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?
• The most important formulas
Forms of energy
Energy occurs in nature in many different forms. These include:
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.
With the help of the EES, many physical tasks and problems can be solved. Here are some examples:
- 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.
- 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.
- 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.
- 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.
The above examples are calculated with typical numerical values and the corresponding formulas.
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.
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.
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
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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