Energy is a very important concept in Physics. We can define energy as the strength to do any kind of physical activity. Therefore, we can say that Energy is the ability to do work. In this article, we will discuss the concept of energy and energy formula physics with examples. Let us learn the concept Scientific Principles. Basic Energy Principles. Energy is the driving force for the universe. Energy is a quantitative property of a system which may be kinetic, potential, or other in form. There are many different forms of energy. One form of energy can be transferred to another form. The laws of thermodynamics govern how and why energy is.

The equation we have introduced defining power as energy divided by time may be rewritten as follows: energy (joules) = power (watts) x time (seconds) This important equation allows you (and the electric company) to calculate how much energy you consume (and how much you have to pay for) Equation (6) is based on the conservation of energy principle, i.e. the work done by the external forces going through a virtual displacement equals the work done by the internal forces due to the same virtual displacement. The external virtual work can b

The energy equation is a statement of the conservation of energy principle. In fluid mechanics, it is found convenient to separate mechanical energy from thermal energy and to consider the conversion of mechanical energy to ther-mal energy as a result of frictional effects as mechanical energy loss. The Formula: We can calculate work by multiplying the force by the movement of the object. W = F × d: Unit: The SI unit of work is the joule (J) Energy: Definition: In physics, we can define energy as the capacity to do work. Formula: For the potential energy the formula is. P.E. = mgh: Uni The formula for kinetic energy states that the kinetic energy of a body is directly proportional to the velocity of a body. There is also a special equation for elastic potential energy, which describes the energy stored in a compressed or stretched elastic material, like a spring, trampoline, or a bow with a nocked arrow By integrating the equation of motion, ∑Ft = mat = mv(dv/ds), the principle of work and energy can be written as ∑U1-2= 0.5m(v2)2-0.5m(v1)2or T1+ ∑U1-2= T2 ∑U1-2is the work done by all the forces acting on the particle as it moves from point 1 to point 2. Work can be either a positive or negative scalar

The total energy of a channel flow referred to a datum is given by equation below: If the datum coincides with the channel bed at the section, the resulting expression is known as specific energy and is denoted as E. thus V2 =dcosθ+α2 The work W done by the net force on a particle equals the change in the particle's kinetic energy K E: W = ΔKE = 1 2mv2 f − 1 2mv2 i W = Δ KE = 1 2 mv f 2 − 1 2 mv i 2. The work-energy theorem can be derived from Newton's second law. Work transfers energy from one place to another or one form to another To calculate Grav. potential energy we use Ep = -G (m1m2/r). In order to have 0 grav. potential energy, the denominator, r, would need to be really massive, so that you were out of the grav, field

Class 15 vf 2=2gh To see how this relates to energy we multiply both sides of the above equation by 1 2 m we get 1 2 mvf 2=1 2 m 2gh 1 2 mvf 2=mgh This means that the object has acquired a kinetic energy equal to m g h. Because we can object to a height h, and then drop is and have is acquire a kinetic energy equal to m g h we call the quantity m g h gravitational potential energy By using principle of conservation of energy, we can apply Bernoulli equation between two points (1 and 2) on the streamline: (Ú)= (Û) P 5 ρg + v 5 6 2g +z 5= P 6 ρg + v 6 6 2g +z 6 But!!, this equation no energy losses ( e.g. from friction) or energy gains (e.g. from a pump) along a stream line, so the final form for Bernoulli equation is. Introduction to Energy Methods Readings: Reddy Ch 4, 5, 7 Learning Objectives Understand the energy formulation of the elasticity problem. Understand the principle of virtual work as the weak formulation of the elasticity problem. Apply energy and variational principles for the determination of de ections and in Although this principle cannot be proved, there is no known example of a violation of the principle of conservation of energy. The amount of energy in any system is determined by the following equation: U T is the total energy of a syste The free energy principle is a formal statement that explains how living and non-living systems remain in non-equilibrium steady-states by restricting themselves to a limited number of states. It establishes that systems minimise a free energy function of their internal states, which entail beliefs about hidden states in their environment

