# Thurs Nov 21 Forces Test Asst: Guided Notes pages

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Thurs Nov 21 Forces Test Asst: Guided Notes pages 228-245
Do THESE YOURSELF, they’re IMPORTANT! ** Note – Momentum is also a SHORT FAST UNIT (because so much is just Concepts derived from Newton’s Laws) – test will be in 2 weeks!

Fri Nov 22 (computer lab???) **collect any Forces stuff left over ** do one of the following … Quick lecture notes on Impulse-Momentum problems & start problems in class (do NOT copy list below; they are on your HW packet, with answers!) pp : #56-60*, 61-62, 64-65, 67-69, 70*, 72a&b&e, 83-84, 85a&b* *problems with asterisks - 60, 70, & 85 - require unit conversions! ***#70 is very important! Do Google Slides 1-6 as seen on handout (do NOT copy notes below; they are all in your HW packet!) Make sure you cover ALL the information listed for full credit! You may have MORE slides, but not less. Make sure you write out the bk problem, show the work (if there is any), and show the correct answer. “C” stands for conceptual problems & you should write the answer in a way that encompasses the question(s). THIS IS ALSO YOUR STUDY GUIDE FOR THIS UNIT! (look at the handout, on the middle of the right column, right after slide 13!) Asst: J = F · t = p problems (“asst 3”)

Impulse-Momentum problems
Question #1 on the test Impulse-Momentum problems 1. Write this across the top of your HW paper: J = F · t = p = pF – pI = mvF – mvI = m(vF – vI) = mv a b c d e f g 2. List the knowns & unknown of the problem (watch the units in problems with an asterisk!) 3. See what parts of eq to use (note a = g, e = c, etc!) and write only those 2 parts down 4. Plug in knowns 5. Solve for unknown Let’s do 56& 57 together; see next slide ….

Start working on your own ppt!
J = F · t = p = pF – pI = mvF – mvI = m(vF – vI) = mv a b c d e f g pp : #56-60*, 61-62, 64-65, 67-69, 70*, 72a&b&e, 83-84, 85a&b*p. 870: #1-3 56) m = kg F = 272 N vI = 0 m/s vF = 62.0 m/s t = ? F · t = mvF – mvI 272*t = 0.058*62 – 0.058*0 Solve for t …. 57a) p = ? 58b) v = ? NOTE: If you get stuck tonight, there are some hints on slide 6 of this ppt!!!! Start working on your own ppt!

Start your ppt: See handout (2013) and/or page 2 of the HW packet

J = F · t = p = pF – pI = mvF – mvI = m(vF – vI) = mv
a b c d e f g

Impulse-Momentum problems hints:
J is the symbol for impulse, p mashed together means “change in momentum” & v mashed together means “change in velocity” #69b: Use t = the INVERSE of 1.5 x 1023 *******

Mon Dec 2 (computer lab; hand out HW packets)
Quick Lecture Notes on the Cons. of Momentum Work on ppts slides 7&8 Asst: # 4 - Conservation of Momentum problems (Note that problem 86 with the asterisk used to read “an octopus thrown on the ice”! – from the Detroit Redwings…) pp : #73f, 74e, 75-78, 80, 82, 86*, 87b, 90b&c, 81c&d (Note #76&77 need unit conversions, as they don’t “match”.) p. 870 chapter 9: #4-9 PS: Slide 7 could also be about air bags &/or “catching an egg” Slide 8, bullet 6 should say “If v is bigger for the bounce, then …”

Conservation of Momentum
“master equation”: p I total = p F total This means the total before = the total after That is the definition of “conservation”, a question on your next test! (Recall 2 other sciencey things, that you learned in chemistry, that are conserved: mass & energy!)

How to solve “Conservation of Momentum” problems:
Write for EACH problem: p I T = p F T ( p initial TOTAL = p final TOTAL ) ***You do NOT need to write out your knowns/unknowns first; you may plug them directly into this expanded equation as you read: m1v1Initial + m2v2Initial + … = m1v1Final + m2v2Final + … Plug in knowns/unknowns for each object, into equation above, for BEFORE the collision. Plug in knowns/unknowns for each object, into equation above, for AFTER the collision. Solve for what you don’t know …

