## Thursday, March 3, 2011

### 3/3/2011 Period 2

The main thing we did today was go over the HW, but there were a lot of very important questions that came up. Our HW had 2 questions that each had a, b, and c.

A said: what are the objects with which the jacket interacts? What object is our system?
When we discussed this in class we came up with the jacket touching the hook, and that the jacket was the point of interest and the system.

B said: Represent the situation with a force diagram.
The force diagram looked a little like this. The hook was pushing the jacket up, and the earth was pulling the jacket down. The arrows have to be equal because if the hook exerted more force than the earth the jacket would keep rising, if the earth was exerting more force than it would pull the jacket down to the ground.

Hook
+70N

Jacket
0N

Earth
-70N

Alexia had a great comment about the hook, she thought that the hook wasn't exerting any force. That's when we had a class discussion and found out that if the hook wasn't exerting any force than the jacket would just fall because there is nothing holding it up..

C said: How would the force diagram change if you put a heavy book in your pocket.
If you put a book in your pocket than the diagram would look more like this. The earth would be exerting more force downward and the hook would be exerting more force upward. The jacket would still be in the same spot because the forces are still equal.

Hook
+100N

Jacket
0N

Earth
-100N

Question 2 was about 2 people carrying a suitcase.

You had to make a # sentence and a force diagram for A. The # sentence i used was +20 + -20 = 0, and this was my force diagram.

People
+20
Suitcase
0N
Earth
-20N
For B you needed to put the total force exerted on the suitcase. That would be again O because 20+(-20) is 0.

C asked how you decided which side you decided was to be positive and which negative. The positive side goes up because the positive side is always represented by going up or right and the negative side is always represented by left or down.

We didn't do anything else in class to today, but we do have 4 Q's for HW.

JB Post #3