Ch6_BarinskyL

toc Chapter 6!

Lesson 1a
Method 2b 1/12/12

1) I understand the concept of work. Work is done upon an object when a force acts upon it, causing displacement. Work is made of 3 ingredients-- force displacement, and cause. I also understand how a waiter carrying a tray is not doing work upon the tray. He is supplying the upward force, which cannot provide the horizontal displacement of the tray. He did push the tray forward for a short amount of time to accelerate it from rest to a walking speed, but once the tray is up to speed, it will not have that forward force. 2) I was a little confused about negative work and how the concept actually makes sense according to the definition of work. I now get that negative work is done on an object when a force hinders displacement. An example of this is a car skidding to a stop (the friction between the tires and the road doing the negative work. 3) Nothing seems unclear to me. 4) I found it interesting how an object can be moving without any work being done on it. In the waiter example, once the tray is moving at constant walking speed, the waiter is not supplying it with a forward push. Only a horizontal force can cause a horizontal displacement, a characteristic of work. I always thought that an object in motion, especially one being carried by a person, had work being done on it.

Lesson 2a
Method 2b 1/30/12

1) I understand the general difference between internal and external forces. External forces include friction, tension, and applied forces, while internal forces include gravity and magnetic and spring force. I also understand how an external force changes the total mechanical energy of an object. This can increase or decreases depending on whether the work is positive or negative, and the work that is done is equal to the change mechanical energy in the object. 2) I was a little confused with the difference between conservative and nonconservative forces, but I now see that nonconservative forces are external forces that are capable of changing the total mechanical energy of an object, and conservative forces are internal forces that can change the form of energy without changing the total amount of mechanical energy. 3) I do not have any questions after completing the reading. 4) I think it is interesting how we are constantly witnessing changes in energy. If something drops, it's potential energy is converting into kinetic energy because of a gravity force. If an object is thrown to a certain height, the kinetic energy from its speed is transformed by the gravity force into potential energy obtained from its height.