EX 1: Car A is 10 kg & going 50 m/s
EX 1: Car A is 10 kg & going 50 m/s. It rear-ends car B, which is 15 kg & was going 25 m/s. If they stick together, what is their final velocity? Just LOOK at the 2 examples at left; you don’t need to copy them down! Note the units must “MATCH”, like in the gas laws in Chemistry. So, the masses can be in grams, if they are ALL in grams. Thus, problems #76&77 (only) need a unit conversion! EX 2: Car A is 10 kg & going 50 m/s. It has a head-on collision with car B, which is 15 kg & was going 25 m/s. If they stick together, what is their final velocity?

pI T = pF T ( pinitial TOTAL = pfinal TOTAL )
(m1v1I + m2v2 I + …. = m1v1F + m2v2 F + …) pp : #73f, 74e, 75-78, 80, 82, 86*, 87b, 90b&c, 81c&d; p. 870 #4-9 Question #2 on the test Let’s do #73f & #74e together right now. #75 you can do on your own. But #76 is tricky ….. (see next slide)

ANSWERS p initial TOTAL = p final TOTAL (pI T = p F T )
(m1v1I + m2v2 I + …. = m1v1F + m2v2 F + …) pp : #73f, 74e, 75-78, 80, 82, 86*, 87b, 90b&c, 81c&d; p. 870 #4-9 ANSWERS

Tricky #76… p initial TOTAL = p final TOTAL
Put “projectile” in kg too! (units must match!) Note: originally the projectile is INSIDE the launcher, so initial mass = , and they’re moving at 2.00 m/s together. They then split up; projectile is launched with velocity of 647 m/s afterwards. NOTE: If you get stuck on any others tonight, there are some hints on slide 15 of this ppt!!!! And start working on your own ppt again! PS: Slide 7 could also be about air bags &/or “catching an egg” Slide 8, bullet 6 should say “If v is bigger for the bounce, then …”

p initial TOTAL = p final TOTAL
Put “projectile” in kg too! (units must match!) Note: originally the projectile is INSIDE the launcher, so initial mass = , and they’re moving at 2.00 m/s together. They then split up; projectile is launched with velocity of 647 m/s afterwards. ANSWER EQUATION WOULD BE……

Catching an egg – tomorrow in class??
Bend legs when land after jumping off desk Catch hard ball barehanded; bunting Passing vs spiking in volleyball Receiving a punch vs karate chopping Others you can think of? Impact Time zzzzzzzzzzzzzzz J = F · t const, so  c OR J = F · t const, so   c Slide 7, Question #4 on the test Note: it is NOT OK to just copy this slide! You must do what the handout says, and use your OWN examples!!!

Bouncing vs sticking J = F · t = p = mv
Slide 8, Question #5 on the test Bouncing Dv bigger (show specific example, using NUMBERS, of why!) so Dp bigger so bigger impulse & bigger force #10 (archery) makes wood move faster & farther #12 (bumpers) so more dangerous to passengers *** Discuss rubber bullets on demonstrators …(look at the size of the bruise!) Sticking Dv smaller (show specific example using #’s of why!) so Dp smaller so smaller impulse & force … Note: it is NOT OK to just copy this slide! You must do what the handout says, and use your OWN examples!!!

Conservation of Momentum problems worked out:

Physics Tues Dec 3 (computer lab)
conservation of momentum Questions on asst 3 or 4; turn in tomorrow See next slide(s) Work on ppts slides 9 & 13 using online book to screen capture problems see second-next slide Asst: Asst 5 – “Concepts” (all of them) on p3 of the asst packet No, they do NOT have to be in complete sentences for the HW! (only for the ppt)

J = p asst p I total = p F total asst #70: known is v (=0.63 m/sec)
Check yours to see if that’s what you have! #4: note north and south!!! #8: both stuck together initially, and at rest! J = p asst p I total = p F total asst Any other ones? Look at Schedules slides #7 & 16 To discuss only:

Glencoe online textbook: (for slides 9 & 13)
Slide 9 & 13 = Questions #1 & 2 on the test use Firefox & see webpage for link: access code: EEDF22594B Use “PrintScrn” button in upper right of keyboard to screen capture (copy) the screen OR …see Shared Classes folder / Out for “Digital Chapter 9” problems & copy the page you want “Paste” into MS Paint from either source. Use toolbar to “cut” from Paint only the problem you want. You may use your HW to help you; and then turn it in! Subscripts in google slides = control comma

Wed Dec 4 (computer lab) Do 2-D problem #1 together QUICKLY
Hand out collision pictures Get out a blank piece of paper, ruler & calculator Turn to 2-D instructions in HW packet Work on ppts slides 11 & 12 whole thing due Friday, printed out see about how to finish & how to print Asst: 2-D asst part 1 (collisions 2-4) **You MAY also want to work on 5-6 and/or #7, which is extra credit!!!! ALL due Friday.

