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Types of ForcesFundamental Forces

There are only four types of forces in the Universe: Gravity, Electromagnetism, Weak Nuclear and StrongNuclear.

Most physicists believe that all four of these forces are actually different manifestations are actually differentmanifestations of a single underlying force.

Electric forces and magnetic forces were not unified until the early part of the 20 th century.Description of Forces

Gravity: the result of an object having mass.o Holds planets and stars together, makes things fall.

Electromagnetism:the force between charges.o Responsible for all familiar everyday forces (except gravity)

Weak Nuclear Forces: known for radioactive decay, where a neuron in an atom spontaneously decays intoa proton, releasing a high-energy electron and an antineutrino.

o Changes one flavour of quark into another. Strong Nuclear Forces: responsible for holding atomic nuclei together.

o Most of the elements on the periodic table have atomic nuclei with multiple protons. The protonsare positively charged and are extremely close together.

Since like charges repel each other, and the strength of the repulsion increases as theseparation decreases, one should expect that the nucleus of an atom will break apart.

Relative StrengthsGravity

Relative Strenght:1 Range: Infinite

Electromagnetism

Relative Strength: 1036 Range: Infinite

Weak Nuclear Forces

Relative Strength: 1025

Range: 1/1000th

the diameter of a typical atomic nucleusStrong Nuclear Forces

Relative Strength: 1038 Range: Diameter of a typical atomic nucleus

Gravitational Force Occurs between any two objects that have mass. The more massive an object is, the more it will attract

other massive objects.

Doubling the mass of one of the objects will increase the gravitational force by a factor of two. Whereas,increasing the separation by a factor of two will reduce the gravitational force by a factor of four.

o Explains why the Sun, which is far more massive than the Earth, exerts far less gravitational forceon us.

Since the force of gravity between two objects depends on the masses of both of these objects, thegravitational force that the Earth is pulling down on you with is equal to the gravitational force that you arepulling up on the earth with.

o When you jump, the fact that you fall back towards the Earth more than it falls up towards you tellsyou not that the Earth exerts more force on you than you do on the Earth, but that the force is moreeffective at moving you because of your smaller inertial mass.

o To quantitatively calculate the force of gravity, a constant of proportionality must be introduced.The symbol used for this constant of proportionality is a capital G, and it is referred to as theGravitational Constant. It is a constant for all matter in the Universe, and it has been constant for at least the last few billion years.

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Formula: Gis a constant of proportionality m1 and m2 are the two masses. R is the separation between the masses.

Electrostatic Force Electromagnetism includes forces generated by the motion of charged particles. However, the electrostatic

force is the electromagnetic force that occurs between any two stationary, charged objects.o There is a significant similarity between the electrostatic force and the gravitational force, but the

electrostatic force acts between charged objects. The more charge an object has, the more it willinteract with other charged objects.

An important distinction between the gravitational and the electrostatic force becomes apparent at this point.Charge can be either negative (electron) or positive (proton).

When the charges are either bot positive or both negative, the resulting force is repulsive. But when one ofthe charges is negative and the other is positive, the resulting force is attractive.

o This differs from the gravitational force, which always produces an attractive force (even with anti-matter).;

The inverse-squared relationship between the electrostatic force and separation is exactly equivalent to theinverse squared relationship between the gravitational force and the separation.

o So if you double the distance between two charged objects, you reduce the electrostatic force by afactor of four. Formula:

k is a constant of proportionality q1 and q2 are the two charges.ris the separation between the objects.

Forces of Convenience Tension Force:String, rope, wire Spring Force:Elasticity returns an object to its original shape Normal Force:Not thats were calling other forces abnormal Friction:When one surface moves with respect to another Drag: Air resistance, fluid resistance.

Tension Force This type of tension isnt responsible for your headache. This type of tension refers to a pulling force

transferred from one end of a medium to the other end of the medium.o The medium can be a rope, a string, a steel I-beam or a human muscle.

Tension is a pull, not a push. A push would cause compression, which is fine for a steel I-beam, but acompression force cannot be transferred from one end of a string to the other.

Tension is caused by either elastic or inelastic deformation of molecular bonds. In this way, the electrostaticforce is fundamentally responsible for this force.

o The difference between elastic deformation and inelastic deformation comes down to a simpledetail.

