Work and Energy

Work and Energy Energy The quantity known as energy is introduced to measure the effect of combining a force with a displacement. The unit of measurement of energy is the Joule (J). An example of energy is when a person pushes a weight. A force is applied to the object that, consequently, moves in the direction of the force. Energy can be defined as the product of the magnitude of the force and the displacement module. In cases where there are forces that impede the movement, the energy takes on a negative value defined as resistant: L=-F*s A force perpendicular to the displacement does not do any work: L = 0 An example is a truck that moves horizontally with the ground as a frame of reference. The weight acting on the truck presents a energy equal to 0. When a force produces a displacement in a different direction, the force is inclined by an angle a. The energy will be calculated as L = F' * s where F' is the component of the force parallel to the displacement If a is an acute angle, there is a force that favours displacement (positive energy), otherwise there will be a force that impede displacement (negative energy). Momentum The momentum of an objects is a vector quantity defined as the product of the mass and velocity of an object. This is a conservative physical quantity, i.e. it remains the same over time in case of absence of external forces applied to the object Potential Energy The potential energy of an objects is the energy it possesses because of its position or orientation relative to a force field. You can also see potential energy as the ability of an objects to transform its own energy into another form of energy, such as kinetic energy. Example: a stretched arc is able to store energy as a result of its position. The potential energy possessed by an objects that is at a certain height (h) on the earth's surface is called gravitational potential energy. Gravitational potential energy: Potential elastic energy: The potential elastic energy acts on the elasticity of a deformed object. The theorem of conservation of mechanical energy: in an isolated system in which there are only conservative forces (dependent only on themselves) the total mechanical energy (sum of kinetic energy and potential energy) does not vary. Newton's law of universal gravitation Newton's law of universal gravitation. In the Universe every particle attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers F is the magnitude of the force between the masses; G is the gravitational constant; m1 is the first mass; m2 is the second mass; r is the distance between the centres of the masses. The gravitational constant is: Kepler's Laws First law of Kepler. The orbit of every planet is an ellipse with the Sun at one of the two foci. The position in which a planet is closer to the Sun is called perihelion, the position of maximum distance is called aphelion. Second law of Kepler. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. In our hemisphere spring and summer are longer than autumn and winter. If the Earth travelled a circular orbit with a constant speed, the four seasons would have the same duration. Third law of Kepler. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.