charged solar particles hurtling across space. If Earth is in the path of the particle stream, our planet’s magnetic field and atmosphere react. When the charged particles from the sun strike atoms and molecules in Earth’s atmosphere, they excite those atoms, causing them to light up.

What does it mean for an atom to be excited? Atoms consist of a central nucleus and a surrounding cloud of electrons encircling the nucleus in an orbit. When charged particles from the sun strike atoms in Earth’s atmosphere, electrons move to higher-energy orbits, further away from the nucleus. Then when an electron moves back to a lower-energy orbit, it releases a particle of light or photon.

What happens in an aurora is similar to what happens in the neon lights we see on many business signs. Electricity is used to excite the atoms in the neon gas within the glass tubes of a neon sign. That’s why these signs give off their brilliant colors. The aurora works on the same principle – but at a far more vast scale.

The aurora often appears as curtains of lights, but they can also be arcs or spirals, often following lines of force in Earth’s magnetic field. Most are green in color but sometimes you’ll see a hint of pink, and strong displays might also have red, violet and white colors. The lights typically are seen in the far north – the nations bordering the Arctic Ocean – Canada and Alaska, Scandinavian countries, Iceland, Greenland and Russia. But strong displays of the lights can extend down into more southerly latitudes in the United States. And of course, the lights have a counterpart at Earth’s south Polar Regions.

The colors in the aurora were also a source of mystery throughout human history. But science says that different gases in Earth’s atmosphere give off different colors when they are excited. Oxygen gives off the green color of the aurora, for example. Nitrogen causes blue or red colors.

So today the mystery of the aurora is not, so mysterious as it used to be. Yet people still travel thousands of miles to see the brilliant natural light shows in Earth’s atmosphere. And even though we know the scientific reason for the aurora, the dazzling natural light show can still fire our imaginations to visualize fire bridges, gods or dancing ghosts.

The fact that an electromagnet ceases to become a magnet when the current is turned off can be used to great effect in large and small machines. For example, a powerful magnet can lift very heavy weights of iron and steel in a factory, but that would be no good if the magnet could not be persuaded to release them. With an electromagnet, the current can be stopped and the load released.

Magnets come in two main types: permanent magnets and electromagnets. As its name suggests, a permanent magnet is always magnetized — think of a kitchen magnet that stays stuck to a refrigerator door for years. An electromagnet is different; its magnetism works only when powered by electricity. Although an electromagnet is more complicated than a permanent magnet, it has useful and important advantages.

One of the most important features of an electromagnet is the ability to change its magnetic force. When no electric current flows through the magnet’s wires, it has no magnetic force. Put a little current in the magnet, and it has a small force. A large current gives the magnet a bigger force, able to lift or pull heavier objects. The ability to turn magnetic force on and off has many important uses, ranging from simple household gadgets to giant industrial machines.

The pulling power of a permanent magnet is limited to the type of metal from which it’s made. Currently, the strongest permanent magnets are made of a combination of iron and a metal called neodymium. Although these permanent magnets are strong, the best electromagnets are more than 20 times stronger.

Small electromagnets are used in electronic locks, such as those found on an automobile or the main door of an apartment building. Scrapyard cranes have powerful electromagnets that lift metal car bodies with ease. Magnetic Resonance Imaging machines use very powerful electromagnets to produce highly detailed images of the human body. The strongest electromagnets are those used in scientific research to study the properties of matter.

An electromagnet can save people who live up several flights of stairs from having to walk down to the front door when the bell rings. They can simply find out who is calling by means of an intercom and then press a switch to let the caller in. The switch turns on a current that activates an electromagnet. The magnet attracts the door latch, pulling it back and allowing the visitor to enter. Then a spring allows the latch to slip back into place.

You can find small permanent magnets in toys, handheld gadgets such as electric razors, and clasps for bracelets and watches. Larger permanent magnets are useful in household appliance motors and in stereo speakers. The electric motors in hybrid vehicles use very strong permanent magnets.

The earth has a core of molten iron and is itself a huge magnet. Its magnetic field acts as though there were a bar magnet running along the axis of the Earth. A compass contains a magnetized needle, which can turn freely. No matter which direction the compass is facing, the needle will turn to point towards the North Pole. The compass can then be rotated so that its north point lines up with the needle and the other directions can be read.

If you’re lost in the woods, your best chance of finding your way might be a tiny magnet. A magnet is what makes a compass point north — the small magnetic pin in a compass is suspended so that it can spin freely inside its casing and respond to our planet’s magnetism. A compass needle aligns itself and points toward the top of Earth’s magnetic field, giving explorers and lost souls a consistent sense of direction.

