How does kinetic energy increase speed

The law of conservation of energy


Table of Contents

• What are forms of energy?
• What does the law of conservation of energy say?
• Examples
• The most important formulas

Forms of energy

Energy occurs in nature in many different forms. These include:

  • kinetic energy of a moving mass,
  • potential energy of a mass in the gravitational field,
  • potential energy of a charge in an electric field,
  • elastic energy of a spring due to its deformation,
  • chemical energy stored in chemical compounds,
  • electrical energy carried by an electrical current,
  • Thermal energy,
  • Light energy contained in radiation.
  • Statement of the law of conservation of energy

    The EES states that the total energy of a closed system is constant. A closed system describes, for example, a box that does not let any energy in or out. This clearly means that all possible forms of energy can be converted into another form, but that the sum of all forms of energy is constant. Energy can neither be destroyed nor generated from nothing. Energy can only be converted from one form to one or more others at a time.

    Illustrative examples

    With the help of the EES, many physical tasks and problems can be solved. Here are some examples:

    1. When a moving car brakes, its kinetic energy is reduced.
      However, the braking process heats up the brakes, their thermal energy
      increases. The thermal energy of the brakes increases by exactly the amount
      by which the kinetic energy of the car decreases. The sum of kinetic
      Energy and thermal energy is constant.
    2. A ball is thrown straight up into the air. At first he owns one
      high speed. However, this becomes smaller until the ball is momentarily
      comes to rest and then falls back down. As the ball rises
      its potential energy increases in the earth's gravitational field. These
      Energy has to be subtracted from the kinetic energy of the ball, the ball
      becomes slower as a result. Again: The sum of the kinetic and the potential
      Energies of the ball is constant.
    3. A motor is z. B. powered by gasoline. By burning
      Gasoline releases chemical energy, some of which translates into the kinetic energy of the pistons
      is used in the engine. However, a large part of the energy is lost as heat
      and cannot be used. If one calculates the released thermal energy
      as well as the kinetic energy of the pistons together, the sum is just that
      chemical energy of the gasoline that was released. The products of combustion
      of gasoline have a lower chemical energy by this amount.
    4. A spring is stretched. This requires a certain amount of energy, which is called
      elastic energy is stored in the spring. Now you put a mass on
      and lets go of the spring, the mass is thrown away and the spring turns
      back to sleep. The elastic energy is converted into kinetic energy
      converted to the mass.

    Bills to these Examples can be found below.

    The most important formulas

    In the following, m is the mass, v is the speed, g is the acceleration due to gravity, k is the
    Spring constant, T the temperature and C the specific heat capacity per mass.
    The kinetic energy of a mass m moving with velocity v is
    E. kin = 12 mv 2 : 1
    The potential energy of a mass m at height h is relative to the reference height h0
    E. pot = mg (h - h 0 ) : 2
    The elastic energy stored in a deformed spring constant is
    E. el = 12 kx 2 : 3
    where x is the deformation of the spring in meters.
    In a mass m with a specific heat capacity C, with a change in temperature
    The following heat energy is absorbed or released by ΔT:
    E ~ th = mC ΔT: 4
    C is a material size and can be looked up in tables.
    These formulas can be used to solve most of the school tasks that require the EES.

    Mathematical application

    The above examples are calculated with typical numerical values ​​and the corresponding formulas.

  • The car with mass M = 1500 kg initially drives with v 1 = 30 m / s. It brakes
    v 2 = 10 m / s. The di erence of the kinetic energies at the beginning and at the end
    is E = 12M (v21-v22) = 600 kJ. This energy is converted into the thermal energy
    Brakes converted. Assume that all of the brakes have a mass
    of m = 10 kg with a specific heat capacity of C = 400 J / kgK
    typical of steel, the brakes would turn around
    ΔT = EmC = 150 K
    heat, e.g. from 20 to 170 ° C.
  • The ball with mass m = 100 g initially moves at a speed of
    v = 10 m / s upwards. He is thus able to use his potential energy
    E = 12 mv 2 = 5Y
    to increase. At the point of greatest height, the ball has no kinetic energy
    more. The ball does not move at this moment. The energy E is now there
    completely in the height change of the ball that
    Δh = Emg = 5m
    if g ≈ 10 m / s 2 is used.
  • A total of one liter of gasoline is burned. The efficiency of the engine
    amount to 30%. That is, 30% of chemical energy is turned into kinetic energy
    the piston is converted, the rest is lost as waste heat and thermal energy
    cannot be used. One liter of gasoline contains about 36 MJ of chemical substances
    look up usable energy in tables. Since only 30% can be used by us,
    only E = 10.8 MJ of energy remain. The remaining 25.2 MJ are directly transferred to
    Heat converted and are not used for the movement of the pistons.
    With the energy E, for example, a mass m = 1000 kg can be achieved
    Δh = Emg = 1080m
    raise. A small car can therefore be more than a kilometer in the air
    be lifted.
  • The spring has a spring constant k = 50 N / m. It is compressed by x = 10 cm and thus stores elastic energy
    E = 12 kx 2 = 0.25J.
    This is completely converted into kinetic energy with a mass of m = 20 g. The
    Mass then moves at a speed of
    v = 2 Em = 10 cm / s.
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