Unit 6 Reflection!

Work:
The first thing we talked about in this unit was about work! Work is the effort put n something that changes its energy. The equation for work is;

work = fd

It is really important to remember that the force and distance of the equation above have to be parallel! That mean that if someone is walking around (on a flat surface) carying a tray he is not doing any work on it. The distance the person is going is perpendicular to the force that is pushing the tray up. Now imagine that someone is walking up the stairs,  the distance now is parallel to the force, that in this case is the weight of the person. Remember that if you were to calculate the work done by the person climbing up the stairs you wold use the distance from the very top straight down, not the diagonal extension of the stairs! What matters in the stairs scenario is how far up from the ground you are, not how much distance you walked. Remember that if you are pushing against a wall, you are not doing work on the wall since its not moving, but you are doing work within yourself inside your body.

Power:

Power is a rate of  how fast work is being done! The equations for power is


Power = work/time

Imagine that there is a race up the stairs and both participants have the same weight (force is constant), which of the competitors would have more power? The one that won the race! The winer of the race would have more power because he would have done the same amount of work (since the distance and force are constant) in a shorter amount of time!

Potential Energy:

Pontial Energy is the energy of possition, is the energy stored in "readiness", the energy that has the potential of doing work. Because Potential Energy is the energy of possition, ther higher something is from the ground the more PE it has! The Equation for Potential Energy is:

PE = mgh

Kinetic Energy:

Kinetic Energy is the energy of movement! The faster something is moving the more kinetic energy it has. The equation for Kinetic Energy is

KE = 1/2mv2

Another important thing to remember about KE is that the change in kinetic energy is equal to work. Therefore...

 change in KE = fd

Work = 1/2 mv2

Conservation of Energy:

Conservation of Energy's simple deffinition is enegy in is equal to energy out! Based on that deffinition we can talk about the relationship between the kinetic energy and potential energy. Let's say that there is a person jumping from a rock to the sea, when the person is on the top of the rock and about to jump he has PE. When he jumps from the rock he loses height and therefore loses PE  (PE = mgh), but since we have established that energy in is equal to energy out, the energy can't be lost right? Right, the energy is being transformed from PE to KE. Now the person does not have as much height but it is a greater velocity therefore no energy is lost, it is only transfered.

The same idea applies to roller coasters, the same hill is always the highest one so that it accumulates enough energy to go through all the elevations. As the car goes down, the PE goes down but KE goes up. If added together at any part of the ride, the KE and PE of the car will equal the PE that the car had on the first hill.

Machines:

Machines can be understood by using the concept of conservation of energy. We said earlier that energy in = energy out and that change in KE = work. That means that the work in also equals the work out! A machine works by increasing the distance you put in in order to decrease the force (work = fd).  To decrease the force you have to exert, you will have to pull for a longer distance and the distance that the object will move will be smaller that the one you pulled. Regardless of how the distances turn out to be the work in wil always equal the work out.

Efficiency: 

In real life no machine works with 100% efficiency. There is always some energy that is transformed in something else, like in heat due to friction. The efficiency of a machine can be meassure by dividing the energy output by the energy input.

Reflection:

The first part of this unit was easy to understand. The concept of work seemed easy enough to understand. On the quizzes about work tough... I fell for every single one of the questions that asked how much work someone walking with their books is doing. No work, the  answer is no work! I found it hard not to be ready to plug in numbers that were given to me before analising if there was any work being done at all. When question asked for me to say what would happened to the work if the velocity was doubled I also got a little confused but after going over on of those questions with my teacher I now know what to do! The questions about the change in kinetic energy, when the velocity went from a number to another also tricked me. I made the mistake of calculating the difference in velocity first and using that number in the kinetic energy formular, while I learned that I should have calculated the kinetic energy of each individually, and then found the difference.  This energy was easy in theory but complicated in practice but after looking back to some quizzes I found out what mistakes I made and now know what not to do!