A compass points north because ll magnets have two poles, a north pole and a south pole, and the north pole of one magnet is attracted to the south pole of another magnet. (You may have seen this demonstrated by a pair of simple bar magnets or refrigerator magnets pushed end to end.)

The Earth is a magnet that can interact with other magnets in this way, so the north end of a compass magnet is drawn to align with the Earth’s magnetic field. Because the Earth’s magnetic North Pole attracts the “north” ends of other magnets, it is technically the “South Pole” of our planet’s magnetic field.

While a compass is a great tool for navigation, it doesn’t always point exactly north. This is because the Earth’s magnetic North Pole is not the same as “true north,” or the Earth’s geographic North Pole. The magnetic North Pole lies about 1,000 miles south of true north, in Canada.

And making things even more difficult for the compass-wielding navigator, the magnetic North Pole isn’t even a stationary point. As the Earth’s magnetic field changes, the magnetic North Pole moves. Over the last century, it has shifted more than 620 miles (1,000 kilometers) toward Siberia, according to scientists at Oregon State University.

This difference between true north and the north heading on a compass is an angle called declination. Declination varies from place to place because the Earth’s magnetic field is not uniform it dips and undulates.

These local disturbances in the field can cause a compass needle to point away from both the geographic North Pole and the magnetic North Pole. According to the United States Geological Survey, at very high latitudes, a compass needle can even point south.

By using charts of declination or local calibrations, compass users can compensate for these differences and point themselves in the right direction.

Like the earth itself, each magnet has a north and a south pole. If it can turn freely, the north pole of a magnet will turn towards the North Pole of the Earth. The south pole of a magnet will be attracted towards the South Pole of the Earth. Confusingly, the Earth’s North Pole actually has a south magnetic pole, which is why the north pole of a magnet is attracted to it. For the rule is that like poles repel each other (push each other away), while unlike poles attract.

The areas of a magnet that have magnetic strength are called “poles”. When you have more than one magnet, like (or same) poles repel, or push, each other. Opposite poles attract, or pull, each other. In other words, the north pole of one magnet will click together with the south pole of another magnet, and two north poles will push each other away. These acts of attraction and repulsion are called “magnetism”, and the magnetic space around a magnet is called the “magnetic field”.

Unless they came marked with “N” or “S,” the poles of a magnet look the same. One easy way to tell which pole is north and which is south is to set your magnet near a compass. The needle on the compass that normally points toward the North Pole of the Earth will move toward the magnet’s South Pole. This works because the needle in a compass is actually a magnet! So the north pole of the compasses’ needle magnet is attracted to the south pole of your magnet.

Another way to tell which is north and which is south is by dangling your magnet from a string. When you dangle a magnet, it automatically turns itself so that one pole is pointing directly north and the other directly south, which is why we call them the “north” and “south” poles.

Remember in the intro when we said all magnets have AT LEAST two poles? Well bar magnets have two poles, so their magnetic fields are called dipole which means–you guessed it–two poles! But some magnets have more than two poles. In fact, some have four, or six, or even eight! (These are called “octopoles”– doesn’t that sound like a cool Play Station™ game?) Some celestial objects, including stars and planets, have magnetic fields, and some of them have more than two poles! We call these fields “multipoles.” Earth is a good example of a dipole magnetic field–we have one North Pole and one South Pole (with a few weaker multipole parts, but let’s not get into that right now).

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When an electric current runs through a wire, it creates a magnetic field. If the wire is wound round and round an iron core, the coil and core become strongly magnetized whenever the current is turned on. The coil of wire is called a solenoid.

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the hole, denoting the centre of the coil. The magnetic field disappears when the current is turned off. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet.

The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field.

Electromagnets are widely used as components of other electrical devices, such as motors, generators, electromechanical solenoids, relays, loudspeakers, hard disks, MRI machines, scientific instruments, and magnetic separation equipment. Electromagnets are also employed in industry for picking up and moving heavy iron objects such as scrap iron and steel.

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Forces act in a straight line unless some-thing changes their direction. A carriage travelling around a fairground ride would go straight on if the track did not take it round in a circle, forcing it to change direction all the time. The forward-acting force keeps the carriage on the tracks, even when it is upside down. This is known as a centrifugal force.

