English Of Physics

DISCUSSION

1.      Concept of Work in Physics

In physics, the word work has difference in meaning with that of used in everyday life. In everyday live, work is defined as anything done by human. While in physics, work is the defined as force acting upon an object that causes the object to displace. Therefore, if a force acts upon an objects but the object does not displace, then it is said that the force does not apply work.

For example, man is pushing a car. The man tries hard to push up the car with all of her energy but the car does not move at all. Whether the man is said to have attempted or doing work? In the everyday life of the man is said to have been doing work, meaning that the man has been doing a task or job. Understanding the work is different in everyday life is different in the sense of work in physics. In physics, work is defined as the product of the force and displacement. So, suppose there is a person who pushed the table but the table is not moved, saying it has not made the work.

If an object is to get the style and experience the movement of the things that gives the style had been doing work. And if an object exerts a force even though the style is great, but the object is not experienced then its displacement is zero or not going work. If an object is the force (F) and the object is experiencing displacement (d) that the direction of the force applied then the force has been doing work (W) of the force applied multiplied by the displacement.
Then the work done by the force can we write in mathematically as follows:

By:
 = joule or work unit Nm
 = Force unit N or kg m/s2
 = displacement unit m

With a quantity  trade above can be negative or positive and can be worth zero.

Example question:
An object on the pull with a force of 40 N to the north and 120 south Newton resulting object is shifted as far as 2 meters. Find work working on this style.

Hint:
F1 = 40 N
F2 = 120 N
Then R= F2-F1 = 120 N - 40 N = 80 Newton
S = 2 meters
Question:
W = .... Joule
Answer:
W = F d = 80 N x 2 m = 160 Joules

2.      Concept of Energy in Physics

Energy and work occupy an important part of our ordinary life and are among the most important topics in physics. Energy exists in several forms such as heat, mechanical energy ( potential and kinetic energy), light, electrical, or other forms.  But in this case, focus of explanation in mechanical energy.

We use energy to do work. The concepts of work and energy are closely tied to the concept of force because an applied force can do work on an object and cause a change in energy. Energy in physics is defined as the ability to do work. When energy is changed from one form to another, the total amount present does not change. The SI unit of energy is the joule (J) or newton-meter (N * m). The joule is also the SI unit of work.

Mechanical energy is the energy that is possessed by an object due to its motion or due to its position. Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position). Objects have mechanical energy if they are in motion and/or if they are at some position relative to a zero potential energy position (for example, a brick held at a vertical position above the ground or zero height position). A moving car possesses mechanical energy due to its motion (kinetic energy). A moving baseball possesses mechanical energy due to both its high speed (kinetic energy) and its vertical position above the ground (gravitational potential energy).

a.       Potential Energy

An object can store energy as the result of its position. Potential energy is the stored energy of position possessed by an object .

For example : A drawn bow is able to store energy as the result of its position. When assuming its usual position (i.e., when not drawn), there is no energy stored in the bow.

Gravitational Potential Energy

As an object falls toward the Earth, the Earth exerts a gravitational force on the object, with the direction of the force being the same as the direction of the object’s motion. The gravitational force does work on the object and thereby increases the object’s kinetic energy.

There is a direct relation between gravitational potential energy and the mass of an object. More massive objects have greater gravitational potential energy. There is also a direct relation between gravitational potential energy and the height of an object. The higher that an object is elevated, the greater the gravitational potential energy. The symbol for gravitational potential energy is Ug. These relationships are expressed by the following equation:

In the above equation, represents the mass of the object,  represents the height of the object and represents the gravitational field strength (9.8 N/kg on Earth) - sometimes referred to as the acceleration of gravity.

To determine the gravitational potential energy of an object, a zero height position must  first be arbitrarily assigned. Typically, the ground is considered to be a position of zero height. But this is merely an arbitrarily assigned position that most people agree upon.

Since the gravitational potential energy of an object is directly proportional to its height above the zero position, a doubling of the height will result in a doubling of the gravitational potential energy. A tripling of the height will result in a tripling of the gravitational potential energy.

Sample Question : A cat had climbed at the top of the tree. The Tree is 20 meters high and the cat weighs 6kg. How much potential energy does the cat have?

The answer is :

From the problem above, we conclusion that gravitational potential energy depend on mass and height.

b.      Kinetic Energy

Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or horizontal motion - has kinetic energy.

