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# Physics - PowerPoint PPT Presentation

Physics. Energy and Work. The Energy. We define energy the ability of a body or of a system to do work and the extent of this work is in its turn the energy measurement. Energy is an inherent, measurable property of matter belonging to the very existence of physical bodies.

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### Physics

Energy and Work

• We define energy the ability of a body or of a system to do work and the extent of this work is in its turn the energy measurement.

• Energy is an inherent, measurable property of matter belonging to the very existence of physical bodies.

• Energy can be neither seen or touched; we become aware of its existence only when it passes from one body to another or when it is used.

• Energy is not consumed, but it manifests itself in different forms.This statement is a cornerstone of physics:The principle of conservation of energy.

• You cannot create new energy, but only transform what is already present in the universe in a form that may prove useful.

• The effect of a force depends on:

• The intensity of the force

• The time it acts

• The displacement of the body

The physical magnitude which correlates force and displacement is called WORK.

• Work is the product of two vectors: force and displacement.This formula is correct only if the displacement takes place exactly in the direction of the resisting force and vice versa, otherwise we should find the parallel component.

• A conservative force is the force exterted by a field of conservative forces. The work that it performs on an object along a path depends only on the start and on the end position, and not on the nature of the route.At a microscopic level all forces are conservative.

• A moving object has an energy called kinetic energy that depends both on the mass and on the speeed.

• Kinetic energy EC of a body of mass m and velocity v is equal to the semifinished product of mass for the square of the speed.

• The work performed on an object changes the kinetic energy, because the applied force changes the speed of the object.

• In the International System (SI), kinetic energy is expressed in Joule. In fact the unit of measurement of the kinetic energy is:(kg) x (m2/s2) = (kg) x (m/s2) x (m) =N x m = J

• It can be shown that the change in kinetic energy of an object on which a force acts is equal to the work done by the force, as it is demonstrated in the following relation:

• An object that has the capacity to do work has energy, for example, a stationary object. A boulder can do work by falling down a rock, the arch can do work if you relax it. So we can say that a rock and an arch have energy position, that is the POTENTIAL ENERGY.

• An object of mass m, at a height h considered a reference level, has a gravitational potential energy equal to the work that the force of gravity can accomplish on the object causing it to fall on the reference plane.

The boxes have the same mass but they are placed at different heights relative to the floor. Boxes 1 and 2 have gravitational potential energy because when they fall they perform a work:L = m*g*h

• The box placed on the cabinet has more energy because different heights relative to the floor. Boxes 1 and 2 have gravitational potential energy because when they fall they perform a work:it can do more work having a height greater than the one posed on the table. Thus, the gravitational potential energy depends on the height respect to the reference plane. Also, the energy also depends on the value of the mass; the larger the mass, the greater is the energy.

Elastic different heights relative to the floor. Boxes 1 and 2 have gravitational potential energy because when they fall they perform a work:Potential Energy

• The elastic energy is the energy that causes or is caused by the elastic deformation of a solid or a fluid.

• In the case of a spring elastic energy is:E= ½ kx2

Where k is the spring costant of the spring (see Hooke law) and x is the elongation of the spring.

Theorem of conservation of mechanical energy different heights relative to the floor. Boxes 1 and 2 have gravitational potential energy because when they fall they perform a work:

• In a system subjected to conservative forces, the total mechanical energy of the system E is defined the sum of the kinetic energy Ecin and potential energy Epot of the system.

• In the presence of only non-conservative forces, the work is equal to the variation of mechanical energy.

Theorem of conservation of mechanical different heights relative to the floor. Boxes 1 and 2 have gravitational potential energy because when they fall they perform a work:energy

• If A is the starting point and B the end point, the work can be expressed as:

• L= EpotA – EpotB or L= EcinB – EcinA

• Then

• EcinB – EcinA = EpotA – EpotB

• and, summing both the members of equality, we obtain:

• EcinB + EpotB = EcinA + EpoA

• Then

• E = Ecin + Epot = COSTANT

• Bibliography different heights relative to the floor. Boxes 1 and 2 have gravitational potential energy because when they fall they perform a work:

• http://it.wikipedia.org/wiki/Pagina_principale

• http://digilander.libero.it/nick47/lvpt.htm

• http://www.sapere.it/sapere/strumenti/studiafacile/fisica/La-meccanica/Le-leggi-di-conservazione/La-legge-di-conservazione-dell-energia.html

• This presentation is created by:

2CLSAI.T.I.S.G. Cardano