Class on Friday Nov. 13th

The first thing we did in class on Friday was talk a little bit about projectile motion. Next we started discussing air resistance (also known as drag force). To demonstrate the effects of air resistance, Mr. Burk had a tube with air in it that contained both a feather and a penny. When he turned the tube upside down the penny fell downward much faster then the feather. Also the penny accelerated while the feather seemed to fall at constant velocity. In order to explain this we thought about the forces acting on the objects as well as the factors that affect air resistance. The only two forces acting on the objects are the gravitational force of the earth and the force of air resistance. The two factors that affect air resistance that we focused on were cross-sectional area and velocity. Since the penny weighs much more than the feather it has a much greater gravitational force. But the penny also has a significantly smaller cross-sectional area then the feather. Cross-sectional area is half of the normal area. We use cross-sectional area because air resistance does not act on the top of the objects. Since the feather falls at constant velocity it's air resistance must equal the gravitational force. When the force of air resistance and the gravitational force are equal it is called terminal velocity. This explains why the feather falls at constant velocity and why the penny falls with an acceleration of a little less than 9.8 m/s^2.

Next we talked about the physics of skydiving:

1. When a skydiver first jumps out of a plane his initial velocity is zero.

2. When he starts falling his velocity is about 10 m/s.

3. He keeps falling at an acceleration of 9.8 m/s^2 until he reaches a terminal velocity of about 200 mi/hr. His acceleration is 0.

4. When he opens his chute his acceleration becomes negative because the force of air resistance is greater than the force of gravity.

5. As his velocity decreases so does the force of air resistance acting on him.

6. As he nears the ground he reaches a new terminal velocity of about 5 mi/hr.

After that we went back to the tube. This time Mr. Burk used a vacuum to suck all of the air out of the tube. With no more air resistance when Mr. Burk turned the tube upside down the feather and the penny fell at the same rate. When he let the air back the feather shot up to the top of the tube because the air was traveling at roughly the speed of sound. Next we watched a video of a feather a hammer being dropped simultaneously on the moon. The feather and the hammer fell at the same rate but they fell very slowly because gravity on the moon is much smaller than on earth.

At the end of class we did a lab. We dropped coffee filters and used the motion sensor to track their terminal velocities. We started by dropping one coffee filter and then we did two, three etc. all the way to seven. We calculated air resistance by finding the mass of the filters and then finding the gravitational force. At terminal velocity air resistance is equal to the gravitational force. We did this lab to determine the effect of air resistance on velocity.

Hw was to graph f(drag) vs velocity as well as to linearize it.

I think Margaret wanted to be the scribe for Monday.