Lecture Notes How to solve a 2-D problem!
See separate Powerpoint for those step-by-step directions! The skectehs of the “answers” are also in that ppt (last slide) Or the very last slide in this ppt too

Thurs Dec 5 (last day in computer lab)
Turn in anything? Review some concepts quickly if haven’t yet See slides and/or and/or webpage about … Impact Time Bouncing vs Sticking Recoil Effect ppt - help with some questions.ppt – you can look at this yourself on the webpage! Finish ppts; due printde Friday in class double-check you have all slides 1-15 done add slides 14 &15 pics from shaerd google doc add some pictures Print out the ppt as “4 slides per page / landscape” (see for details) Asst: (A) 2-D collisions 5-6 plus xc7 if you want; (B) finish & print Google slides

Conserved, etc (slide 11) Slide 11, Question #6 on the test Note: it is NOT OK to just copy this slide! You must do what the handout says, and use your OWN examples!!! “Constant” means that each individual object never changes “Conserved” means the TOTAL before & TOTAL after are the same (one object can give some to stuff to another object) The impulse-change-in-momentum theorem says J = F t = p. Thus if F = 0, then J = 0, and p = 0 too, which means the total momentum does not change. But, in order for F to be = 0 (and thus pinitial TOTAL= pfinal TOTAL) then the system must be “closed” and “isolated” Closed: (just like if I close the door) nothing enters or leaves the system Isolated: no EXTERNAL forces on the system (like friction!) All of the problems in Asst 4 must have been closed & isolated or we couldn’t have used pinitial TOTAL= pfinal TOTAL!

Recoil Effect #22, etc Who cares if there’s “nothing to push against”?
Slide 12, Question #7 on the test Recoil Effect #22, etc Who cares if there’s “nothing to push against”? By Newton’s third law: if spacecraft pushes gasses out backwards, gasses push spacecraft forwards. By conservation of momentum: Total initial momentum equals zero; then, total final momentum MUST equal zero also; So, if momentum of gasses one way = 10,000 kg·m/s; then momentum of spacecraft other way = 10000 kg·m/s for a final total of 0 kg·m/s!! 8 PTS ON THE TEST, MUST EXPLAIN RECOIL BOTH WAYS TO GET ALL THE POINTS!!!! Note: it is NOT OK to just copy this slide! You must do what the handout says, and use your OWN examples!!! Don’t be surprised by the loud noise!

Cat is example of Recoil Effect too! **How does the cat conserve momentum? pIT = 0, so pFT must = 0 too ….!?!? This video reminded me of the cat example with a little twist:

Fri Dec 6 (back in classroom)
“Catch” eggs ??? Talk about & collect … Printed Google Slides 2D stuff Staple your papers all together, with the pictures handout stapled to the bottom; If you did the extra credit – write me a note VERY CLEARLY on the top of the first data sheet! Concepts (don’t take the handouts home; it’s a class set, and we can post them on the webpage!) Guided Notes ???? (you’ll get them back Tuesday, when we go over Angular Momentum) Bill Nye: Momentum (if time start it) Asst: Review worksheet YES! You may write on it! Both sides due Mon!! Note: you’ll be working on the experiment write-up Tues & Wed evening; the test is on Thursday

Guided Notes Review (then collect)
9) … But when it comes time to USE the Impulse-Momentum theorem, we will expand it out into all its parts: J = F · t = p = pF – pI = mvF – mvI = m(vF – vI) = mv (***write this on a notecard and start memorizing it ASAP!!! 10) Note that the units of impulse are Newton-seconds (from the formula J = F · t) and that the units of momentum are kilogram*meter/second (from the formula p = m · v). 11)…Because F · t = p, we know how to save lives. If we want to reduce the force on an object, we will make the contact time as long as possible. That’s why we have air bags in cars. 18)…In order for momentum to be conserved, there are 2 conditions that are mandatory. The system has to be both closed and isolated. (Question #6 on the test! Know each defn!) 24) – 25) MEMORIZE THOSE SENTENCES! 30) & 31) note we will only do conservation in 2 dimensions with vectors, not with equations (question #10 of the test) 32) – 39) We will go over these next week when we talk about Angular Momentum (Tuesday), which is question #9 on the test

Mon Dec 9 Conservation of Momentum experiment
When finished all items in middle of table On top of photogate box = labpro & cable, 3x5 cards, spring scale and tape Asst: hand-write: Title, Introduction, Purpose, Materials (see correct page in HW packet) Tomorrow back in math computer lab to analyze data! UNIT TEST IS THURSDAY!

