Use these variables to select a formula. x = t Average speed = 100 m/s = distance / time t = x/v = ~2 m / 100 m/s = 0.02 s or 20 milliseconds

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1 The speed o a nere mpulse n the human body s about 100 m/s. I you accdentally stub your toe n the dark, estmatethe tme t takes the nere mpulse to trael to your bran. Tps: pcture, poste drecton, and lst knowns and unknowns. Use these arables to select a ormula. x x x = a = t t t Aerage speed = 100 m/s = dstance / tme t = x/ = ~ m / 100 m/s = 0.0 s or 0 mllseconds t t t

2 Acceleraton Aerage acceleraton = change n elocty/tme a t Instantaneous acceleraton t = t a = lm t 0 t Unts: m/s/s = m/s Lke elocty and dsplacement, acceleraton s a ector (has drecton and magntude). The sgn o acceleraton ndcates whch drecton ts elocty changes. Poste acceleraton means speedng up when mong n the poste x drecton OR slowng down when mong n the negate x drecton.

3 Claryng the Sgns Can someone ge me an example when an object s nstantaneous elocty and nstantaneous acceleraton to be o opposte sgn at some nstant o tme? Ex: car mong n +x drecton but slowng down a +x

4 Acceleraton = change n elocty oer some tme Class requency AA Hold power button, press A twce Consder the ollowng stuatons: a car slowng down at a stop sgn a ball beng swung n a crcle at constant speed a bratng strng (ex: plucked gutar strng) a person drng down a straght secton o hghway at constant speed wth her oot on the accelerator Q03 In how many o the stuatons s the object acceleratng? A. 0 B. 1 C. D. 3 E. 4 Don t orget there s homework tonght and eery class day except test nghts.

5 Moton at Constant Acceleraton Specal case when a does not change wth tme (most problems n ths class, oten you can at least approxmate as constant) Smply Notaton: t = t t = 0 x = x x = x o = = o You don t need to know how to dere equatons. Just need to be comortable usng them. a = t t a t o = = + o at ag = Smlar deratons lead to more equatons: + o x = t o + 1 at = o + a x

6 Constant acceleraton equatons. Only use acceleraton constant (most problems) Mght hae a problem you hae to break nto steps (e.g. beore/ater brakes) Whch ormula to use? Constant a or not x t = + o at = o + a x ag = + o x = t o + 1 at Pro Tp #3: Lst what you know and need to know n arable orm 1 equaton wth one unknown s solable. equatons wth two unknowns s solable. Pro Tp # 4: Practce helps you pck best ormulas! = nal elocty o = = ntal elocty

7 Plannng a Strategy A certan car s capable o acceleratng at a rate o 0.60 m/s. How long does t take or ths car to go rom a speed o 55 m/h to a speed o 60 m/h? What are our pro tps? Draw pcture and rame. Lst the knowns & unknowns Want: t Know: o,, a Make sure your knowns hae correct unts/sgns. = o +a t rearrange: t = (- o )/a Can we just plug n our numbers? Wll need to conert m/h to what? (or m/s to what?) Whle could do ether, I nd easer to stck to SI unts.

8 Results o HW Questonnare Many o you hope to learn exactly what I plan to teach! Teach you what physcs s about Sharpen your analytcal thnkng sklls Stmulate your curosty about the world Also prepare many o you or the MCAT/DAT Many people hope to use sklls n ther own elds (e.g., occupatonal therapy, pharmacy, bology, geology, psychology, exercse physology)

9 Purpose o the book s to: Prode background or lectures (repetton) Be a resource or detaled explanatons (such as deratons o ormulas) Be a reerence and a study gude Oer practce problems and solutons Help teach problem solng

10 How many hours you thnk t wll take to learn all you need to know rom ths course? (Include eerythng: lectures, homework, etc.) Our rst test s Jan 31(< weeks)! Now s the tme to start gettng ready. Not enough To get an A n ths course, t s lkely that you wll need to spend >1 hours per week. (3:1 rule) Don t wat untl the test to get prepared.

11 Whle chasng ts prey n a short sprnt, a cheetah starts rom rest and runs 45 m n a straght lne, reachng a nal speed o 7 km/h. (a) Determne the cheetah s aerage acceleraton durng the short sprnt, and (b) nd ts dsplacement at t = 3.5s.

12 Clcker Answers Chapter/Secton: Clcker #=Answer Ch.A: 1=E, =E Today: 3=D