Unit 5 Blog Post

Unit 5 Blog Post

            This unit we learned about work, power, kinetic energy, potential energy, the conservation of energy, and machines. I thought this unit was particularly cool because throughout the chapter we could see how all the physics concepts related to one another.

            Work is the effort exerted on something that will change its energy. Work equals force times distance. [Work=Force X Distance] Work is measured in Joules. The force and distance must be parallel to one another in order for there to be work done.

            For example, this is a waiter carrying a tray. I automatically assumed that work was being done on the tray. However, there is NO work being done on the tray itself. The force, which is the tray, being pulled down by gravity is not parallel to the horizontal distance the tray is moving.

However, there is work being done on the tray when the waiter lifts the tray. The force of the tray moving upward is parallel to the vertical distance the tray is lifted.

            Here is another example of work. You have the option to take the steeper hill or the more gradual side to get to the field hockey field, yet both sides will take you the same vertical distance. Therefore, the steeper side of the hill and the gradual side would require you to do the same amount of work because work is dependent upon force and the VERTICAL distance you traveled and both hills have the same vertical distance.

            The next concept learned in this unit is power. Power depends on work. Power is equal to the amount of work done per time it takes to do it. Power is measured in watts. The faster work is done, the more power there is.

[Power=Work/Time Interval] Power is measured in Watts. The amount of power is dependent upon how quickly work is done. The faster work is done the more power there will be. But remember the amount work done does not factor any time unit; it is power that depends on work and time.

            In class we had a lab involving walking/running up and down the stairs. The amount of work done when we walked up the stairs versus when we ran up the stairs was the exact same amount because the force and distance did not change. However, the power of walking versus the power of running up the stairs changed because the amount of time it took to go up and down the stairs changed. There was more power when we ran up the stairs because it took less time to complete the work. The faster the work, the more power there is.

            The next concepts we learned about were kinetic and potential energy.  Energy is the ability to do WORK. These two energies come from mechanical energy, which is the energy due to the position of something or the movement of something. Mechanical energy can be in the form of potential, kinetic, or a sum of the two energies. Potential energy is energy that is stored and held in a stored state that has POTENTIAL of doing work. Potential energy is equal to the combination (multiplied) of mass, gravity, and height.

[Potential Energy= Mass X Gravity X Height] A simple example of potential energy is a ball sitting on the top of a cliff about to fall. Another example involves a bow an arrow. When a bow is drawn, energy is stored in the bow. 

            Kinetic energy is the energy of motion. Potential energy can change into kinetic energy. The change in kinetic energy of a moving object depends on the mass of the object as well as its speed.

[Change in Kinetic Energy=1/2mass X velocity^2] Anything in motion has kinetic energy. According to the work energy theorem, work equals change in kinetic energy. Potential energy and kinetic energy are both measured in watts because they are both forms of energy and energy depends on work. Whenever work is done, energy is exchanged. 

            The next concept we learned was the conservation of energy. The conservation of energy states that energy cannot be created or destroyed; it may be transformed from one form into another; but the total amount of energy never changes. When you think of a system, like a swinging pendulum, there is one thing that is neither created nor destroyed, and that would be energy. The energy may change form, for example, it may turn into heat, however, that does not mean the energy is lost. Take a water dam for instance. The water behind a dam has energy that may be used to power a generating plant below, where it will be transformed to electric energy. The energy will then travel through wires to homes where it can be used for everyday uses. Because we know energy does not disappear or appear, it transforms, we can assume that kinetic and potential energy can transform into one another. Here is an example that can help to explain this concept.; Imagine a ball at the top of a cliff about to fall off,  at the top JUST before the ball is falling, it has 10,000J of PE and it has 0 KE. However, as the ball falls, its potential energy decreases and its kinetic energy decreases. However, the total amount of energy remains at a constant 10,000J. Right before the ball hits the ground, its potential energy has decreased to 0J and its kinetic energy has increased to 10,000J. Here we see the transformation of energy.

            When you think of a machine, you might assume it’s a complicated device. However, machines can be very simple. A machine is a device for multiplying force or simply changing the direction of force. Machines reduce or change force but NOT energy and work. The principle of machines comes from the conservation of energy. When you put work in there is an equal amount of work out.

[Work in=Work out] Since work equals force X distance…

[Force in X distance in = force out X distance out] Suppose you are loading a box to a truck. It will take much more force to load the box with a shorter distance than it would be if you were to add a ramp.

Work in (the ramp)= fd

Work out (without the ramp)= fd

Work in=Work out

fd=fd

An ideal machine would work with 100% efficiency. However, all machines have some sort of transformation of thermal energy. If we put in 100 J of work and get out 98 J of work, that machine is 98% efficient. It did not LOSE energy, it just wasted 2% of its energy because that 2% was transformed into heat.

As I mentioned before, this unit ties a lot of its concepts together. This is an example to show this connection of terms:

A 10kg car accelerated from 10m/s to 20m/s in 2 seconds. In that time it traveled 10m.

The change in Kinetic energy the car experience?

Change in KE= 1/2mv^2

Change in KE before= Change in KE after

Change in KE final- Change in KE initial = Change in KE

1/2mv^2 final - 1/2mv^2 initial= Change in KE

½ 10(20)^2- ½ 10(10)^2= Change in KE

½ 10(400)- ½ 10(100)^2= Change in KE

½ 4000 - ½  1000= Change in KE

2000-500= Change in KE

1500J= Change in KE

How much work was done?

Work= Change in KE

Work= 1500J

What was the force that caused the car to accelerate?

Work= fd

1500=F(10)

Force=150N

What was the power during the acceleration?

Power=Work/Time

Power=1500/2

Power=750 Watts

This unit I started out strong with the concepts of work and power, however, I struggled a lot with potential and kinetic energy. I was confused with how they could transform into one another. I especially struggled with the questions asking about the potential, kinetic, and total energy throughout the fall of an object. However, after struggling on an open note quiz, I realized that I needed to clarify whatever was confusing me. So, I looked at the Eureka videos, which helped to clarify my confusions.

            My problem solving skills this unit stayed fairly steady. Once I had clarified my confusions with kinetic and potential energy it was easier for me to make connections between all the concepts. The six problems we did in class were a really useful tool for bringing these concepts together. After going over these problems, my problem solving skills were strong and clear. Although I had a high effort in homework and I always take diligent notes, I did poorly on an open note quiz. This was because although I had the notes and homework, I didn’t have as clear of an understanding as I should have. So, next unit my goal is to catch my confusions like this early on so that I don’t face another poor open note quiz. 

Our podcast had some technical difficulties but our group worked well together, we all collaborated our ideas and my ideas about work were clarified and corrected like on the waiter example. Instead of saying there was no work being done (when the waiter was carrying the tray) I was corrected to say that there was no work being done on the TRAY.