The Free Energy Principle states that a system in nonequilibrium steady state with its environment must minimise its free energy. We'll need another few posts to unpack everything there, but at minimum we now know what the 'free energy' term is and why minimising it might be useful! Conclusion, Summary, and Anki Card The equation for kinetic energy is KE = 0.5 • m • v2 where m is the mass of the object (with standard units of kilograms) and v is the speed of the object (with standard units of m/s). The total mechanical energy possessed by an object is the sum of its kinetic and potential energies Here's another way of looking at energy balance in real life. Eating just 150 calories more a day than you burn can lead to an extra 5 pounds over 6 months. That's a gain of 10 pounds a year. If you don't want this weight gain to happen, or you want to lose the extra weight, you can either reduce your ENERGY IN or increase your ENERGY OUT

- Uncertainty principle of Heisenberg, 1927. In quantum mechanics, the uncertainty principle (also known as Heisenberg's uncertainty principle) is any of a variety of mathematical inequalities asserting a fundamental limit to the accuracy with which the values for certain pairs of physical quantities of a particle, such as position, x, and.
- We look into the history of kinetic
**energy**and how it became a classical concept in physics.We invoke the Work-**Energy****principle**and see how it becomes usefu.. - All ideal and real gases obey these energy conservation formulas. Energy change formula First law of thermodynamics for cyclic process. Internal energy changes in the cyclic process, dU = 0. Therefore, according to the first law, q = w. Hence from the energy conservation principle, heat is completely converted into work for the cyclic process
- Conservation of Energy - Formula - Equation. Let us assume the one dimensional elastic collision of two objects, the object A and the object B. These two objects are moving with velocities v A and v B along the x axis before the collision. After the collision, their velocities are v' A and v' B.The conservation of the total momentum demands that the total momentum before the collision.
- The principle of energy conservation states that energy is neither created nor destroyed. It may transform from one type to another. Like the mass conservation principle, the validity of the conservation of energy relies on experimental observations; thus, it is an empirical law

Solved Examples for Energy of a Wave Formula 1) Find the total energy of a wave with the values, A = 20 meters, ω = 40 Hz, λ = 50 meters and μ = 100 ? Use Energy of a Wave Formula. Answer : U total = 1/2 (100 × 20 × 20 × 40 × 40 × 50 Mr. Andersen defines the terms energy, work and power. He also uses a simple example to calculate both work and power.Intro Music AtributionTitle: I4dsong_l.. The most common energy generated in a dynamics problem is a force, F, acting through a distance (or more acurately, a change in position, r).This written in equation form as Δ U = F•dr. The dot product is needed since only the force in the direction of the motion will produce work

- The conservation of energy principle (the energy balance): The net energy transfer to or from a system during a process be equal to the change in the energy content of the system. Energy can be transferred to or from a closed system by heat or work . Control volumes also involve energy transfer via mass flow. the total rates of energy transfer int
- KE = ½mv². where m is the mass of the object, g is the height of the object, g is the gravitational field strength (9.8m/s²), and v is the average velocity of the object.The formula for potential energy states that the potential energy stored in an object is directly proportional to the position of an object with respect to a defined 0 point
- Finally, using dynamical principles, such as the conservation of energy or conservation of momentum, and the two principles at the core of special relativity (i.e., the relativity principle and the light principle), one shows that the dynamical principles require that after \(B\) suffers the interaction in which it absorbs or emits energy, its.

Einstein's equation E=mc² pops up on everything from baseball caps to bumper stickers. It's even the title of a 2008 Mariah Carey album. But what does Albert Einstein's famous equation really mean? For starters, the E stands for energy and the m stands for mass, a measurement of the quantity of matter. Energy and matter are interchangeable The free electron model of metals has been used to explain the photo-electric effect (see section 1.2.2).This model assumes that electrons are free to move within the metal but are confined to the metal by potential barriers as illustrated by Figure 2.3.1.The minimum energy needed to extract an electron from the metal equals qF M, where F M is the workfunction E = mc2, equation in German-born physicist Albert Einstein 's theory of special relativity that expresses the fact that mass and energy are the same physical entity and can be changed into each other. In the equation, the increased relativistic mass ( m) of a body times the speed of light squared ( c2) is equal to the kinetic energy ( E) of. ** The Poseidon Principles: A Groundbreaking New Formula for Navigating Decarbonization**. For the past 18 months, Rocky Mountain Institute (RMI) has worked behind closed doors with an unprecedented coalition to establish the Poseidon Principles. Today, June 18, 2019, the Principles become public as 11 banks representing approximately $100 billion. The work-energy principle. There is a strong connection between work and energy, in a sense that when there is a net force doing work on an object, the object's kinetic energy will change by an amount equal to the work done: Note that the work in this equation is the work done by the net force, rather than the work done by an individual force

