Physics 131- Fundamentals of Physics for Biologists I

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10/3/2012 - Fundamenals of Physics for iologiss I Professor: Wolfgang Loser 10/3/2012 Miderm review -How can we describe moion

1 10/3/ Fundamenals of Physics for iologiss I Professor: Wolfgang Loser 10/3/2012 Miderm review -How can we describe moion (Kinemaics) - Wha is responsible for moion (Dynamics) wloser@umd.edu Movie of he Day Cells on edge Quiz review Miderm Review 2 Prof W. Loser 1

2 10/3/2012 Quiz review Quiz D D C 8 C D C 3 C D C 5 C 4 C DE C 8 C D C 8 CD DE 10 CD D Correc C,D D Main Incorrec,E C 3 Objec acceleraes o he righ 1. F12 larger 2. F21 larger 3. same 100% 0% 0% F12 larger F21 larger 10/3/ same Prof W. Loser 2

3 10/3/2012 ball hrown in he air will ake 1. Longer o move up hen o come down 2. Longer o fall down 3. Same ime o reach op as o come back down 100% 0% 0% Longer o move... Longer o fall... Same ime o r... Two dense objecs (so air drag can be ignored) are sho sraigh up a he same ime from he same heigh. Objec is sho wih a speed of 1 m/s, objec wih a speed of 2 m/s. Which akes longer o come back o is saring poin? 63% oh a same ime 4. NEI 38% 0% 0% oh a same... NEI Prof W. Loser 3

4 10/3/2012 baleship simulaneously fires wo shells a enemy ships. If he shells follow he parabolic rajecories shown, which ship ges hi firs? (from E. Mazur) boh a he same ime need more informaion 0% 33% 44% 22% boh a he sa... ed more info... The ball on he funnel car will land 1. In he funnel 2. head of he funnel 3. ehind he funnel 4. Depends on speed of car 5. Don know In he funnel 71% head of he f... 0% 0% 0% ehind he fun... 29% Depends on spe... Don know Prof W. Loser 4

10/3/2012 REVIEW 10/3/2012 9 Foohold ideas: Modeling he world wih mah We use mah o model relaionships and properies in physical, chemical or biological sysems.

5 10/3/2012 REVIEW 10/3/ Foohold ideas: Modeling he world wih mah We use mah o model relaionships and properies in physical, chemical or biological sysems. (1) Mahemaical processing allows us o make predicions (2) from he model ha we can inerpre (3) for he physical sysem. Very non-naural physical sysems such as lasers, ransisors have been designed via such model based exrapolaion. In biology, seps 1 and 3 are sill very hard iological Sysem iological Sysem Prof W. Loser 5

6 10/3/2012 Foohold Ideas: Esimaion Quanifying experience Use 1-digi arihmeic Do figure ou your esimaions by saring wih somehing you can plausibly know and scale up or down Do check your answer o see if i s reasonable You will learn useful numbers for biology 11 Numbers Useful numbers (people) Number of people on he earh ~7 billion (7 x 10 9 ) Number of people in he US ~ 300 million (3 x 10 8 ) Number of people in he sae of Maryland ~ 5 million (5 x 10 6 ) Number of sudens in a large sae universiy ~30-40 housand (3 x 10 4 ) 9/5/12 12 Prof W. Loser 6

7 10/3/2012 Useful numbers (disances) Macro Disances Circumference of he earh Radius of he earh* ~24,000 miles (1000 miles/ime zone a he equaor) 2/π x 10 7 m Disance across he US ~3000 miles Disance across DC ~10 miles 13 Foohold iology Numbers Numbers ha we do NOT know from personal experience bu ha we need o build an inuiion for living sysems io Scales Size of a ypical animal cell ~10-20 microns (10-5 m) Size of a bacerium, chloroplas, or miochondrion Size of a medium-sized virus ~1 micron (10-6 m) ~0.1 micron (10-7 m) Thickness of a cell membrane 9/5/12 ~10nanomeer = 0.01 micron (10-8 m) Number of molecules per m 3 in a 1 nanomolar soluion 1 14 Prof W. Loser 7

