Bes es pracice: Take he pas es on he class websie hp://communiy.wvu.edu/~miholcomb/phys11.hml I have posed he key o he WebAssign pracice es. Newon
Previous Tes is Online. Forma will be idenical. You migh wan o pracice problems wih only his formula shee.
Main Ideas in Class Today Inroducion o Falling Apply Old Equaions Graphing Free Fall Solve Free Fall Problems Pracice:.45,.47,.53,.59,.61,.63,.69, Muliple Choice.1
Freely Falling Objecs Refers o objecs which are no held, bu are free o fall near he Earh s surface (oher cases Ch. 7) All objecs fall wih he same consan acceleraion due o graviy in he absence of air resisance!
No Air on he Moon When Asronau David Sco dropped a feaher and a hammer on he moon, which hi he ground firs?
+y Acceleraion Due o Graviy We ofen ignore air resisance, which ypically has a small effec (ecepions: feahers, parachues, big surface area/weigh). Acceleraion due o graviy poins owards he ground (owards he cener of Earh) Magniude is g = +9.8 m/s near he surface of Earh In free fall, a = ± g depending he direcion y is defined (I like o always define +y as up for consisency) o =1 m Wha does ha mean abou he sign of acceleraion, velociy and displacemen? Thus, in his figure, a, velociy and are all negaive (wha happens if I flip +y)! f = m = f o = m 1 m = -1m
These are he only formulas you need for Used: To Ge: Tes 1 and hey will be given! a = v f f v i i v = vo + = vo + v a 1 a v = vo + a = f f i i Derived in book, bu you don need o know how o derive hese formulas vo v v = + Average of numbers In free fall, he only acceleraion is due o graviy: a=±g (consan) We also generally use y insead of (because verical moion) +y Define y down o use a=+g Define y up o use a= -g +y v = v o y = vo g 1 g v = vo g y Same formulas jus differen leers A lo of confusion abou physics is jus due o calling he same hing by differen leers
Same syle as some concepual quesions on he es If you oss a ball upward wih a cerain iniial speed, i falls freely and reaches a maimum heigh h. By wha facor mus you increase he iniial speed of he ball for i o reach a maimum heigh 4h? Similar o Homework A. B. 3 C. 4 v = v + o = vo + a 1 a v = vo + a Jus because i s a concepual problem, doesn mean you can use numbers. D. 8 E. 16 Q9
A small bag is released from a helicoper ha is descending seadily a 1.5 m/s. (a) Afer 4. s, wha is he speed of he bag? (b) A 4 s, how far is i below where i was when i was dropped? Similar o Homework v = v + o a = v + o 1 a v = vo + a
No Talking This Time! y A ball is ossed sraigh up ino he air. Which of he following represens he signs ( ) of he acceleraion as i moves upward, reaches is highes poin, and falls back down? Moving upward Highes poin Falling back down A. + - B. - - C. - - - D. None of hese Q1
+y Graphing Freely Falling Bodies y a y = -g = -9.8 m/s Equaion of a sraigh line y = m + b ground m is slope, b is y-inercep 1 y = y o + v oy 1 g v y = v oy g y (displacemen) y -5-1 -15 1 5 5..5 1. 1.5..5 3. v y (y velociy) v y..5 1. 1.5..5 3. -5-1 -15 - - -5
You May Talk This Time y A ball is ossed sraigh up ino he air. Which of he following represens he signs ( ) of he acceleraion as i moves upward, reaches is highes poin, and falls back down? Moving upward Highes poin Falling back down A. + - B. - - C. - - - D. None of hese Q1
+y Acceleraion Due o Graviy Acceleraion due o graviy poins owards he ground (owards he cener of Earh) Magniude is g = 9.8 m/s near he surface of Earh In free fall, a = ± g depending he direcion y is defined NOT ON WHICH DIRECTION YOU ARE GOING! o =1 m f = m
Common misconcepion ha objec sops a op of pah (only looks ha way because i slows down before urning) y 1 5 y = y v 1 o + oy g vy = voy g..5 1. 1.5..5 3. -5-1 -15-1 5 a = 9.8 m/s v y 1 5..5 1. 1.5..5 3. -5-1 -15 - -5 Noe: y is a maimum a he same insan when v y = a y (m/s ) -1-15..5 1. 1.5..5 3. -5 Noe: a y = -9.8 m/s always, even a op of he rajecory!
If you drop an objec in he absence of air resisance, i acceleraes downward a 9.8 m/s. If insead you hrow i downward, is downward acceleraion afer release is A. less han 9.8 m/s. B. 9.8 m/s. C. more han 9.8 m/s. Q11
Similar o Homework A baseball is hrown up in he air a an iniial velociy of. m/s. (a) How high up does i go? (b) How long is i in he air if you cach i a he same heigh you iniially le go of he ball? v vo + = a = v o + 1 a v = vo + a
Concepual Problem A person sanding a he edge of a cliff hrows one ball sraigh up and anoher ball sraigh down a he same iniial speed. Neglecing air resisance, he ball o hi he ground below he cliff wih he greaer speed is he one iniially hrown v = v + o = vo + A. upward. B. downward. v = v o + a C. neiher hey boh hi a he same speed. a 1 a Jus because i s a concepual problem, doesn mean you can use a formula o help you hink abou i. Q1
A glider is on an inclined, fricionless rack. The -ais poins downhill. A = he glider is a = and moving uphill. Afer reaching he high poin of is moion, i moves downhill and reurns o =. High poin of moion Which of he following v graphs (graphs of velociy vs. ime) bes maches he moion of he glider? Glider a = = Q13 v v v v v A. B. C. D. E.
A glider is on an inclined, fricionless rack. The -ais poins downhill. A = he glider is a = and moving uphill. Afer reaching he high poin of is moion, i moves downhill and reurns o =. High poin of moion Glider a = = Which of he following a graphs (graphs of acceleraion vs. ime) bes maches he moion of he glider? Q14 a a a a a A. B. C. D. E.
Injury from Falling I s no he falling ha hurs, bu he sopping Skydiving a a=9.8 m/s is fine When we sop, a is much more han 9.8 m/s How do rea a problem wih acceleraions?
Concussions: The Graviy of I All Pun inended Traumaic brain injury resuls when he head undergoes a very large acceleraion. hp://www.ncbi.nlm.nih.gov/pmc/aricles/pmc155415/ Generally, an acceleraion less han 8 m/s lasing for any lengh of ime will no cause injury, whereas an acceleraion greaer han 1 m/s lasing for a leas.1 seconds will cause injury. So wha do we need o know in order o ge ime and acceleraion?
Challenging Applicaion: Child Falling from Bed Suppose a child rolls off a bed ha is 1. m above he floor. If he floor is hardwood, he child's head is brough o res in ~. mm (carpe ~ 1. cm, 5 more). Calculae he magniude (and duraion) of he acceleraion for hardwood o deermine he risk of injury. Why is his a challenging problem?
Figuring Ou Which Formula (main formulas given on eam, see websie) v f f i i = = o + vo + 1 a v = vo + a v = v + o Eq. A Eq. B Eq. C Eq. D a Lis your knowns and unknowns: consan velociy, v o, v, a v o, v, a, Almos easier o find which formula does no have he variable you are NOT considering Same sraegy applies o mulipar quesions, you jus do one par a a ime. Someimes need answer from a o answer b
Clicker Answers Chaper/Secion: Clicker #=Answer Ch.A: 1=D, =E, 3=D, Ch.B: 4=B, 5=C, 6=A, 7=C, 8=E Ch.C: 9=A, 1=C, 11=B, 1=C, 13=E, 14=A