Conflict Exam Issue. Sorry, Can t do it. Please see Kevin Pitts if you have any additional questions or concerns about this. Office is 231 Loomis

Size: px

Start display at page:

Download "Conflict Exam Issue. Sorry, Can t do it. Please see Kevin Pitts if you have any additional questions or concerns about this. Office is 231 Loomis"

Douglas Daniel
6 years ago
Views:

1 Conflict Exam Issue. Soy, Can t do it I was told that: Students can only be excused fom the scheduled final fo illness, death in the family o eligious holiday. No exceptions. Please see Kevin Pitts if you have any additional questions o concens about this. Office is 3 Loomis

2 Fist midtem is next week! Thusday June 7 th I would eally like it if you could give us some advice on whee to find moe pactice poblems. I kind of get this stuff, but I'd eally like to pactice moe than the ten questions we get on smat physics. Can you ecommend anywhee else to go? Look at pactice exams. Optional HW poblems

3 Today s Concepts: Wok & Kinetic Enegy Physics Lectue 7

4 Stuff you asked about: How to assign the coect signs to wok Eveything seemed to be difficult liked the into suggested.. I would like to focus on exactly how and when to apply the mathematical concepts and equations that wee intoduced. Those deivations wee intense! Thee wee expessions flying all ove the place and it was all ove befoe I knew what was going on. This was much hade than most othe pelectues/checkpoints. Please go ove the apple poblem I don't eally undestand how the way the integals ae used to give the kinetic enegy theoem This pe-lectue was definitely the most difficult and confusing that we've discussed so fa in this class. I think the concepts themselves ae not too difficult to undestand, but the math behind how we got to the equations is difficult to follow. It'd be nice to go ove that, so then we can maybe bette undestand the equations themselves if we know how they ae deived.

5 Wok-Kinetic Enegy Theoem W F dl No ticky integals on exams but you need to undestand them conceptually You ae intellectually matue enough to lean whee fomulas come fom. Ae we eve going to have to integate anything, o ae the integals just thee to make sue we undestand whee the math we will be doing is coming fom?

6 Wok-Kinetic Enegy Theoem The wok done by foce F as it acts on an object that moves between positions and is equal to the change in the object s kinetic enegy: W TOT NET WTOT F dl K K mv Integating is just adding up a bunch of pats to get a whole moe on this late.

7 The Dot Poduct I keep mixing up when the wok is positive, negative o zeo. Can you go ove the dot poduct and how it woks with wok. W > 0 W < 0 F F D D a a v v Speeding up, K inceasing Slowing down, K deceasing

8 Wok-Kinetic Enegy Theoem: -D Example If the foce is constant and the diections aen t changing then this is vey simple to evaluate: ca F d In this case W F dl F d = Fd since cos(0)= This is pobably what you emembe fom High School.

9 Clicke Question A lighte ca and a heavie van, each initially at est, ae pushed with the same constant foce F. Afte both vehicles tavel a distance d, which of the following statements is tue? (Ignoe fiction) ca F d W= Fd same van F d K= W Same too A) They will have the same velocity B) They will have the same kinetic enegy C) They will have the same momentum

10 Deivation not so impotant F dl K Concept vey impotant A foce pushing ove some distance will change the kinetic enegy. F dl q W K F dl

11 Wok done by gavity nea the Eath s suface mg

12 Wok done by gavity nea the Eath s suface W W W... W TOT mg dl mg dl... mg dln N dl N dl mg dl dl dy dx mg

13 Wok done by gavity nea the Eath s suface W W W... W TOT mg dl mg dl... mg dln mgdy... mgdy mgdy N mgy N dl dl N y mg dl Wg mgy

14 d dq d F g W F( ) d GM em d GM m e GM em

15 Close to the Eath s suface: ~ ~ R e : Wg GM em GM em GM m R e e mg -y So: W g = -mgy Same!!

16 Wok-Kinetic Enegy Theoem If thee ae seveal foces acting then W is the wok done by the net (total) foce: WNET K W W... You can just add up the wok done by each foce W NET W TOT

17 Clicke Question Thee objects having the same mass begin at the same height, and all move down the same vetical distance H. One falls staight down, one slides down a fictionless inclined plane, and one swings on the end of a sting. What is the elationship between thei speeds when they each the bottom? H Fee Fall Fictionless incline Sting A) v f > v i > v p B) v f > v p > v i C) v f = v p = v i

18 Checkpoint A ca dives up a hill with constant speed. Which statement best descibes the total wok W TOT done on the ca by all foces as it moves up the hill? A) W TOT > 0 B) W TOT = 0 C) W TOT < 0 Only /3 got this ight

19 Clicke Question A ca dives up a hill with constant speed. How does the kinetic enegy of the ca change as it moves up the hill? A) It inceases B) It stays the same C) It deceases

20 Clicke Question A ca dives up a hill with constant speed. The acceleation of the ca: A) Points up the hill B) Points down the hill C) Is zeo

21 Clicke Question A ca dives up a hill with constant speed. The net foce on the ca: A) Points up the hill B) Points down the hill C) Is zeo

22 Reminde We know two expessions that involve W TOT WTOT K W TOT F dl NET

Checkpoint A ca dives up a hill with constant speed. Which statement best descibes the total wok W TOT done on the ca by all foces as it moves up the hill?

23 Checkpoint A ca dives up a hill with constant speed. Which statement best descibes the total wok W TOT done on the ca by all foces as it moves up the hill? W W K F dl A) W TOT > 0 B) W TOT = 0 C) W TOT < 0 some of the situations, like the ca moving uphill with constant velocity, ae confusing as I have always thought of changing the position of an object can change its total enegy (Potential enegy) TOT

24 Fist you said the total wok in lifting an apple up one shelf was zeo, then in the vey next slide you said wok depended the change in height. How was wok not done on the apple then? Hand does positive wok. Gavity does negative wok. The sum of these is zeo since W tot = W hand + W gavity = K = 0

25 Checkpoint A box sits on the hoizontal bed of a moving tuck. Static fiction between the box and the tuck keeps the box fom sliding aound as the tuck dives. a S The wok done on the box by the static fictional foce as the tuck moves a distance D is: A) Positive B) Negative C) Zeo Less that half got this ight

26 Fom Last Lectue A box sits on the hoizontal bed of a moving tuck. Static fiction between the box and the tuck keeps the box fom sliding aound as the tuck dives. a S If the tuck moves with constant acceleating to the left as shown, which of the following diagams best descibes the static fictional foce acting on the box: A B C

displacement ae in the same diection so the wok is positive.

27 Checkpoint a F S D The wok done on the box by the static fictional foce as the tuck moves a distance D is: A) Positive B) Zeo C) Negative A) The foce and the displacement ae in the same diection so the wok is positive. B) Wok is change in kinetic enegy. C) Since the movement is negative the wok is also negative.

28 Wok done by a Sping W F dl = Aea unde the Foce vs. displacement gaph

29 Can you claify the diections of positive o negativity fo spings when they ae compessing and uncompessing? Use the fomula to get the magnitude of the wok Use a pictue to get the sign (look at diections) In this example the sping does negative wok since F and x ae in opposite diection. The axes don t matte.

Potential Energy and Conservation of Energy

Potential Energy and Conservation of Energy Potential Enegy and Consevation of Enegy Consevative Foces Definition: Consevative Foce If the wok done by a foce in moving an object fom an initial point to a final point is independent of the path (A