What Greatly Affects Kinetic Energy?

Definition of Kinetic Energy

Kinetic energy is defined as the energy associated with the motion of an object. The formula for kinetic energy is:

Kinetic Energy = 1/2 x mass x velocity²

Kinetic energy depends on two variables: the mass and the velocity of the object. An increase in either the mass or the velocity of an object will result in an increase in its kinetic energy. The velocity component is squared in the formula, so velocity has a greater impact on kinetic energy than mass (Definition of kinetic energy). The formula shows that doubling the velocity of an object will quadruple its kinetic energy, while doubling the mass of the object will only double its kinetic energy.

Mass

Mass has a direct proportional relationship with kinetic energy, meaning that as mass increases, kinetic energy also increases [1]. This is because kinetic energy is calculated using the formula: Kinetic Energy = 1/2 x mass x velocity squared. Since mass is directly multiplied in the formula, an increase in mass leads to a direct increase in kinetic energy, assuming velocity is held constant. Doubling the mass of an object moving at the same velocity will double its kinetic energy. Overall, the more massive an object is, the greater its kinetic energy will be at a given velocity.

Velocity

Kinetic energy has an exponential relationship with velocity, as shown in the kinetic energy formula: KE = 1/2mv². This means that as velocity increases, kinetic energy increases at a much greater rate. Doubling the velocity of an object quadruplets its kinetic energy. This nonlinear relationship occurs because velocity is squared in the formula. For example, a car traveling at 20 mph has 4 times the kinetic energy of a car traveling at 10 mph. This exponential increase makes sense intuitively – the faster something moves, the more energy is required to stop it and the more damage it can cause in a collision. At extremely high velocities like those achieved by particles in particle accelerators, even small objects carry tremendous amounts of kinetic energy.

As the Physics Classroom notes, “This equation reveals that the kinetic energy of an object is directly proportional to the square of its speed. That means that for a twofold increase in speed, its kinetic energy increases by a factor of four.”¹ Increases in velocity can come from adding kinetic energy to an object via work, gravity, or other means. A traveling object’s kinetic energy will continue growing exponentially as long as its velocity keeps increasing.

Gravity

Gravity greatly impacts an object’s kinetic energy. The force of gravity accelerates objects downward, increasing their velocity and thus kinetic energy. As an object falls, its gravitational potential energy is converted to kinetic energy. The relationship can be described by the gravitational potential energy equation: Kinetic Energy = Gravitational Potential Energy – Work Done Against Friction. The longer an object falls vertically, the greater its velocity and kinetic energy become. This is because the object is accelerating at 9.8 m/s² due to Earth’s gravitational field. Gravity enables objects to gain immense amounts of kinetic energy that can then be transferred upon impact. For example, a meteor gaining kinetic energy through gravitational acceleration can create a massive crater when it collides with Earth’s surface.

Friction

Friction is a force that acts between two surfaces or objects that are moving relative to each other. Friction converts some of the kinetic energy into thermal energy, effectively reducing the total amount of kinetic energy in a system. When two surfaces are in contact, the irregularities of the surfaces can become temporarily “hooked” together, increasing friction. This resistive force works against the motion and causes moving objects to slow down. According to the work-energy theorem, because friction does negative work against the direction of motion, it removes some of the mechanical energy that an object or person expends and converts it to thermal energy. The net work equals the change in kinetic energy. So the greater the friction, the more kinetic energy is converted into thermal energy, and the less kinetic energy remains. This reduces the speed and kinetic energy of moving objects.

Inelastic Collisions

In an inelastic collision, part of the kinetic energy is changed to some other form of energy in the collision. Many collisions in everyday life are inelastic collisions. Some examples of inelastic collisions include a ball hitting the floor and staying there, a car hitting a parked car, or two sticky balls colliding and sticking together.

In an inelastic collision, kinetic energy is not conserved. Some of the initial kinetic energy before the collision is lost or transformed into other forms of energy such as heat, sound, and deformation during the collision. The loss in kinetic energy is related to the coefficient of restitution (e) of the colliding objects. The coefficient of restitution is a measure of bounciness of an object, with a perfectly elastic collision having a coefficient of 1.