The Bernoulli Equation The Bernoulli equation is an equation for flow based on the law of conservation of energy, which states that the total energy of a fluid or gas at any one point in a flow stream is equal to the total energy at all other points in the flow Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Although Bernoulli discovered that pressure decreases when the flow speed increases, it was actually Leonhard Euler who created Bernoulli's equation Substitute the last expression for acceleration into the expression for work to obtain W = m ( v f 2 − v o 2) or W = (1/2) mv f 2 − (1/2) mv o 2. The right side of the last equation yields the definition for kinetic energy: K. E. = (1/2) mv 2 Kinetic energy is a scalar quantity with the same units as work, joules (J)

22.55 Principles of Radiation Interactions The Bethe Formula for Stopping Power. Using relativistic quantum mechanics, Bethe derived the following expression for the stopping power of a uniform medium for a heavy charged particle: - dx dE = 2 2 2 2 4 4 0 β π mc k z e n ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − − 2 2 2 2 (1 ) 2 ln β β β I mc. ko. To derive the relativistic energy formula we assume that the mass-energy principle holds good under relativity. According to the work-energy theorem, it states that the net work done on a system goes into kinetic energy. In other words, we say the change in kinetic energy can be evaluated by calculating the work done on the system or the object. principle (mass conservation in this case) to a fluid element fixed in space. Non-conservative forms For isothermal (constant temperature) incompressible flows energy equation (and therefore temperature) can be dropped and only the mass and linear momentum equations are solved to obtain the velocity and pressure fields Work-Energy Principle. The work-energy principle is a general principle which can be applied specifically to rotating objects. For pure rotation, the net work is equal to the change in rotational kinetic energy:. For a constant torque, the work can be expressed as. and for a net torque, Newton's 2nd law for rotation gives Combining this last expression with the work-energy principle gives a.

The energy equationis a statement of the con-servation of energy principle. In fluid mechanics, it is found convenient to separate mechanical energyfrom thermal energyand to consider the con-version of mechanical energy to thermal energy as a result of frictional effects as mechanical energy loss.Then the energy equation becomes th Principle of (Real) work and (Real) energy (for conservative systems) Real external work done = Real internal energy stored => only 1 unknown displacement/rotation can be solved for 1 applied force/moment. Note : This principle provides 1 scalar equation for the whole structure Examples: Example : A= 0.1 m 2; E = 210 Gpa ; P = 1KN A B C 3 m 4 m 5 ** Bernoulli's equation is a form of the conservation of energy principle**. Note that the second and third terms are the kinetic and potential energy with m replaced by ρ. In fact, each term in the equation has units of energy per unit volume. We can prove this for the second term by substituting ρ = m/V into it and gathering terms Uncertainty Principle Formula Questions: 1) Assume an electron is confined to a atom of size 0.4 nm, what is the energy average of the particle in the atom? Answer: From the equation above we find Δp, which is the average momentum of the particle in the atom (-24) Kg*m/s. The energy is given by Δp 2 /2m, where 9.10938356 × 10-31 kg is the. Using these values, and the formula for conservation of energy, the final kinetic energy can be found: The kinetic energy of the moon rock immediately before it hits the surface of the moon is 5.00 J. 2) A block of wood on a table is forced against a horizontal spring. This compresses the spring, so that it has 18.00 J of elastic potential energy

