When you first learn about energy, the first concept you will come across is the law of conservation of energy. It states that energy can neither be created nor destroyed.
That will be hard to absorb for a normal person. But you will see the wisdom and logic in it if you care to delve a bit deeper. The law tells us that the total energy of an isolated system remains constant. The energy it holds can only be transformed from one form to another.
This also means we cannot learn about kinetic energy in isolation. When kinetic energy is created, it is, in fact, changing from another form of energy – potential energy. When kinetic and potential energy are taken together, it is called the total mechanical energy of the object.
That said, how can the kinetic energy of an object be less than zero or negative? In classical Newtonian mechanics, this is an alien concept – something that is inconceivable. However, negative kinetic energy is a common concept in quantum mechanics.
This article tries to answer the question – Can kinetic energy be negative? This article explores the concept of kinetic energy, especially, negative kinetic energy. Here, you will learn everything you need to know about negative kinetic energy.
What is kinetic energy?
The kinetic energy of an object is defined as the energy possessed by it by virtue of its motion. It’s the energy of motion. In other words, as a body moves, it acquires kinetic energy. To understand this better, it is important to know about potential energy.
Potential energy is the energy contained in an object because of its position in a system and physical properties. When the object is stationary, it has the maximum potential energy and zero kinetic energy. When a force acts on the object and it starts moving, it loses its potential energy and gains in kinetic energy.
The kinetic energy of a moving object is maximum when its velocity of motion is the maximum. As the object starts slowing down, its kinetic energy decreases and potential energy increases.
Formula for calculating kinetic energy
Consider an object of mass “m” moving on a surface with “v” velocity. Let the frictional force of the surface be “F”. The object is bound to slow down and stop. Let the distance traveled by the object before stopping be “S”.
In the beginning, the kinetic energy of the object was maximum and this was utilized to do work against the frictional force. In the process, the object loses all its kinetic energy. So, here, the kinetic energy is equal to the work done by the object against the frictional force over the distance.
i.e., Kinetic energy = Work = Force x Distance = FS
Let the initial velocity v₁ of the object be v. The final velocity v₂ of the object is zero.
By Newton’s second law of motion, F = ma, where m is mass and a is the acceleration of the object
In this instance, as the body is slowing down, the acceleration is negative.
a = -F/m
Newton’s third law of motion gives us v₂² = v₁² + 2aS
That is, 0² = v² + 2 (-F/m) S
-2FS/m = -v²
FS = ½ mv²
Kinetic energy Ek = ½ mv²
Unit of kinetic energy
The SI unit of kinetic energy is Joule. 1 Joule is equal to 1 kg.m²/s². The CGS unit of kinetic energy is erg, which is equivalent to 1 gm.cm²/s². · Ergs are much smaller than Joules.
1 erg = 10⁻⁷ Joule
One Joule is the amount of work done when a force of 1 Newton is exerted over a distance of 1 meter.
How is kinetic energy calculated?
The equation for calculating kinetic energy as explained above is Kinetic energy Ek = ½ mv², where m is the mass and v is the velocity of the object.
This means the kinetic energy of a moving object is directly proportional to the mass of the object and the square of the velocity of its motion.
Examples of calculating kinetic energy
If an object of mass 25 kg is moving at a velocity of 10 m/s,
Mass of the object m = 25 kg and velocity v = 10 m/s
Kinetic energy Ek = ½ mv² = ½ x 25 x 10² = 1250 Joules
If a 500gm object is moving at 80 cm/s velocity,
Mass m = 500 gm and v = 80 cm/s
Mass needs to be expressed in kg and velocity in m/s to get the answer in the SI unit joule.
m = 500 gm = 0.5 kg, v= 80 cm/s = 0.8 m/s
Kinetic energy Ek = ½ mv² = ½ x 0.5 x (0.8)² = 0.16 Joule
Can kinetic energy be negative?
The short answer is no.
In classical Newtonian mechanics, kinetic energy is the energy possessed by an object by virtue of its motion. It is a scalar quantity. This means it has only magnitude and no direction. That implies that it can only be positive and cannot be negative.
The kinetic energy of an object with mass m and moving with a velocity v is calculated using the equation Ek = ½ mv². As both mass and velocity are positive quantities and cannot be negative, their product will also not be negative. This means kinetic energy cannot be negative.
However, this is in classical Newtonian mechanics. There are other branches of physics in which matter and energy need not comply with the classical definitions. For example, in quantum mechanics, energy is a more complex concept and may have negative values.
Negative kinetic energy is a common concept in quantum mechanics.
Exploring the concept of negative kinetic energy
While in Newtonian mechanics, energy can only be positive or zero, in quantum mechanics, it can be negative. Consider this example in quantum mechanics.
The Uncertainty Principle in quantum mechanics states that it is not possible to know both the position and speed of a particle, such as an electron or a proton, with perfect accuracy at the same time. The more we try to fix the particle’s position, the less we know about its speed and vice versa.
According to this principle in quantum mechanics, a vacuum of space may appear spontaneously with virtual particle-antiparticle pairs and last for a short time. Some of these particles existing in a virtual state may carry negative kinetic energy. Being in a virtual state means the position of the particle remains uncertain within the region of space.
Certain specific types of waves, such as water waves or waves formed by a stretched string, also offer examples of negative kinetic energy. The kinetic energy at certain regions of the wave can be negative at times, even as the potential energy in these regions remains positive. When added, the total energy will remain constant.
