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Physics Lesson 5.1.2 - What is Energy?
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Welcome to our Physics lesson on What is Energy?, this is the second lesson of our suite of physics lessons covering the topic of Work and Energy. Types of Energy, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
What is Energy?
When one gets tired or is sick, we often say phrases words like "I have no energy left today to go at work" or "I cannot move from the bed as I lack energy". When we move, we must carry our body, we exert a force F at our feet. In this case, we can displace ourselves by Δx metres. Therefore, it is obvious that we must do some work in order to go somewhere even though we only carry our own body weight.
However, everybody knows from practice that we need to eat well if we want to walk long distances (displacements). We can conclude that we need a considerable amount of Energy supply from food in order to convert it into work while walking. From here, we can deduce the meaning of the Energy concept. Thus,
"Energy (E) is the ability of an object or system to do work."
This means more energy an object possesses more work it can do. If no energy is left, the object cannot do work anymore. Hence, the maximum work an object or system can do is equal to the energy it possesses.
Mathematically, we can write:
Equation 2
E = WmaxIn general, objects and systems (especially living organisms) keep some energy for other processes besides for boing work. They do not consume all their energy only for doing work. Therefore, we can write:
Equation 3
W = ∆E = Einitial - Efinal = E1 - E2From the above equation, it is easy to understand that when all the initial energy goes for doing work, there is no energy left at the end (E2 = 0). Therefore, we have
Equation 4
W = ∆E = Einitial - Efinal= E1 - 0
= E1
= Wmax
The last part of the equation 4 is simply the equation 2. Hence, we can say that eq. (2) represents a special case of eq. (4).
Remark!
Work and Energy are both scalar quantities, i.e. they are physical quantities that don't have direction but only magnitude.
Units of Work and Energy
It is obvious that since Work represents a change (difference) in Energy, they must both have the same unit. Since Force is measured in Newtons [N] and Displacement in metres [m], the unit of Work is [N ∙ m]. It is otherwise known as Joule [J]. Hence,
As stated before, Work and Energy have are measured using the same unit. Therefore, the unit of Energy is Joule [J] as well. This is the official (SI) unit of Energy.
In Thermodynamics (especially when dealing with food (chemical) energy and thermal energy, another unit is often used. It is known as Calorie [cal]. The conversion factor between Joule and Calorie is
Not always, the direction of force is in the direction of motion. In this case, we have to use only the component of force that lies in the direction of motion. Look at the figure below:
In this case, only Fx = F ∙ cos α contributes to the motion. Therefore, the work done by the force F will be
For example, if the force F in the above figure is equal to 50 N and the angle is 300 (cos 300 = 0.86), the work W done by the force F to move the object by Δx = 40 m is
=F × cos α × ∆x
=50N × 0.86 × 40m
=1720 J
Remark!
This is still a 1-D motion regardless the fact that the direction of force F is not in the direction of motion. You must consider only the force component that lies in the direction of motion. You can use the 1D Motion Work Calculator to calculate and check your own results when performing this Physics equation.
Example
A 50 kg girl took 1200 kcal energy from food. What linear distance can she travel using this amount of energy? Take g = 9.81 m/s2. Round up the result to the nearest whole number.
Solution
Here the linear distance represent the displacement. First, we have to convert cal to J. Also it is known that the prefix (k) stands for kilo, i.e. thousands. Thus,
= 1 200 000 cal
= 1 200 000cal × 4.18 J/cal
= 5 016 000 J
Also, it is known that Weight = Mass ∙ Gravity. Here Weight represents the force to be exerted by the girl in order to make herself move. Thus,
= 50kg × 9.81 m/s2
= 490.5 N
(From the Newton's Second Law, it is known that 1N = 1 kg × m / s2)
Hence, from the equation (1)
after rearranging, we obtain:
= 5 016 000 J/490.5 N
= 10 226 m
In reality, most of the energy generated by the consumption of foods goes for vital processes such as for keeping the body temperature constant, maintaining a normal blood flow, for breathing etc. Only a small portion is available for doing work.
You have reached the end of Physics lesson 5.1.2 What is Energy?. There are 5 lessons in this physics tutorial covering Work and Energy. Types of Energy, you can access all the lessons from this tutorial below.
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