Tues Dec 10 (computer lab)
Go over Angular Momentum (fill in worksheet as we go over it) Discuss Experiment write-up quickly Get excel-data template from “Out” folder & type in data, fill down ( home) Go over Review worksheet (in red pen) & turn in Asst: work on hand-written write-up – Procedure & R&C (leaving a blank for actual results to be filled in later)

Angular Momentum (There are at least 20 points worth of stuff listed here; you only need 7!)
Angular momentum does NOT mean things at angles; it means things going in CIRCLES or rotating. For things moving in straight lines … J = F · t = p, which becomes the following for things moving in circles: Jrotational =  · t = L This math statement says that (a) Jrotational, a rotational/angular/circular impulse is caused by (b)  · t, a torque delivered over time, which (c) L, causes the object’s angular momentum to change. Torque is defined to be a rotational/angular/circular force. (open a door, spin the wheelie-chair) Torque has the symbol,  , pronounced “tau” and has the units [N·m]. Torque can be found by:  = r · F , where r stands for the radius of the object, like a door handle’s distance from the door’s hinges. (It’s more effective to push out on the edge, than close to the hinges!) L stands for the change in angular momentum; angular momentum of an object or system changes if there is a torque on the object (which happens if the system is NOT closed and/or isolated in a rotational/angular/ circular sense). Just plain ole’ L, or angular momentum, can be found in 3 ways: L = r · p or L = r · mv (where r stands for the radius of the object,). The units are [kg·m2/s]. Also: L = I ·  , or “angular mass” · “angular velocity”. If the system is closed and isolated in a rotational/angular/circular sense, then since  = 0, L = 0 also. Thus angular momentum is conserved, L initial TOTAL = L final TOTAL, or LI T = L F T, Examples: the cat (“angular recoil”), rotating ice-skater & planets around suns (L = I ·  conserved)

Experiment = Recoil Effect
Any questions on what goes in the Intro? See sent yesterday for more details than handout has The point of the experiment was to prove the Conservation of Momentum, specifically the recoil effect: Total initial momentum equals zero; then, total final momentum MUST equal zero also; So, if momentum of cart A = + 10 kg·m/s; then momentum of cart B other way = 10 kg·m/s for a final total of 0 kg·m/s!! Let’s see if it works ….

Conservation of Momentum data (Go to Shared/Out folder to find this template!)

Wed Dec 11 (copy down both days!)
***Did you your data home yesterday? Need to print out from Excel & attach to hand-written Experiment write-up Hand back papers; Guided notes sheet for tomorrow’s HW Finish going over Review worksheet (in red pen) Go over test structure See next 2-3 slides More review for test… Look at a student’s ppt, AND/OR … Look at old powerpoint to show.ppt, AND/OR … Take a look at sample quiz Asst (a): STUDY - Guided Notes, your own PowerPoint slides, Concepts***, Review wksht, Assts 3 & 4 problems (memorize 2 formulas), quiz questions on last slide of Agendas … Asst (b): Do another 2-D problem Asst (c) finish write-up … Were your cart’s momenta conserved??? Thurs – Dec 12 Test (need protractor & ruler) Asst: Guided Notes on work, power, and energy pp: , pp

Review Sheet Get out a RED PEN – so we can finish going over the Review sheet (fill-in, mc ??) Newton’s Cradle … more examples 4 from one side? 1 from each side at same time? 2 from each side at same time? 1 from one side + 2 from other? What about 2 from one + 3 from other?