The velocity change in an inelastic collision can be calculated using the conservation of momentum principle and the coefficient of restitution. The kinetic energy after the collision is decreased, following the equation:

Kinetic Energy After = (1 – Coefficient of Restitution^2) x Kinetic Energy Before


(Source: https://openstax.org/books/college-physics-2e/pages/8-5-inelastic-collisions-in-one-dimension)

The kinetic energy lost in an inelastic collision depends on how “inelastic” the collision is. The more inelastic the collision, the more kinetic energy that will be lost or transformed into other forms of energy.

Elastic Collisions

In an elastic collision, kinetic energy is conserved. This means the total kinetic energy of the system before and after the collision is the same. During an elastic collision, the colliding objects compress each other like springs and then rebound without any loss of kinetic energy. A classic example is two billiard balls colliding on a pool table. The kinetic energy of the first ball is transferred to potential energy as it compresses the second ball through the collision. This potential energy is then transferred back to kinetic energy as the balls rebound and separate.

According to the physics.stackexchange.com, “In an elastic collision (for objects >> in mass than typical molecules) energy moves from kinetic to potential then back to kinetic.” This transfer of energy from kinetic to potential and back is what allows kinetic energy to be conserved overall in an elastic collision [1].

Chemical Energy

Chemical energy stored in molecules can be converted into kinetic energy through chemical reactions. This commonly occurs through a process called combustion, in which the chemical energy stored in a fuel is released through an exothermic reaction with oxygen ^[1]. The energy released from the breaking and formation of chemical bonds is transferred into the kinetic energy of the combustion products, increasing their speed and motion.

A familiar example is the combustion of gasoline in a car engine. The chemical energy stored in the gasoline molecules is converted into kinetic energy of the expanding gases, which push on the pistons of the engine and propel the car forwards. The greater the amount of stored chemical energy in the fuel, the more kinetic energy can be produced through combustion.

Another example is metabolism in living organisms. The chemical energy stored in food molecules like glucose and fats is released through metabolic reactions and converted into kinetic energy that allows organisms to move and function. Chemical energy from ATP drives most cellular processes that require mechanical energy. The conversion of chemical energy to kinetic energy takes place in every movement, breath, and heartbeat.

Thermal Energy

Thermal energy, often referred to as heat energy, is the total kinetic energy of all the molecules within an object. When an object is heated, its molecules gain kinetic energy and begin moving faster and vibrating more. This increase in molecular motion corresponds directly to an increase in thermal energy and temperature (1).

Adding thermal energy in the form of heat thus increases the kinetic energy of the molecules in an object. The faster the molecules vibrate and move, the more kinetic energy they collectively possess. This relationship is intuitive – heating an object increases molecular motion, which in turn increases the total kinetic energy (2).

Some examples of thermal energy increasing kinetic energy are:

• Heating water on a stove causes the water molecules to vibrate faster, increasing their kinetic energy.
• Warming up metal in a furnace increases the metal atoms’ kinetic energy as they vibrate more rapidly.
• Sunlight striking a rock heats the rock by boosting the kinetic energy of the rock’s molecules.

In summary, adding thermal energy in the form of heat is one of the most direct ways to increase the kinetic energy of an object or system of particles. The increased molecular motion corresponds to higher speeds and more vibrating atoms, resulting in greater total kinetic energy.

(1) https://www.school-for-champions.com/science/thermal_energy.htm
(2) https://www.houstonisd.org/cms/lib2/TX01001591/Centricity/Domain/5364/Thermal%20Energy.pdf

Electrical Energy

Electrical energy can be converted into kinetic energy through electric motors. When current flows through the coils in an electric motor, it generates a magnetic field that causes the motor’s rotor to spin. This spinning motion is kinetic energy. For example, electrical energy is converted into kinetic energy in electric vehicle motors to propel the vehicle forward. Other common examples include electric fans, blenders, and elevators, which all use electric motors to convert electrical energy into kinetic energy to spin the fan blades, blender blades, and move the elevator cab.

According to the Fire2Fusion article Kinetic Energy Conversion, “A best example to demonstrate the conversion of kinetic energy to electrical energy is the wind mill. A wind turbine is a machine for converting the kinetic energy of the wind into electrical energy.” The motion of the wind provides kinetic energy that can be harnessed by wind turbines and converted into electricity.