Saturday, June 22, 2013




1.1 Background

"Some of the problem is often more difficult than what it seems" (Young, 2002:164). As you try to find a new pace arrows released from a bow. You use Newton's laws and all problem-solving techniques that we've learned, but you are having difficulty. After the archers shooting arrows, bowstring give varying styles that depend on the position of the arc. As a result, a simple method that we've learned enough not to  its speed. Never fear, there are still other methods to solve these problems.
The new method that we will soon see the use of ideas and energy work. We will use the concept of energy to study the physical phenomena that range very widely. We will develop the concept of work and kinetic energy to understand the general concept of the energy and we'll see how the conservation of energy appears.

1.2 Problem Formulation
1.2.1 What is a business?
1.2.2 What is energy?
1.2.3 What is the power, the power unit, and efficiency?

1.3 Objectives
This paper is intended to help improve the understanding of the concepts of effort and energy that will allow us to solve the problems that previously could not be solved by mechanics.


Business 2.1

2.1.1 Definition of Business
What is the difference in the business of everyday life with the physics? In everyday life, the effort can be interpreted as an activity with exertion, mind, or body to achieve certain goals. Enterprises can also be interpreted as the work to achieve certain goals.
In physics, understanding the business similar to the business sense in everyday life. Similarity is in terms of activities with exertion. Business understanding in physics always involves force or energy. If something (human, animal, or machine) doing business that do business then it should issue a certain amount of energy to produce movement.
NurAzizah (2007:46) states "effort is the product of the force to the displacement experienced by earlier styles.
So, if an object is given a stylish yet not experienced displacement object, then the object is said to attempt to zero ".
For example, a machine doing business when lifting or moving something. Someone carrying bricks to the second floor of a building has been doing business.
When walking, your leg muscles do business. However, if you only hold an object so that the object is not moving, it is not doing business. Someone who already hold a large stone so it does not roll down any attempt, even if that person has exerted all his strength to hold the stone. So, in physics, work related to the motion of an object. When we push or pull objects, we expend energy. Business that we do look at the displacement of the object.

2.1.2 Business Conducted by Constant Style
Work done by a constant force (large or direction) is defined as the multiplication of displacement  point with force components in the direction of the displacement (Halliday, 1985:174).
To move an object whose mass is larger and at longer distances, greater efforts are needed anyway.
On the basis of this fact, a business is defined as the product of force and displacement that occurs (NurAzizah, 2007:46).
If the business symbolized by W, force F, and the displacement s, then:
Neither force nor displacement is a vector quantity. In accordance with the concept of dot product between two vectors, then the business is a scalar quantity W (Halliday, 1985:176).
If the angle formed by the force F with displacement s is θ, then the size of the business can be written as: W = (F cos θ). S (NurAzizah, 2007:47).

Force component F sin θ is said to not do business because there is no displacement in the direction of the component.
Satriawan (2008) concluded as follows.
Of the business equation formula, it can be said that the work done by a force:
a. Directly proportional to the magnitude of the force,
b. Directly proportional to the displacement of the object,
c. Depends on the angle between the direction of the force and the displacement of the object.
If we review our business formula equation more carefully, we get some special circumstances associated with the direction of the force and the displacement of the object is as follows:
a. If θ = 00, the same style or direction coincides with the direction of displacement of the object and cos θ = 1, so that the work done by the force F can be expressed:
W = F. s cos θ
W = F. s. 1

b. If θ = 900, then the direction of the force F perpendicular to the direction of displacement of the object and cos θ = 0, so W = 0. So, if the force F acting on an object and the object moves in the direction perpendicular to the direction of the force, said the force was not doing business.
c. If θ = 1800, then the direction opposite to the direction of force F and the displacement of the object value of cos θ = -1, so that W has a negative value. It can be interpreted that style or that it did not make an effort and do not expend energy objects, but getting energy. An example is an object that is thrown vertically upward. During the upward moving objects, gravity direction opposite to the displacement of the object object. It can be said that gravity is a negative thing to do business.
Another example is an object that is driven on rough surfaces and moving objects as shown in Figure 2.2. On the thing worked two styles, the force F and the frictional force fk the direction opposite to the direction of displacement of the object.

If the displacement of the object as far s the force F doing business: W = F. s, while the friction force fk do business: W = fk. s
d. If s = 0, then the force did not cause objects to move. That means that W = 0. So, although there are forces acting on an object, but if the object was not moving then, that style was not doing business.
2.1.3 Business Unit
Satriawan (2008) states that.
In the SI unit of force is the newton (N) and displacement units are meters (m). Thus, a business unit of the multiplication of the unit of force and displacement units, ie newton meters or joules. Joule unit Presccott chosen in honor of James Joule (1816 - 1869), a British scientist who is famous in his research on the concept of heat and energy.