This section covers Work, Energy and Power using maths. Work Done. Suppose a force F acts on a body, causing it to move in a particular direction. Then the work done by the force is the component of F in the direction of motion × the distance the body moves as a result. Work done is measured in joules (which has symbol J). So if we have a constant force of magnitude F newtons, which moves a. Concepts of work, kinetic **energy** and potential **energy** are discussed; these concepts are combined with the work-**energy** theorem to provide a convenient means of analyzing an object or system of objects moving between an initial and final state The law of conservation of energy is one of the basic laws of physics along with the conservation of mass and the conservation of momentum. The law of conservation of energy states that energy can change from one form into another, but it cannot be created or destroyed.Or the general definition is: The total energy of an isolated system remains constant over time The equation for relativistic energy looks like this. E =. mc2. √ (1 − v2 / c2) Applying the correspondence principal to give us the classical equations is not so easy here. Once again, at low speeds the denominator is one, but the numerator we're left with is something new. Something with no classical counterpart * (1) Choose the appropriate fundamental physical principles from the laws of physics, such as (a) Mass is conserved*. (b) F =ma (Newton's 2nd Law). (c) Energy is conserved. (2) Apply these physical principles to a suitable model of the ﬂow. (3) From this application, extract the mathematical equations which embody such physical principles

* Specific energy curves of different discharges at a given channel section At a smaller depth, y1 Q can be delivered only by a higher velocity and, a higher specific energy*. The state of rapid and shallow flow through a section is known as supercritical flow or rapid flow. At a larger depth the same discharge may be delivered through th The mathematical description of Gibbs energy is as follows. (14) G = U + p V − T S = H − T S. where G is the Gibbs energy of the system. The fundamental thermodynamic equation for Gibbs Energy follows directly from its definition 14 and the fundamental equation for enthalpy 8: (15) d G = d H − d ( T S The basic equation for kinetic energy is shown in Equation 4.2.8: If all the initial potential energy has changed into kinetic energy, it must be true that the potential energy at the start of the process equals the kinetic energy at the end of the process. To this end, it can be deduced that Mechanical energy is the sum of the potential and kinetic energies in a system. The principle of the conservation of mechanical energy states that the total mechanical energy in a system (i.e., the sum of the potential plus kinetic energies) remains constant as long as the only forces acting are conservative forces

- Bernoulli's Equation. As per Bernoulli's principle, Pressure energy (P.E) + Kinetic Energy (K.E) + Potential Energy (Pt.E) = Constant. That means, P.E + LK.E + Pt.E = Constant. P 1 + 1/2ρv 1 2 + pgh 1 = P 2 + ½ ρv 2 2 + pgh 2 = Constant. where v is the fluid velocity, ρ is the fluid density, h is relative height, P is pressur
- The Bernoulli's equation can be considered to be a statement of the conservation of energy principle appropriate for flowing fluids.. It is one of the most important/useful equations in fluid mechanics.It puts into a relation pressure and velocity in an inviscid incompressible flow.The Bernoulli's effect causes the lowering of fluid pressure (static pressure - p) in regions where the.
- This principle is generally known as the conservation of energy principle and states that the total energy of an isolated system remains constant — it is said to be conserved over time. This is equivalent to the First Law of Thermodynamics, which is used to develop the general energy equation in thermodynamics.This principle can be use in the analysis of flowing fluids and this principle is.

** Two things differentiate the momentum principle from the work energy**. First, it is technically a vector equation because the momentum of an object depends upon its direction of movement. Second. Box 1. The free-energy principle Free-energy is a function of a recognition density and sensory input. It comprises two terms; the energy expected under this density and its entropy. The energy is simply the surprise about the joint occurrence of sensory input y and its causes #. The free-energy depends on tw

- Conservation of energy, principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant. The first kind of energy to be recognized was kinetic energy, or energy of motion.In certain particle collisions, called elastic, the sum of the kinetic energy of the particles before collision is equal to the sum of the kinetic energy of the.
- This principle of work and its relationship to kinetic energy is a core mechanical physics concept. The work-energy theorem describes the direct relationship between work and energy. Its formula shows that net work done by forces acting on a particle causes a change in that particle's kinetic energy
- The principle of virtual work has nothing to do with linear elasticity, but it is convenient to start thinking about energy methods by reviewing a principle that you've probably seen before. Let be a state of stress satisfying. Furthermore, let be a displacement field on and define. Then. This is known as the Virtual Work equation
- Buy Now $49. What if you could have endless energy and feel totally optimal in your health every single day? The Formula 4 Maximized Energy teaches you how to build a strong foundation of energy reserves. There's no learning curve. Just watch and listen and unlock the secrets to unlimited energy