      When we do work on a ball by throwing it, what becomes of this work ? Let us suppose we apply the constant force F to the ball for a distance x before it leaves our hand, as Figure under. The work done on the ball is therefore, since ,

The mass of the ball is m. As we throw it, its acceleration has the magnitude

According to the second law of motion,

We know from the formula  that when an object starting from rest ( ) undergoes an acceleration of magnitude  through a distance , its final speed  is related to  and  by

 

Substitute  for  , we find that

 

Which we can rewrite as

The quantity on the left-side, , is the work our hand has done in throwing the ball.The quantity on the right-hand side, , must therefore be the energy acquired by the ball as a result of the work we did on it. This energy is kinetic energy, energy of motion. That is, we interpret that preceding equation as follows.

Work done on ball = kinetic energy of ball

The kinetic energy of an object of mass m and velocity  is therefore

Where  is mass of object,  is velocity.

Example :

Find the kinetic energy of the car.

Figure 9. Car drove with velocity

The answer is :

3.      Relation Between Work and Energy

a.       Relation Between Kinetic Energy and Work

Let’s examine how can we derive an equation that expresses the relationship between work and kinetic energy. Suppose we continue to impose force F on a cart in motion. In a direction parallel to that cart’s velocity. That cart has mass m and starts with an initial, uniform velocity of v.

Figure 11. Block drove until r m

Since F is defined as constant, the cart is experiencing uniform acceleration. Therefore, if we represent the cart’s acceleration with , we know that the following must be true :

Substitute using Newton’s second law :

And we will get the following

Then if we simply multiply both sides by 1/2 , we are there.

In this case, in which work is done on a system and the only change in the system is in its speed, the work done by the net force equals the change in kinetic energy of the system ( Work-Kinetic Energy Theorem). Work-Kinetic Energy Theorem applies to positive and negative work. If work, W, performed on positive object ( direction of net force in the same direction with displacement). But if work done is negative ( direction of net force is opposite direction with displacement, then  decreases. If total of work is zero, so kinetic energy of an object is constant.

b.      Relation Between Potential Energy and Work

To develop mathematical relationship between work and gravitational energy, start with the equation for work.

From the force of gravity on a mass near Earth’s surface is given by :

Substitute  into work equation :

Since gravitational potential energy is vertical displacement so the equation will be :

4.      Concept The Law of Conservation Mechanical Energy

The energy that is in us to connect with the energy in the universe. We will refer a little to the energy laws of physics, to parse and identify how energy works. Law of Conservation of Energy reads: "Energy can be transformed from one form to another but can not be created nor destroyed (energy conversion)".

Because energy is conserved, the energy in the universe is the number never changes, no waxes and wanes. There is only a change of energy from one form to another.

In the physical sciences are used today, counting the amount of energy over the energy changes that occur in an object / material. Because every material has energy.

The energy of a material can be calculated through the processes or specific causes, such as the energy of a moving object, the result of the combustion energy, the energy of a chemical process, electrical energy, and others.

Mechanical energy

What if a moving object has a certain height? So the answer is it has potential energy and kinetic energy. The second is the amount of energy called mechanical energy. In other words, the sum of the kinetic and potential energies—the total mechanical energy E—remains constant. This is an example of the principle of conservation

Em = Ep + Ek

An object moving in the gravitational field will apply the law of conservation of mechanical energy.

Em = Ep + Ek = eternal          and      Ep1 + Ek1 = Ep2 + Ek2

5.      Concept of Power in Physics

Power is the ability to convert a form of energy into a form of energy. For example, if there is a 100-watt light that efficiency is 100%, then every second lamp will change the 100 joules of electrical energy into light energy to 100 joules of light. The greater the power of the tool, the greater the ability of the instrument to change a form of energy into another form of energy. Mathematically, the relationship between power, work and time is defined as follows:

P   = Power (watt)

W  = Work (joule)

 t    = Time (s)

Example question: A man who climbed the ladder of mass 60 kg for 4 second. If the vertical height of the ladder is 4 feet, calculate the man power in watts, and the amount of energy needed to climb the ladder. Suppose the acceleration of gravity (g) = 10

Known  :    m  : 60 kg                h  : 4m

                t   : 4s                            g  : 10 

Large Power :

 

 

REFERENCES

Halliday-Resnick-Walker 8^th edition. Fundamental of Physics.