1 joule = 1 Nm
since 1 N = 1 kg. m/s2
then 1 joule = 1 Kg. m/s2 x 1 m
1 joule = 1 Kg. m2/s2
For larger businesses, typically used units of kilo joules (kJ) and mega joules (MJ).
1 kJ = 1000 J
1 MJ = 1,000,000 J

2.1.4 Calculating the Business of Graphic Styles and Displacement
If the forces acting on a massive object and its direction remains the graph between the F and the displacement s is a straight line parallel to the horizontal axis s, as in Figure 2.3.
Figure 2.3 Graph of the displacement force F s if the magnitude and direction of F remains
Of graphs F - s, equal to the area of ​​business up bounded by a line graph with horizontal axis s.

Business: W = wide shaded area
Thus, from the diagram F - s can be concluded that the work done by the force F is equal to the area bounded up a line graph with horizontal axis s (NurAzizah, 2007:47).

2.1.5 Business Conducted by Some Style
In real life almost never the case we find an object only works on a single style. For example, when you pull a block along the floor. In addition to pull you provide, the beam also works the other styles such as: friction between the block and the floor, the wind resistance force, and normal force.
So, the work done by the resultant some styles that have the same capture point is equal to the sum of work done by each force. If the two work on a body style then the work done is:
W = W1 + W2
If there are more than two styles:
W = W1 + W2 + W3 + ...... + Wn
or W = ΣWn

2.1.6 Negative Business
A child pushes a beam with his hands. In accordance with Newton's third law, it can be concluded that the forces acting on the beam and hand in this case is equal but opposite in direction, ie FAB =-FBA. Negative sign indicates the opposite direction. If efforts by the hand on the beam is positive (because the direction of the displacement of the beam), then the attempt by the beam on hand is negative.

2.2 Energy
Energy plays a very important in the life of this nature. Energy states the ability to conduct business. A system (human, animal, or object) is said to have energy if you have the ability to do business.
Energy possessed by, the objects are moving is called the energy of motion or kinetic energy, while the energy possessed by an object because of the position or state of matter is called potential energy.
2.2.1 Kinetic Energy
How much energy is possessed by the objects with a certain mass and moving at a certain speed? For example, we threw a ball of mass m.
If the force exerted on the ball was constant at F and can move it's out of our hands so far, then according to Newton's second law, the acceleration of the ball obtained by:

It is known that a stationary object, if obtained through the acceleration a distance s, then finally speed can be expressed by the equation:
V2 = 2 a. s
If a is replaced with, the above equation becomes:

F. s is the magnitude of the work done by our hand at throwing the ball, while ½ m. V2 is the amount of energy that got the ball, hereinafter called kinetic energy. Thus, if the kinetic energy Ek is expressed by the symbol:

Ek = kinetic energy (J)
m = mass (kg)
V = velocity (m / s)
Thus, the kinetic energy of an object that has a mass m and velocity V, is ½ m. V2. Because m is expressed in units of kg and V in units of m / s, then in units of joules (J). Business Law and Kinetic Energy
An object whose mass m moving with velocity V1, while the position of objects in A, fixed work force F in the direction of the motion. After t seconds, the position of objects in so far as s B of A and the velocity is changed to V2.

Because the force F, uniformly accelerated moving objects, so that the relationship applies:
| S = V1. t = ½ a. t2 | (a)
Because V2 = V1 + a. t, then:
| | (B)

With a substitution equation to equation b obtained:

Effort force F for moving objects from A to B is:

So, the work done by a force on an object is equal to the change in kinetic energy of the object.
Satriawan (2008) concluded that.
Businesses can be positive and can also be negative. Therefore, the kinetic energy can also be positive or negative. So, there are the following two possibilities:
1) If W> 0 then Δ Ek> 0
That means that the work done by a force equal to the addition of the kinetic energy of objects.
2) If W <0 then Δ Ek <0
That means that the work done by a force equal to the reduction in kinetic energy of objects.

2.2.2 Potential Energy
Satriawan (2008) states "the general potential energy is energy stored in an object or in a specific situation". Examples of potential energy contained in a waterfall, in coal, in our body there is the potential energy.
Potential energy stored in the water that are useful over a new cliff when converted into heat energy through combustion. Potential energy in our bodies would be useful if we turn it into energy of motion is done by the muscles of our body.
"In a more narrow sense, or the mechanics, the potential energy is the energy possessed objects because of the position or state of the object" (NurAzizah: 2007). Examples of potential energy in this sense is the gravitational potential energy and elastic potential energy. Gravitational potential energy possessed by objects that are at a certain height from ground level. While the elastic potential energy possessed by such a stretched rubber slingshot. Elastic potential energy in this new rubber slingshot useful when the strain is released, causing changes in elastic potential energy into kinetic energy (the slingshot pebble thrown in). Gravitational Potential Energy
Objects that are at a height h has the potential to do work for m. g. h. Therefore, be said that the object has a gravitational potential energy.
So, the higher the position of objects on the ground, the greater the potential energy.
Thus, we define that the gravitational potential energy of an object is the product of its weight anda h, so it can be written:

Ep = gravitational potential energy (J)
m = mass of object (kg)
g = acceleration due to gravity (ms-2)
h = height of the object of the reference soil (m)
What if the object trajectory is not vertical but tilted upward as shown 2.6? To simplify matters, we suppose that the appointment of the object through a straight line from A to B.
W = F. s
W = m. g. sin θ. s