A stone whirling in a horizontal plane on the end of a string tied to a post on the ground is continuously changing the direction of its velocity and, therefore, has acceleration toward the post. This acceleration is equal to the square of its velocity divided by the length of the string. According to Newton’s second law, acceleration is caused by a force, which in this case is the tension in the string. If the stone is moving at a constant speed and gravity is neglected, the inward-pointing string tension is the only force acting on the stone. If the string breaks, the stone, because of inertia, will keep on going in a straight line tangent to its previous circular path; it does not move in the outward direction as it would if the centrifugal force were real.

Although it is not a real force according to Newton’s laws, the centrifugal-force concept is a useful one. For example, when analyzing the behaviour of the fluid in a cream separator or a centrifuge, it is convenient to study the fluid’s behaviour relative to the rotating container rather than relative to the Earth; and, in order that Newton’s laws be applicable in such a rotating frame of reference, an inertial force, or a fictitious force (the centrifugal force), equal and opposite to the centripetal force, must be included in the equations of motion. In a frame of reference attached to the whirling stone, the stone is at rest; to obtain a balanced force system, the outward-acting centrifugal force must be included.

Centrifugal force can be increased by increasing either the speed of rotation or the mass of the body or by decreasing the radius, which is the distance of the body from the centre of the curve. Increasing the mass or decreasing the radius increases the centrifugal force in direct or inverse proportion, respectively, but increasing the speed of rotation increases it in proportion to the square of the speed; that is, an increase in speed of 10 times, say from 10 to 100 revolutions per minute, increases the centrifugal force by a factor of 100. Centrifugal force is expressed as a multiple of g, the symbol for normal gravitational force (strictly speaking, the acceleration due to gravity). Centrifugal fields of more than 1,000,000,000g have been produced in the laboratory by devices called centrifuges.

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A magnetic field is the area around a magnet in which its magnetic force operates. A magnetic object that is placed within the field will be attracted or repelled by the magnet. When iron filings (tiny slivers of iron) are placed near a magnet, they line up to show its magnetic field. In fact, each tiny piece of iron has become a small magnet. The mini-magnets show how strongly each part of the large magnet attracts them.

Magnetism is one aspect of the combined electromagnetic force. It refers to physical phenomena arising from the force caused by magnets, objects that produce fields that attract or repel other objects. A magnetic field exerts a force on particles in the field due to the Lorentz force, according to Georgia State University’s Hyper-Physics website. The motion of electrically charged particles gives rise to magnetism. The force acting on an electrically charged particle in a magnetic field depends on the magnitude of the charge, the velocity of the particle, and the strength of the magnetic field.

All materials experience magnetism, some more strongly than others. Permanent magnets, made from materials such as iron, experience the strongest effects, known as ferromagnetism. With rare exception, this is the only form of magnetism strong enough to be felt by people.

Magnetic fields are generated by rotating electric charges, according to Hyper-Physics. Electrons all have a property of angular momentum, or spin. Most electrons tend to form pairs in which one of them is “spin up” and the other is “spin down,” in accordance with the Pauli Exclusion Principle, which states that two electrons cannot occupy the same energy state at the same time. In this case, their magnetic fields are in opposite directions, so they cancel each other. However, some atoms contain one or more unpaired electrons whose spin can produce a directional magnetic field. The direction of their spin determines the direction of the magnetic field, according to the Non-Destructive Testing (NDT) Resource Center. When a significant majority of unpaired electrons are aligned with their spins in the same direction, they combine to produce a magnetic field that is strong enough to be felt on a macroscopic scale. 

Magnetic field sources are dipolar, having a north and south magnetic pole. Opposite poles (N and S) attract, and like poles (N and N, or S and S) repel, according to Joseph Becker of San Jose State University. This creates a toroidal, or doughnut-shaped field, as the direction of the field propagates outward from the north pole and enters through the south pole. 

The Earth itself is a giant magnet. The planet gets its magnetic field from circulating electric currents within the molten metallic core, according to Hyper-Physics. A compass points north because the small magnetic needle in it is suspended so that it can spin freely inside its casing to align itself with the planet’s magnetic field. Paradoxically, what we call the Magnetic North Pole is actually a south magnetic pole because it attracts the north magnetic poles of compass needles.

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Most objects have more than one force acting on them at any one time. If an object is not moving, it is said to be in equilibrium, meaning that all the forces acting on it cancel each other out.

If both dogs are pulling with equal force, the shift will not move. It will be in equilibrium. Of course, if the forces on it re too strong the fabric itself will tear apart.

Often, the forces acting on an object are not balanced but combine together to have a certain effect, called the resultant. If the direction and size of all the forces acting on an object are known, the resultant can be calculated.

