Category Forces of Nature

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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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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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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