Energy is defined as the capacity to do work. The energy in the universe exists in many forms, and energy can be transformed from one form to another. But the energy transformations have a constraint that is based on the principle of conservation of energy. This principle states that energy can not be created and can not be destroyed Energy Balance principles are critical to understanding how radiation from the sun causes evaporation of water from: the ocean, the ground, and crop plants. Our crops are in the hot sun all day and use a lot of water to stay cool. There is no easy way to measure the amount of water they need, but we can calculate it using energy balance principles For steady flow process, net quantity of energy contained within the system will never change with respect to time. Therefore according to the principle of conservation of energy, we will have following statement and energy equation for a steady flow process of breastfed and formula-fed infants (634, 701 and 900 kcal/d at 6-8, 9-11 and 12-23 months, respectively; Butte et al., 2000), because resting metabolic rate is higher in formula-fed infants. However, because these guiding principles are targeted primarily at populations i

Value of strain energy per unit volume corresponding to the yield point of the material under tension test will be given by following equation as mentioned here. Uy = (1/2E) x σ t 2 Where σ t is the principle stress at elastic limit under tension tes Derivation of Kinetic energy formula and energy principle. Last Post; Jun 9, 2014; Replies 5 Views 2K. Kinetic energy formula. Last Post; Nov 11, 2013; Replies 2 Views 1K. R. Kinetic energy formula. Last Post; Nov 28, 2012; Replies 5 Views 5K. K. Is the classical formula for kinetic energy wrong? Last Post; Feb 6, 2016; 2. Replies 43 Views 6K. O Define mass-energy equivalence. mass-energy equivalence synonyms, mass-energy equivalence pronunciation, mass-energy equivalence translation, English dictionary definition of mass-energy equivalence. n

* Conservation of Energy in the motion of simple pendulum*. In a simple pendulum with no friction, mechanical energy is conserved. When a simple pendulum oscillates with simple harmonic motion, it gains some kinetic energy because of this type of motion. As the pendulum swings back and forth, there is a constant exchange between kinetic energy and gravitational potential energy On this slide we derive a useful form of the energy conservation equation for a gas beginning with the first law of thermodynamics. If we call the internal energy of a gas E , the work done by the gas W , and the heat transferred into the gas Q , then the first law of thermodynamics indicates that between state 1 and state 2 Units in Bernoulli calculator: ft=foot, kg=kilogram, lb=pound, m=meter, N=Newton, s=second. Bernoulli (Energy) Equation for steady incompressible flow: Mass density ρ can be found at mass density of liquids and gases. g = acceleration due to gravity = 32.174 ft/s 2 = 9.806 m/s 2.. The steady state incompressible energy equation (also known as the Bernoulli equation) models a fluid moving from. The energy balance equation (E S = E I - E O) is a statement of the principle of energy conservation. Components of intake Energy intake includes 3 major macronutrient groups—carbohydrate, protein, and fat—and a smaller component from alcohol Schrodinger equation in spherical coordinates 4.2.2 . Angular momentum operator 4.2.3 . 4.4.3 Pauli exclusion principle . the solutions to the energy eigenvalue equation (i.e. the time-independent Schrodinger equation) are now only a discrete set of possible values (a discrete set os energy levels)..

The Uncertainty Principle applied to time and energy has an interesting application: it's used to assign a lifetime to very short-lived particles. In essence, the 'spread' around their mean energy (ΔE) is used to calculate their lifetime through the ΔEΔt = ħ/2 equation STEADY FLOW ENERGY EQUATION. First Law for a Control Volume (VW, S & B: Chapter 6) Frequently (especially for flow processes) it is most useful to express the First Law as a statement about ratesof heat and work, for a control volume.; Conservation of mass (VW, S & B: 6.1). Conservation of Energy (First Law) (VW, S & B: 6.2). * Bernoulli's Equation 18*.Equivalent to the Conservation of Energy principle. a. P/ρ ~ potential energy Æ PRESSURE HEAD b. V2/2 ~ kinetic energy Æ VELOCITY HEAD c. gz ~ gravitational potential energy Æ ELEVATION HEAD 19.Bernoulli's Principle shows the relationship between pressure, velocity

Kinetic energy is created when a force does work accelerating a mass and increases its speed. Just as for potential energy, we can find the kinetic energy created by figuring out how much work the force does in speeding up the body. Remember that a force only does work if the body the force is acting on moves in the direction of the force Qualitatively, the wave function provides a probability distribution for each observable we want to measure (think energy, momentum, position, total energy, etc). Deriving the Heisenberg Uncertainty Principle. The derivation of the generalized uncertainty formula will be covered at a later stage