There are many forces operating on this balloon. The force of gravity is pulling it downwards towards the ground. The hot air is creating an upward force called lift. The pushing force of the wind is blowing the balloon along. The friction force of the air on the balloon is slowing it down. If the size and direction of all the forces are known, the direction and speed of the balloon can be worked out.

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A device that can change the direction of a force is called a machine. It may be very simple, such as a lever. This can change a force pushing down on one end of the lever into a force pushing up on an object at the other end of the lever. A bicycle is able to move along because the crankshaft changes the force of your feet pushing down into the turning force of the wheels going round.

An applied force affects the motion of an object. An applied force can be a push, pull, or dragging on an object. The push can come from direct contact, like when objects collide or from a force field like magnetism. The pull seems to only come from a field at a distance, like gravity or magnetism. Dragging can occur when sliding an object over the surface of another.

The action from a force can cause an object to move or speed up (accelerate), to slow down (decelerate), to stop, or to change direction. Since any change in velocity is considered acceleration, it can be said that a force on an object results in the acceleration of an object.

When a force acts on an object that is stationary or not moving, the force will cause the object to move, provided there are no other forces preventing that movement. If you throw a ball, you are pushing on it to start its movement. If you drop an object, the force of gravity causes it to move.

If an object is initially stationary, it accelerates when it starts to move. Acceleration is the change in velocity over a period of time. The object is going from v = 0 to some other speed or velocity. Likewise, if an object is already moving and a force is applied in the same direction, the object will speed up or accelerate. For example, a gust of wind can speed up a sailboat.

A force applied at an angle to the direction of motion of an object can cause it to change direction. A side wind will cause an airplane to change its direction.

It is possible that the object keeps going at the same speed, if the force is applied perpendicular to the direction of motion. But the velocity of the object changes. Speed is how fast the object is going, while velocity is speed plus direction.

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Friction is a force that slows the motion of two surfaces when they move across each other. An engine has many moving parts. If they rub against each other, creating friction, the efficiency of the engine is affected. The friction creates heat and the engine needs more energy to work. The parts also wear down as they come into contact. To reduce the friction in an engine, a lubricant, such as oil, is put between the moving parts.

There are also times when friction is useful. For example, if there were no friction between our feet and the ground, we would fall over. This can happen when a floor is polished or there is ice on the ground. There is less friction between our feet and these surfaces, so we easily slip.

The grooves on a tyre help to push water on the surface of the road out of the way. This means that there is more friction between the tyre and the road, preventing the car from skidding.

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Inertia is the resistance of any physical object to any change in its velocity. This includes changes to the object’s speed, or direction of motion. An aspect of this property is the tendency of objects to keep moving in a straight line at a constant speed, when no forces act upon them.

Inertia comes from the Latin word, iners, meaning idle, sluggish. Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems. Isaac Newton defined inertia as his first law in his Philosophiae Naturalis Principia Mathematica, which states:

The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.

In common usage, the term “inertia” may refer to an object’s “amount of resistance to change in velocity” or for simpler terms, “resistance to a change in motion” (which is quantified by its mass), or sometimes to its momentum, depending on the context. The term “inertia” is more properly understood as shorthand for “the principle of inertia” as described by Newton in his first law of motion: an object not subject to any net external force moves at a constant velocity. Thus, an object will continue moving at its current velocity until some force causes its speed or direction to change.

On the surface of the Earth, inertia is often masked by gravity and the effects of friction and air resistance, both of which tend to decrease the speed of moving objects (commonly to the point of rest). This misled the philosopher Aristotle to believe that objects would move only as long as force was applied to them.

The principle of inertia is one of the fundamental principles in classical physics that are still used today to describe the motion of objects and how they are affected by the applied forces on them.

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There are forces acting on us — and everything on our planet — all the time. The push and pull of forces is what keeps things where they are or starts them into motion. Forces enable something to stay the same size and shape or to change size and shape. They can slow down a moving object or speed it up, or change the direction of its motion. Whenever energy is being used, forces are at work.

In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI of newtons and represented by the symbol F.

The original form of Newton’s second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

Concepts related to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in Fluids.

There are many different kinds of force. They are affecting everyday objects around us all the time. Here are just some of the many forces that we experience.

An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is applied force acting upon the object. The applied force is the force exerted on the desk by the person.

The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed “downward” towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation:

The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force – sliding and static friction. Though it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.

The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said to obey “Hooke’s Law”), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.