The concepts and scenarios involving negative kinetic energy are quite abstract and complex and cannot be explained using classical mechanics. In classical Newtonian mechanics, kinetic energy can never be negative.
Significance of negative kinetic energy
The concept of negative kinetic energy is applicable only in certain branches of physics and certain contexts. For example, in quantum mechanics and for specific types of waves. In these instances, the concept of energy turns complex and counterintuitive. It doesn’t behave as per our classical understanding and expectations.
The Uncertainty Principle in quantum mechanics allows the momentary existence of particles in certain regions with negative kinetic energy if their position remains uncertain within the region. This comes from the concept of the duality of matter both as a particle and as a wave. Wave-particle duality is one of the fundamental properties of matter that accounts for the matter existing as a particle in one moment, while it exhibits wave-like properties the next moment.
The significance of negative kinetic energy goes beyond this. It accounts for phenomena like resonance and wave interference. In such instances, the negative kinetic energy comes with positive potential energy, leading to the constant total energy.
Even as we discuss the significance of negative kinetic energy, it is worth noting that this concept doesn’t apply to classical mechanics where the laws of classical Newtonian mechanics are applied to macroscopic bodies and systems. In classical mechanics, the kinetic energy can only be positive or zero.
When is kinetic energy negative?
Kinetic energy can never be negative in classical mechanics as it is directly proportional to the mass and the square of the velocity of the object. However, in other branches of physics like quantum mechanics, negative kinetic energy is possible in certain situations.
Let’s consider the scenario when the object is moving in the opposite direction to the initial motion. Then, the object is bound to have negative kinetic energy as the displacement is negative. The same is true when the object is losing energy due to the frictional force acting on it, resulting in its speed to reduce.
When searching for examples of negative kinetic energy in action, you may not find any in classical mechanics as it concerns only macroscopic bodies that behave according to Newton’s laws. It is not hard to find examples of negative kinetic energy in quantum mechanics where this scenario occurs due to the wave-particle duality of matter.
In quantum mechanics, particles of matter may exhibit wave-like properties in certain circumstances. The Uncertainty Principle helps us gain a better understanding of the behavior of particles. It states that determining the position and momentum of a particle accurately is not possible. The particle will exist in a virtual state in which its position can only be estimated within a certain region. In such instances, kinetic energy can be negative.
The concept of negative kinetic energy also arises when studying certain waves. Water waves and waves formed on a stretched string are examples. In the waves, the kinetic energy may be negative in certain regions, even as the potential energy is positive. This will ensure that the total energy remains constant. Such behavior will result in phenomena like resonance and wave interference.
What is meant by negative kinetic energy?
As discussed above, negative kinetic energy is not possible in classical mechanics. However, it is a possibility in other branches of physics like quantum mechanics. Let’s understand the relationship between kinetic energy and potential energy when the kinetic energy is negative.
Kinetic energy is the energy acquired by an object by virtue of its motion while potential energy is the energy it possesses by virtue of its position. As per the law of energy conservation, energy cannot be created or destroyed; it can only be transformed from one form to another. As such, kinetic energy and potential energy are closely linked to each other.
For an isolated object with no interference from external forces, the sum of kinetic energy and potential energy is constant. This sum is called total mechanical energy.
The potential energy of an object is determined by its position with respect to the reference point. This means it can be negative. The total energy can be negative when there is negative energy density. Negative energy density means there is more negative energy than positive energy in the region. This may happen when both kinetic energy and potential energy are negative. Or, when the magnitude of the negative kinetic energy is greater than that of the positive potential energy.
As negative kinetic energy doesn’t arise in classical mechanics and is seen only in quantum mechanics, you will come across negative total energy also only in quantum mechanics. Because you need a negative kinetic energy for a negative total energy.
Whether the kinetic energy is positive or negative, the relationship between kinetic energy and potential energy and the law of energy conservation hold true.
FAQs on negative kinetic energy
1. What makes negative kinetic energy significant?
Negative kinetic energy comes in handy to explain the behavior of particles in certain branches of physics like quantum mechanics. It also helps in making sense of wave behavior in diverse mediums.
2. Can we observe negative kinetic energy in real-life situations?
As the motion and energy of everyday objects are typically covered by classical mechanics, it is not common to come across negative kinetic energy in real-life situations. However, it is common in quantum mechanics and the behavior of waves. A real-life example of negative kinetic energy can be found in certain regions of water waves and the waves formed by a stretched string.
3. Is negative kinetic energy a violation of the laws of physics?
Not at all. Whether kinetic energy is positive or negative, it still follows all the laws of physics. The negative value for kinetic energy arises as a result of the laws in quantum mechanics and wave-particle duality.
4. Is it possible to convert negative kinetic energy to some other form of energy?
Yes. This is possible. Whether positive or negative in value, kinetic energy can be converted to other forms of energy, like potential energy. The law of energy conservation will always hold true. The total energy of an isolated system will remain constant, even when it has negative kinetic energy.
When you confront the question “Can kinetic energy be negative” for the first time, your answer is bound to be “no”. Because in classical mechanics, this is not possible. Kinetic energy is a scalar quantity with only magnitude and no direction. So, it can either be positive or zero.
However, when you consider other branches of physics like quantum mechanics, you will realize that this is a quite common concept. The fundamental principles of quantum mechanics and wave-particle duality will lead to the possibility of a negative value for kinetic energy.