Momentum Test Structure:
Question 1: a series of Impulse-Momentum problems from asst 3 (did you note the important ones?) Question 2: a series of Conservation of Momentum problems from asst 4 (did you note the important ones?) Questions 3-8: Conceptual problems See the GUIDED NOTES, CONCEPTUAL PROBLEMS KEY, the REVIEW WKSHT, your own PPT’s! Newton’s 3 laws & what they become in momentum general impulse & momentum concepts impact time short vs long (J = F t , with J constant) bouncing/sticking in collisions (KNOW v & p bigger for bounce!!!) conservation of momentum concepts; difference between constant and conserved (defn of); where does the momentum go / how is it transferred? recoil effect stuff (know how to describe it BOTH ways) Question 9: Angular Momentum (see notes sheet) Question 10: a 2-d problem for you to do Bring a ruler & protractor!

How to solve a 2-D problem! Last question (#10) on the test
Measure distances & angles Calculate velocity & momentum Draw start point for momentum vectors Draw Ainitial PLUS Binitial Go BACK TO START Draw Afinal PLUS Bfinal Does pinitial TOTAL = pfinal TOTAL ????? The question before that (#9) says “Tell me anything you know about Angular Momentum.” (6 pts + XC possibilities)

CONCEPTS you should know (but they were all on your ppt – have you printed that out yet? – and we’ve gone over all of them already!): If you look at a falling ball, it speeds up, so it is gaining momentum, so its momentum is not conserved. It speeds up because there is an external force acting on it: gravity. That means it is not an isolated system. (Momentum can only be conserved if the system is both closed and isolated.) OK, so, what is causing the external force? Well the earth, of course. OK, define your system to be ball + earth. As you know from the last unit, if the ball falls down,. the earth moves up (equal and opposite forces). Similarly, if the ball has downward momentum, then the earth has upward momentum, so that TOTAL the momentum is CONSERVED if you are looking at both the ball and the earth TOGETHER. CONSTANT means that thing A's and thing B's velocity and momentum never change. CONSERVED means thing A can give momentum to thing B (or vice versa) so that the TOTAL momentum is always the same. CONSTANT means nothing changes in the system. ie: thing A has the same velocity/momentum always, thing B has the same velocity/momentum always. CONSERVED means the TOTAL of thing A and thing B remains the same, but thing A can give momentum to thing B, or vice-versa. If the system is BOTH isolated AND closed (know the definitions of those for test) then the impulse equals zero. Since J = ft = delta p, if J = o then delta p must ALSO equal zero. If delta p equals zero for a system, then momentum is CONSERVED. You must be able to describe the RECOIL EFFECT in BOTH of the following ways: By Newton’s third law, if spacecraft pushes gasses out backwards, gasses push spacecraft forwards. (3 points) By conservation of momentum, say… Total initial momentum equals zero; momentum of gasses one way + spacecraft other way (which are equal and opposite) equal zero too. (5 points) Now, the above explanation was for a space craft going forwards when the gasses go back out backwards. On the test you will have your choice of ANY recoil example to use: a hose, a gun, anything.

How much time is left? (Collaboration Day)
Only 5 minutes, take the very small practice quiz on the next slide! (Grade it at home tonight) More time? Look at old powerpoint to show.ppt, AND/OR Put students’ names in a box (if you want to) & pull a name at random to view their ppt Homework: (a) STUDY - Guided Notes, your own ppt, Concepts, Review wksht, Assts 3 & 4 problems, quiz questions on last slide of Agendas! (b) finish HAND-WRITTEN write-up … Were your cart’s momenta conserved??? ****Did you your data home yesterday, so you can print it out from Excel (not Word, that’s for AP) & attach it to your write-up?

5-minute self quiz (key next slide)
Write out EACH of Newton’s 3 laws (in order), and what they become in our Momentum unit. If a thrust of 35 Newtons is used to change the velocity of a kg craft by 0.63 m/s. How long should the thrusters be applied? A kg hockey puck moving 35.0 m/s strikes an octopus sitting on the ice. The octopus has a mass of kg. Find their velocity as they slide off together. Do one where they do NOT stick together at end, but at beginning instead?

1) You should be able to see that on your own ppt that you printed out 6 slides per page & put in your Physics binder!! 2) m = kg F = 35 N v = 0.63 m/s t = ? F · t = m v 35*t = 72000*0.63 Solve for t …. 3) p I total = p F total 0.115* *0 = ( )*vF Solve for vF …. NOTE: Momentum is a LOWER CASE p !!!

2D answers Let’s discuss #7’s two strobe pictures… Yeah! XC Yeah!
Boo  2D answers Boo  Yeah! Boo  Yeah! XC Yeah! Let’s discuss #7’s two strobe pictures…

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