Physics 1050: Concepts of Physics

Second Homework Assignment, Due January 26, 2010

10 points

To receive credit, your completed assignment must be submitted to me by the beginning of lecture on Tuesday, January 26.

Do one, and only one, of the following:

1. Explain vector adddition.

You return to your room after an evening of studying. Your roommate makes small talk for a little while, then begins, “So, you’re taking physics. I have a question for you. I understand how, if two people are pulling on an object in opposite directions, the total force is just equal to the stronger pull minus the weaker one, and in the direction of the stronger one. I also can figure out that it two people push in the same direction, the total force is just the sum of their two forces in that same direction. But I can’t quite figure out—how do you find the total force if they’re pushing or pulling at some angle to each other?”

Explain it to your roommate. Make your explanation complete and correct, and at the same time understandable to someone who has not taken physics and is not familiar with vectors. Write your answer as the script of the dialogue between you and your roommate. (Not “I would explain that forces are vectors, which combine according to rules revealed to Isaac Newton by the Flying Spaghetti Monster…”)

An adequate answer will be several paragraphs long.

2. Explain vectors in free-fall.

A commonly-held misconception about objects in free-fall is that, if they begin a trajectory with some horizontal motion, the trajectory curves downward to eventually begome straight down.

• Explain how free-fall trajectories actually evolve. Include in your explanation how the vertical and horizontal components of velocity change with time.
• Identify the “kernel of truth” in the misconception. Why might people who have observed ballistic trajectories believe this misconception? Include in your explanation how the direction of the velocity changes with time.
• Justify the actual chracteristics of free-fall trajectories. In particular, explain how the force(s) acting on projectiles in free-fall do not cause objects initially moving horizontally to eventually fall straight down.
3. Analyze nonfree fall.

When an object falls through a fluid, such as air or water, it is affected by a force of drag as well as gravity.

• Explain how and why the object’s speed increases as its acceleration decreases.
• Explain how and why when the object’s speed is at its maximum (“terminal speed”), the net force acting on it is zero.
• Predict what happens to the object’s speed and acceleration if it is initially thrown downward at a speed greater than its terminal speed. Is this even possible?
4. Learn the song.

Learn the song “What’s a Guy Gotta Do.” (The one I did in class, not the original by Joe Nichols.) Demonstrate that you have learned it by either performing it for me or sitting down in front of me and writing the words from memory.

5. Practice thinking about forces.

Work pages 17–20 in the workbook.

But don’t stop there and expect me to grade your work. Check your own work against the answers on pages 138–140. Did any of the answers in the back surprise you? If so, could you eventually convince yourself to agree with the book, or does it still not make sense?

You do not need to turn in your worked pages. Instead, write and submit your answers to these questions:

• Did you miss any questions? If so, which ones?
• For all questions you missed, explain why you selected the answer that you selected.
• Do you understand why the answer in the back is correct? If so, explain what someone answering the question should have done to obtain the correct answer.
• If you did not miss any questions, instead write a paragraph explaining how to calculate the acceleration of an object when forces such as friction and drag act on it in addition to the action of gravity or an external push.

Take your choice of these tasks. Do not submit answers to more than one of them. Just choose one, and submit it.

The point of homework is to help you learn. Getting help is permitted, and even encouraged. Ask questions if you are stuck.

Warning: Do not turn in paper that has ragged edges! If you write your work in a spiral-bound notebook, trim the edge before submitting it. Papers with ragged edges will be returned unmarked.