Here Comes The Sun

After yesterday's gloom, the sunshine today is just dandy...let's hope it lasts!

Honors Physics discussed the first and second condition of equilibrium and then turned towards reviewing the homework problems. Most folks have difficulty with equilibrium problems, so don’t feel bad if you pulled your hair a bit while doing homework. What I tend to do is always draw out the situation, with all weights, forces, distances and angles labeled. Then, write the expression for the first condition of equilibrium (sum of all forces equals zero). This may take two expressions if some forces have both an x- and y-component. Then, write the expression for the second condition of equilibrium (sum of all torques equals zero). When working with torques, remember that you can choose any axis of rotation that you wish – if the object is in rotational equilibrium, then it is in equilibrium around any axis that you might choose. Pick an axis through which one of your unknown forces directly passes – this means the force does not contribute to torque around that axis. This eliminates one unknown variable from your net torque calculation. Solve for the other unknown and then return to the expression for the first condition of equilibrium to get the other force. Takes a bit of practice (and patience), but that’s why I gave you another problem like this as part of the homework. Look over what you did today and use the techniques to help you plan your attack on tonight’s problem. Tomorrow, we’ll review rotational equilibrium and then dive into Newton’s 2nd Law for Rotation.

Physical Science B began their exploration of conservation of energy. Whether in a living or physical system, energy is neither created nor destroyed, it is only converted into different forms/types. The total number of joules of energy you begin with, you will end with; however, the energy may or may not be useful for the work you need to do. When we look at usable energy for work in physics, we concentrate on mechanical energy, for our purposes the sum of the kinetic and gravitational potential energy an object possesses. Often, there is not complete conservation of energy when one is transformed into the other, because friction causes some of the energy to be transformed into heat. This will lead us to a discussion about efficiency later in the section. We then worked a few practice problems on the board for conservation of energy and will have more practice with these tomorrow in class.

Physical Science E conducted a lab investigation on conservation of energy, using a ball thrown into the air as the studied system. It took groups a bit to get a good graph of the throw, but when analyzed, these graphs nicely showed the continuous back and forth conversion of kinetic and potential energies. The total conservation of energy was pretty good – we can’t expect it to be perfect due to energy loss from friction and air resistance. We’ll go into conservation energy in more detail tomorrow and begin to work on some mathematical ways that conservation of energy can be used to analyze motion.

Physics F and G underwent the painful examination procedure for Chapter 7. Tomorrow, we pick up with torque, extending our study of rotational motion. We are going to pick and choose from this chapter, however, and will leave some topics in the dust. Torque, center of mass, machines, mechanical advantage and efficiency will be the highlights. We’ll demo conservation of angular momentum, but don’t worry about the mathematical analysis.

Homework

Honors Physics A: Read 8.3 and complete Practice 8C and #21 of the Chapter Review
Physical Science B: Review today’s problems in preparation for tomorrow’s practice
Physical Science E: Complete lab write up
Physics F and G: Read Section 8.1 and complete Practice 8A #1 and 2 and, also, #1,2,3,6, 7 and 8 of the Chapter Review