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 C5Chapter C5Applying Momentum Applying Momentum

ConservationConservation

Isolated systemsIsolated 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)

Steps in solving problems – Steps in solving problems – RequiredRequired!!

1) List all the data 2) Draw a figure (if helpful)3) State the general principle(s) that will

solve the problem4) State the specific situation that applies to

this problem (This step may not always be necessary in some simple problems.)

5) Write the formula6) Solve the problemThese steps are to be used on all homework

and test problems.

A 10,000 rocket ship traveling 40m/s A 10,000 rocket ship traveling 40m/s explodes into two pieces. The front half explodes into two pieces. The front half (8,000kg continues in the same direction (8,000kg continues in the same direction

with a velocity of 60m/s. What is the with a velocity of 60m/s. What is the velocity of the back half?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

8,000kg2,000kg

40m/s

10,000kg? 60m/s

8,000kg2,000kg

Conservation of momentumConservation of momentum• Momentum before = momentum after• Momentum of whole ship before the explosion =

sum of the momentum of the two pieces after.

0

0

0

00

0bfi v

bv

f

v

T MMM

MT = total mass of ship

vi = initial velocity of ship

Mf = mass of front of ship

Mb = mass of back of ship

vf = velocity of front of ship

vb = velocity of back of ship = ?

?

b

ffiTb M

vMvMv

vb = - 40 m/s

40m/s

10,000kg?

8,000kg2,000kg

40m/s

10,000kg?

8,000kg2,000kg

Example: A 600 kg car traveling 20 m/s west Example: A 600 kg car traveling 20 m/s west collides with a 800 kg pickup. The two stick collides with a 800 kg pickup. The two stick

together after the collision traveling 30 together after the collision traveling 30 degrees south of west at 25 m/s. How fast and degrees south of west at 25 m/s. How fast and

in what direction was the pickup traveling in what direction was the pickup traveling before the collision? 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 Problems • C5B.1, C5B.2, C5B.7, C5S.3, C5S.5,

C5R1• Problems due Monday

Add points to your test scoreAdd 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 worksHow the system works• You must email (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.)

ExampleExample• 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%!