Kinetic Energy Concerns Movement of Matter
Kinetic energy is the energy of a moving object according to the equation KE = ½ mv². This equation is valid provided that the speed of the object does not approach the speed of light. Motion and kinetic energy can be a direct result of a force can be applied to a object. Also, releasing potential energy can create kinetic energy, such as with springs and chemical reactions. Most forms of energy where material is moving are kinetic energy.
Questions you may have include:
- What does the equation KE = ½ mv² mean?
- How can a force create kinetic energy?
- What are other forms of energy?
Kinetic energy equation
The energy of a moving object is called kinetic energy and is abbreviated as KE. The properties of kinetic energy are that the greater the mass of a moving object, the greater its energy will be. Also, the faster it goes, the greater its energy. That energy is proportional to the square of the velocity.
The equation for calculating the kinetic energy of an object is
KE = ½ mv²
where:
KE is the kinetic energy, usually measured in joules
m is the mass of the object, usually measured in kilograms
v is its speed or velocity of the object, measured in meters per second
v² is the velocity squared or v times v
½ mv² is one-half times m times v²
A joule equals a newton-meter. Since a newton equals a kilogram-meter per second squared (kg-m/s²), you can see that a joule equals kg-m²/s². It is not usually designated that way, but you can see it holds by multiplying the units.
Example with different masses
If two objects were going at the same velocity, but one object had twice the mass of the other, then the energy of the heavier object would be 2 times the energy of the lighter object.
If M = 16 and m = 8
then KEM = ½*16*v² = 8v²
and KEm = ½*8*v² = 4v²
thus KEM = 2KEm
Example with different velocities
Now suppose the lighter object in the example above was going twice as fast as the heavier object. Say the object with m = 8 was going at v = 4 m/s and the object with M = 16 was going at V = 2 m/s. Then the energies would be:
KEmv = ½ mv² = ½ * 8 * 4 * 4 = 128
KEMV = ½ MV² = ½ * 16 * 2 * 2 = 64
Thus, the effect of higher velocity is greater than that of a greater mass.
Force results in kinetic energy
When a force is applied to an object, it accelerates the object according to the equation
F = ma
where:
F is the force, measured in newtons
m is the mass, usually measured in kilograms
a is the acceleration, usually measured in meters/second/second or m/s²
ma is m times a
Acceleration is defined as the change in velocity over a period of time. Thus, if the force is applied to an object for a certain amount of time, it will reach the velocity v and have a kinetic energy of KE = ½ mv².
Work
Work is defined as the result of a force applied over a distance, W = Fd. Since force is in newtons and distance is in meters, work is in newton-meters, which equals the units of energy, joules.
Other forms are also kinetic energy
Often, you will hear about other forms of energy, such as heat and electrical energy. In reality, they are also kinetic energy.
Heat energy
Heat is the movement of molecules. It is the sum of their kinetic energy. In many physics textbooks, they look at heat as some sort of substance and heat energy as something independent of kinetic energy. In our lessons, it is just another form of kinetic energy.
Electrical energy
Electrical energy is the movement of electrons. That is kinetic energy. The voltage in an electrical circuit is the potential energy that can start electrons moving. Electrical forces cause the movement to occur.
Chemical energy
A chemical reaction puts atoms and molecules in motion. Heat and radiation energy are often the result of a chemical reaction, which releases chemical potential energy.
Light energy
Light consists of the movement of waves and/or light particles (photons). It is usually formed when atoms gain so much kinetic energy from being heated that they give off radiation. This is often from electrons jumping orbits and emitting moving photons.
Summary
Kinetic energy is the energy of a moving object according to the equation KE = ½ mv². Motion and kinetic energy can be a direct result of a force can be applied to a object. Releasing potential energy can create kinetic energy, such as with springs and chemical reactions. Heat, electrical energy and light are forms of kinetic energy.