Model Post for 1/4/07 - Energy and Power

We started today's class with a review of last night's homework, which was a problem set on "power". The most difficult problems were on the back page, where we had to recognize that we could change the power equation P=W/t into P=Fd/t and then into P=Fv, where v is the velocity of the object. So if you know the force being applied, and the velocity, you can calculate power. You don't need the actual distance traveled.

We then went over two topics - one connected to yesterday's video that mentioned the dot product between two vectors, and the other related to graphing. The dot product that we saw yesterday provides a way to calculate the component of the force in the direction of motion when we calculate work, and today we saw the equation we should use - W = F x cos(theta) x d, where F is force, d is the distance over which the force acts, and theta is the angle between the applied force and the direction of motion. Here's the equation clipped from the online notes:












We worked through an example of this and then we discussed the idea of graphing force versus distance and how the area under that curve becomes the work done, shown below:
Next we began learning about energy and the different forms it can take, and most importantly, the definition - the ability or capacity to do work. It has the same units as work (the joule), and is directly related to it. Energy is conserved, meaning it does not get created or destroyed, and only gets exchanged between objects, systems, or forms. The total is always the same. Kinetic energy (the energy of motion) and potential energy (stored energy) are the two types that make up mechanical energy, and their sum (KE + PE) equal the total mechanical energy of an object or system.

We did a few examples of calculating KE, and then finished up with a review of the weekend blog assignment, which is to read the existing posts on our "Home Base" blog, and the model post you are now reading, and leave a "thoughtful" comment on one of them. Monday we will start writing scribe posts for real.