The energy of a moving object is of course still larger -- in Newtonian physics by an amount given by the well-known kinetic energy formula (1/2) m v 2. The correct expression according to relativity is E = g m c 2. for the total energy, and hence E = (g - 1) m c 2. for the kinetic energy, where g is the same relativity factor used previously the change of the kinetic energy of the particle. The Bernoulli equation is a mathematical statement of this principle. In fact, an alternate method of deriving the Bernoulli equation is to use the first and second laws of thermodynamics (the energy and entropy equations), ra-ther than Newton's second law * 1 Strain Energy Strain energy is stored within an elastic solid when the solid is deformed under load*. In the absence of energy losses, such as from friction, damping or yielding, the strain energy is equal to the work done on the solid by external loads. Strain energy is a type of potential energy If the kinetic energy is decreased so that K = 0 the body will be stationary, but will still possess energy m 0 c 2 . In other words the body contains energy E 0 when stationary relative to its frame and will have mass m 0 . This is called the rest mass. This is shown as: where: This, then, completes the derivation of E = mc 2 for a body at rest

- There are two ways to begin a mechanical energy problem. The first is to begin with the equation. KE 1 + PE 1 = KE 2 + PE 2 - W nc. and to fill into all relevant terms. You can also begin with an energy chain (track the energy throughout the problem) and write a term in your equation for each term in the chain
- Formation Energy Calculation As the first approximation, treat the vacancy as an interior surface: where R is the radius of the hole. The corrected vacancy formation energy: ∆E =4πR2∆σ c f c corrected ∆Ef =∆E +∆E T. R. Mattsson and A. E. Mattsson, Physical Review B, 200
- The Bernoulli Equation: Concept and Derivation Now, consider the movement of a particle along a pathline in an ideal fluid, and define Equations 9 and 10 are statements of the principle of conservation of energy as it applies to a single fluid particle at two different locations and two different times. The two equations expres

The three main equations representing the relationships between energy, work, and force. Work and energy in physics share a close relationship. According to the work-energy principle, an increase in a rigid body's kinetic energy is caused by an equal amount of work done on that body by a force applied to that body. In more mathematical terms. Work-Energy Principle --The change in the kinetic energy of an object is equal to the net work done on the object. Energy can be defined as the capacity for doing work. The simplest case of mechanical work is when an object is standing still and we force it to move. The energy of a moving object is called kinetic energy

d = depth of flow (ft. or m). For some applications, it may be more practical to compute the total energy head as a sum of the water surface elevation (relative to mean sea level) and velocity head. Equation 6-8. WS = water-surface elevation or stage (ft. or m) = z + d. Specific Energy Equation Principle of work and energy (continued) The principle of work and energy cannot be used, in general, to determine forces directed normal to the path, since these forces do no work Note that the principle of work and energy (T 1 + U 1-2 = T 2) is not a vector equation! Each term results in a scalar value Both kinetic energy and work have the.

More than 200,000 people agree: Precision Nutrition is different. Our world-class experts have spent the last 15 years working 1-on1 with thousands of nutrition coaching and certification clients. With this research and experience, we've uncovered an exact formula for getting results It can only be placed within one shelf. Each energy level is labeled with the quantum number n (n=1, 2, 3,...) and the energy of a particular level can be determined by the following: En=-Rh (1/n.

Calculation of hydroelectric power and energy Principle. The principle of hydro electricity generation is quite simple. Circuit waterworks provides the necessary pressure of water supplied to the turbine blades, which drives a generator, producing electricity. Formula to calculate hydropower. How to calculate output power of a hydroelectric. The frictional resistance to this motion is 10 N and g = 9.81 m/s2. Using a) d'Alembert's principle, then b) conservation of energy principles, find: i) the work done in moving the load as described. ii) the maximum input power provided by the pulling device Conservation of Energy formula. Total energy = kinetic energy + potential energy conservation of energy equation. Consider a body of mass m placed at a point p which is at a height h from the ground. P.E of the body at A =mgh K.E of the body at point A =0 The total energy of the body at point P=K.E +P.E =0 + mg