Thursday, 03/13/2008 Physics 4AG 17:25-21:05+++

Hi mates and my dearest Professor Mason,

Unfortunately, I am the one chosen to be writing the blog this time. As English is my 2nd language, please correct me if you see any grammar mistake or error. Thank you!

1) A wonderful quiz at the beginning of the class, it was about 30 mins. (Including the analysis by Professor Mason) It makes me starting to imagine about a flea that weights 100g for the rest of the night.

2)Discussion 1: A cart with a ball shooter move horizontally with a constant velocity, then shoot out a metallic ball with a constant velocity of during the movement. Will the ball fall a) back into the cart? b) fall behind of the cart? c) fall in front of the cart? The result is a); the ball will fall back into the cart. It is because the cart and the ball have the same horizontal velocity, so the distances they move are the same.

3) Discussion 2: Same situation as Discussion 1, but we used a very light plastic ball instead of the metallic ball. Same question was asked, and we got the same result. The reason is same, too. The cart and the ball have the same horizontal velocity, so the distances they move are the same.

3) Discussion 3: According to the situation above, more information was given. The cart is moving horizontally at a velocity of 1 m/s. The ball was given a velocity of 2 m/s by the ball shooter. After how long will the ball fall back into the cart? Answer was 0.4 s.

4) Discussion 4: Add acceleration to the cart in the situation above. Where will the ball fall off at? In front of the cart, behind of the cart, or into the cart? Answer was behind the car. The reason is the velocity is changing to be bigger, but the velocity of the ball in horizontal is staying constant. Therefore, in a same amount of time, the cart moves farther than the ball does.

5) Discussion 5: A box A weights 850 g was placed on the floor and with a box B weights 650 g placed on its center top. Friction between box A and B is 0.49 and 0.325 between box A and the floor.
-Draw 3 free-body diagrams for A, B and the whole system.
-Try to find the minimum force that can make the whole thing start moving. It was about 14.65 N.
-How much force do we need to apply to box A then box B will start to slide off?
-After how long will the box B fall off from the edge of box A? It was about 0.44 s.

Lab for “Force and Motion” was started at about at 20:35.
-We use video camera to measure the acceleration of a cart while pulling by a cord with heavy mass at the other end.

It was a funny Lab and it was a interesting class.

Hopefully my summary will be useful. Professor Mason, could you please correct my mistake or fill up anything that I forget to put in? Thank you very much!


Ching Han