From images obtained by equation 2.6:
sin θ = h: s or h = s. sin θ

So that:

It turns out the same equation obtained existing formulation. Thus, the gravitational potential energy does not depend on length of the track, but it just depends on the position of the end. It can be stated also that the gravitational potential energy possessed by an object at a particular position depends only on the height difference between the position of the object.
Now we consider a body of mass m, initially located at point A at a height h from the reference plane. If the object is released, the object will move vertically downward due to gravity. To reach point B which height h2 (h2 <h1), gravity of objects perform operations of:
W = m. g (h1 - h2)

m. g. h1 = gravitational potential energy at the position in A (J)
m. g. h2 = gravitational potential energy at the position in B (J)
From the above equation can essentially stated that the work done by a force equal to the weight of an object is its potential energy reduction.
More briefly, the above statement can be formulated:

In this case, there are three possible price W, as follows:
1) W> 0 (positive) and Ep <0 (negative) means business with a potential energy reduction.
2) W <0 (negative) and Ep> 0 (positive) means business with a potential energy increase.
3) W = 0 and Ep = Δ 0 (negative) means, potential energy fixed objects. This can happen if the displacement of the object in the horizontal plane.

Elastik Potential Energy Spring
The work done by the spring force to move objects from one position to the deviation = x1, to position 2 with deviation = x2, is:

In general, we can express the elastic spring potential energy formula (Epelastik) as follows:

2.2.3 Mechanical Energy
Satriawan (2008) states "the mechanical energy is the sum of potential energy and kinetic energy of an object at a time".
Mechanical energy is formulated:

Em = mechanical energy (J)
Ep = potential energy (J)
Ek = kinetic energy (J) Law of Conservation of Mechanical Energy
Figure 2.7 depicts an object that falls freely from a height. Here, the object affected only by gravity, which is a conservative force. Objects up at point A at a height hA has speed VA. After arriving at point B, at an altitude of HB objects moving at the speed of VB.
If the object gravity w = m. g, gravity businesses for falling objects from A to B is:

Based on business law and the kinetic energy is obtained:

Equating the above two equations is obtained:

The above equation can also be written as follows:

Thus, the law of conservation of energy states that, if an object is affected only conservative forces so that wherever mechanical energy position is constant (fixed).
2.3 Power
Power is defined as work done by an object per unit time. Thus, the power (P) is calculated by dividing the effort (W) which is conducted on an interval length of doing business (t).
Since business is the multiplication of force with displacement (W = F.Δx), then the equation can be written as follows:

2.3.1 Power Unit

Effort in SI units are joules (J), while the unit of time is seconds (s). So the SI unit for power is:

The SI unit of power is the watt (W) in honor of James Watt (1734-1819), an engineering expert from Scotland who successfully invented the steam engine. Thus:
One watt is the power Therefore, power is often expressed in SI units are larger, ie kilowatt (kW) and megawatt (MW).
1 kW = 105 W = 1000 W
1 MW = 106 W = 1000000 W
In everyday life, especially in engineering equipment, such as pumps, automobile engines and motors, power expressed in hp (horse power), or pk (paarde kracht) or hp (horse power).

2.3.2 Efficiency
The evidence suggests that the energy converter is not possible to change all the energy it receives into useful energy. Most energy will be transformed into energy which is not useful.
Satriawan (2008) states "if the energy received by the converter input and the energy we call the energy converted into useful forms we call output, then the efficiency is defined as the quotient of output and input multiplied by one hundred percent".
Efficiency can be written by the equation:

For example, the incandescent light bulb, which is a tool that converts electrical energy into light energy. A 100 W light bulbs means it receives 100 J of electrical energy within 1 second. If 100 J of energy received only 40 J of energy is converted into light, it is said that the efficiency of light is equal to:

That is, just as much as 40% of electrical energy is converted into light energy (useful energy). And as much as 60% of the electrical energy that is received is converted into heat energy (energy that is not useful).
If efficiency is expressed by the power, the efficiency of the above equation can be written by the equation:


3.1 Conclusion

Effort is the product of the force exerted by the displacement experienced by the object. Effort in SI units are joules (J).
Energy states the ability to perform possessed by moving objects is called kinetic energy, while the energy possessed by the object due to its position is called potential energy.
Power is the rate of work done per unit time or large businesses. The SI unit of power is the watt (W)

3.2 Advice
For those readers are advised that this paper can serve as a medium of learning in order to improve understanding of the effort and energy. And for other writers hoped that this paper can be cultivated in order to refine further papers that have been made previously.


Nurazizah, Siti. 2007. Acuan Pengayaan Fisika SMA Kelas XI Semester 1. Solo: Nyata Grafika Media Surakarta.
Resnick, Halliday. 1985. Fisika Jilid 1 Edisi Ketiga. Jakarta: Erlangga.
Satriawan, Mirza. 2008. Materi Fisika Dasar, (Online), (, diakses 10 november 2009).
Young, Hugh D & Roger A Freedman. 1999. Fisika Universitas Edisi Kesepuluh Jilid 1. Jakarta: Erlangga.

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