Fall 08 C5 problems: C5B.1, C5B.2, C5B.7, C5S.3, C5S.5, C5R1 are due Monday Problems like these will be on the test. If you have difficulty with them be sure to get this resolved before the test. Chapter 4 problems should be in the box now Tomorrow will be a day to finish labs, to work problems and ask questions about the on-line tests. Monday will be a review day. The practice problem will be due Tuesday. The C3 – C5 test will be Wednesday.

New policy (on an experimental basis.) Late problems will be discounted 50%! Problems on homework will receive 80% of the credit even if you do not know how to solve them provided: –You list all known data and write a clear sentence explaining what the problem asks. –You explain the point that is hanging your up, what you need to know to solve the problem but don’t know. 60% credit will be given on tests provided the same steps are followed.

Chapter C5 Applying Momentum Conservation

Isolated systems To avoid outside interactions (forces), the system must be isolated Momentum does not flow into or out of isolated systems, but does with systems that interact with their surroundings. It is possible to have systems on the surface of the Earth that act like isolated systems. –One such system is a friction free flat surface. –Objects on this surface interact only with each other.

Few systems are isolated in reality (float in space) –These would have to be an infinite distance from all other objects in the universe They may be in a system (like the frictionless surface) that under some conditions function as though they were isolated. (functionally isolated) The process may take place so rapidly that there is no time for interactions with the surroundings (momentary isolation)

Solving Problems (book’s suggestions) 1) Translate the words to a conceptual model – mathematical symbols –Most important step here is often drawing a good figure –Clearly stating the data that is known and the unknown are part of this step.

2) Building a conceptual model The part of the problem where 90% of the physics takes place Decide what theories or principles apply to the problem. What approximations are necessary or useful in solving the problem

3) Find the algebraic solution Work through the problem using symbols. Plug in the numbers last (being careful to put units in each step) to obtain the answer. (The important thing is not to do the algebra with the numbers, use symbols.)

4) Check the results to see if they make sense. Are the units consistent with what was calculated? –(Distance should not be in seconds, etc) Check signs and magnitudes to see if they are realistic. –(A bowl of chili with a mass of kg!)

More on building the conceptual model Start from the fundamental ideas of physics and work toward the equations –Do not start with the equations This is the difference between the way an expert attacks the physics problem and a novice.

More on building the conceptual model Not all of these steps may be necessary in all cases Make a helping diagram to help you see more clearly the interactions, etc. Write the basic principle that applies in an English sentence. Use the helping diagram and the statement of the principle to construct the most fundamental mathematical equation. –The author of the text inverts these two steps, you may do which works best for you.

More on building the conceptual model Cancel any symbols that appear on both sides of the equation. Compare symbols with your data, and circle those that are unknown. If the number of unknowns is equal or less than the number of equations, you can solve the problem.

Steps in solving problems – Required! 1)List all the data 2)Draw a figure (if helpful) 3)State the general principle(s) that will solve the problem 4)State the specific situation that applies to this problem (This step may not always be necessary in some simple problems.) 5)Write the formula 6)Solve the problem These steps are to be used on all homework and test problems.

A 10,000 rocket ship traveling 40m/s explodes into two pieces. The front half (8,000kg continues in the same direction with a velocity of 60m/s. What is the velocity of the back half? What principle is necessary to solve this problem? – Because the system is in space we can apply the conservation of momentum 40m/s 10,000kg ?60m/s 8,000kg 2,000kg 40m/s 10,000kg ?60m/s 8,000kg 2,000kg

Conservation of momentum Momentum before = momentum after Momentum of whole ship before the explosion = sum of the momentum of the two pieces after. M T = total mass of ship v i = initial velocity of ship M f = mass of front of ship M b = mass of back of ship v f = velocity of front of ship v b = velocity of back of ship = ? v b = - 40 m/s 40m/s 10,000kg ? 8,000kg 2,000kg 40m/s 10,000kg ? 8,000kg 2,000kg

Example: A 600 kg car traveling 20 m/s west collides with a 800 kg pickup. The two stick together after the collision traveling 30 degrees south of west at 25 m/s. How fast and in what direction was the pickup traveling before the collision? General principle: Because the collision is very rapid the total momentum before the collision is equal the total momentum after the collision. Principle applied to this problem. The momentum of the two stuck together after the collision is equal the vector sum of the momenta of the car and the pickup before the collision

Problems C5B.1, C5B.2, C5B.7, C5S.3, C5S.5, C5R1 Problems due Monday

Add points to your test score Students who have the most success after college have said the technique that they learned that most contributed to that success was how to work in groups. To give additional incentive to work in groups points will be added to the test scores of all students who work in groups and help each other be more successful.

How the system works You must (or hand me a paper) with the names of the people in your group before 8:00 on the day of our test. If the average of the group is better than their average on the chapter 1 test, the increase in the group average will be added to the score of each person in the group (up to max of 10 points.) If the group’s average decreases, there is no penalty. Groups may have between 2 and 4 people. (larger groups must be approved.)

Example On test one Judy’s score was 90, Fred’s was 70 and Jim’s was 38. (Ave = 67) Judy, Fred and Jim study together. On the Chapter 2 test Judy’s score is 92, Fred’s 76 and Jim’s 60 (Ave = 76) 9 points will be added to each person’s score. Judy’s score becomes 101%, Fred’s 85% and Jim’s 69%!