In the case of a pulling on a rope that doesnt break, the deformation is elastic. If it doesnt, as in the case ofpulling a heavy box with a string of liquorice, the deformation is inelastic.

Spring Force Is very similar to the force of tension in that it is caused by deformation of molecular bonds. A significant

practical difference is that the spring force can push or pull.

A spring is not always a helical coil. In fact, a spring is deformed from this equilibrium shape, the greater theforce that will be exerted by the spring.

o Inelastic deformation: If a spring is stretched to the point of having the molecular bondspermanently slip with respect to one another, the shape of the spring will be permanently.

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But other materials, such as glass, also make excellent springs. The down side is thatinelastic deformation of glass results in the glass breaking into pieces.

The further you drag the spring away from its relaxed position, the greater is the spring force. This meansthat less work is done to extend a spring the first centimetre, than is done to extend the spring the nextcentimetre.

o This is true for compression as well. I is also true for other types of springs, such as cantilevers,leaf springs or torsion springs. It is this property that defines a spring.

Normal Force

Normal mans that the force is acting at a right angle to a surface. For instance, if you push your handagainst a vertical wall, the wall pushes back horizontally against your hand. That horizontal force is at a rightangle to the surface of the wall, and this, it is a Normal force.

In reality, the wall must deform slightly in order to push back at you. But most walls are rigid, and do nothave to flex very much to provide a large normal force.

The normal force much like the spring force, is caused by deformation of molecular bonds. If you have everstepped onto an icy pond, only to have the ice crack, you have experienced the inelastic deformation of asurface that is providing the Normal Force.

What Causes Friction Friction is caused by two surfaces getting into close enough contact for atomic and molecular bonds to form.

In fact, the force of friction between two slides of rough glass is much less than force of friction between twoslides of glass that have been polished smooth. The smooth glass allows for more direct electromagneticinteraction.

Similarly, super clean and polished metals can be welded together permanently simply by placing them incontact with each other. This is referred to as a cold weld.

The Standard Model In this Standard Model, friction is proportional to the normal force acting between the two surfaces. The

constant of proportionality is called the coefficient of friction.

The coefficient of friction only depends on the molecular characteristics of the materials in contact. Theunfortunate downside to this is that tables of coefficient must be produced for all relevant pairs of surfacematerial.

o Glass slides easily on wood, but glass on glass is much stickier. If the objects are not moving with respect to each other this is known ascoefficient of static friction. When two surfaces are moving relative to one another this is known as coefficient of kinetic energy.

Kinetic Friction It always works to oppose the relative motion between the two surfaces in contact. This means that the

force of friction is always perpendicular to the normal force.o In most cases, this means that kinetic friction acts to reduce the speed of an object.

Formula: This is the friction that you get when the two surfaces are already moving relative to each

other.

Static Friction

If you push an object that is at rest, for a range of forces, it will not move at all. However, once you reach aparticular magnitude of force, the adhesion between the surfaces is broken, and motion begins.

The force of friction is dependent on the normal force, but the coefficient of proportionality is the coefficientof static friction.

There is a very important difference between the equation for kinetic friction and the equation for staticfriction.

The force of static is less than or equal to coefficient of static friction multiplied by the normal force.

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o For a push that is far less than this maximum value, the force of static friction is simply equal to theforce necessary to keep the surfaces from moving with respect to one another.

Formula: This is the friction that keeps things from starting to move. Notice that this is an inequality.

Rolling Friction

As the wheels rolls, a new part of the tire must deform. This is why tires on cars heat up while you drive.This deformation slows down the motion of the rolling object and is called rolling friction.

Formula: The coefficient of rolling friction is much smaller than the coefficient of kinetic friction. Dragging a bicycle with its brakes on is much harder than rolling it along.

Drag Force It opposes motion of an object trying to get through it. It does this by getting in the way. A piece of paper

falling to the ground has to move the air in front of it out of the way so that it can fall. The bowling ball has todo the same thing, but the air that is in the way does not slow the massive bowling ball down as much asthe relatively light piece of paper.

Air friction, like friction, it opposes the relative motion of the object with respect to the fluid.