8 10/3/2012 Foohold ideas: Coordinaes in space In order o specify he posiion of somehing we need a coordinae sysem. The coordinae sysem includes: Picking an origin Picking perpendicular direcions for he axes of he coordinae sysem Choosing a measuremen scale Each poin in space in hen specified by hree numbers: he x, y, and z coordinaes. a posiion vecor an arrow drawn showing he displacemen from he origin o ha posiion. 15 Velociy: Predicing he fuure posiion verage velociy is defined by v r vecor displacemen ime i ook o do i Insananeous velociy is wha we ge when we consider a very small ime inerval (compared o imes we care abou) v dr d Noe: an average velociy goes wih a ime inerval. Noe: an insananeous velociy goes wih a specific ime. 16 Prof W. Loser 8

9 10/3/2012 Foohold ideas: cceleraion verage acceleraion is defined by a v change in velociy ime i ook o do i Insananeous acceleraion is wha we ge when we consider a very small ime inerval (compared o imes we care abou) a d v d Noe: an average acceleraion goes wih a ime inerval. Noe: an insananeous acceleraion goes wih a specific ime. 17 Newon 0: Newon s Laws n objec responds o he forces i feels when i feels hem. Newon 1: n objec ha feels a ne force of 0 keeps moving wih he same velociy (which may = 0). Newon 2: n objec ha is aced upon by oher objecs changes is velociy according o he rule Newon 3: When wo objecs inerac he forces hey exer on each oher are equal and opposie. ne F a F ype 18 m F ype Prof W. Loser 9

10/3/2012 Foohold Idea: Spring Forces If you pull on a spring from boh sides i changes is lengh. T T T kl ( L = srech or squeeze) Tension force T. Holds for LL objecs ineracing pulled by a spring!

10 10/3/2012 Foohold Idea: Spring Forces If you pull on a spring from boh sides i changes is lengh. T T T kl ( L = srech or squeeze) Tension force T. Holds for LL objecs ineracing pulled by a spring! 19 Foohold Ideas: Fricion Fricion is our name for he ineracion beween wo ouching surfaces ha is parallel o he surface. I acs o oppose he relaive moion of he surfaces. Tha is, i acs as if he wo surfaces are sicking ogeher a bi. Normal forces adjus hemselves in response o exernal forces. So does fricion up o a poin. f f max Saic saic N Fricion can oppose moion or cause i. f Sliding N kineic kineic saic 20 Prof W. Loser 10

10/3/2012 Foohold ideas: Viscosiy Viscosiy is a resisive force ha an objec feels when i moves hrough a fluid as a resul of he fluid sicking o he objec s surface.

11 10/3/2012 Foohold ideas: Viscosiy Viscosiy is a resisive force ha an objec feels when i moves hrough a fluid as a resul of he fluid sicking o he objec s surface. This layer of fluid ries o slide over he nex layer of fluid and he fricion beween he speeds ha layer up and so on. The resul is a force proporional o he velociy of he objec. viscous F fluidobjec 6R objec v 21 Foohold ideas: inerial Drag force The drag (inerial force) is a resisive force fel by an objec moving hrough a fluid. I arises because he objec is pushing fluid wih i, bringing i up o he same speed i s going. The resul is a force proporional o he densiy of he fluid, he area of he objec, and he square of he objec s velociy. drag F fluidobjec Cd fluid objec v 2 22 Prof W. Loser 11

10/3/2012 Foohold Ideas: Graviy Every objec (near he surface of he earh) feels a downward pull proporional o is mass: where g W Em mg is referred o as he graviaional field.

12 10/3/2012 Foohold Ideas: Graviy Every objec (near he surface of he earh) feels a downward pull proporional o is mass: where g W Em mg is referred o as he graviaional field. This is a pforce even hough nohing ouching he objec is responsible for i. The graviaional field has he same magniude for all objecs irrespecive of heir moion and a all poins. The graviaional field always poins down. I is measured o be g 9.8 N/kg 23 s he car above is driven around he rack a consan speed, he ne force on he car is zero. 1. lways 2. Never 3. Someimes 0% 0% 0% lways Never Someimes Prof W. Loser 12

13 10/3/2012 Two objecs 1 and 2 are ineracing. They are isolaed from oher objecs. Objec 2 has wice he mass of objec 1. The graph on he lef shows he force ha 2 exers on 1, ploed agains ime. Which of he graphs o he righ of i correcly shows he force ha 1 exers on 2? F on 1 by 2 F on 2 by 1 F on 2 by 1 F on 2 by 1 F on 2 by 1.. C. D C 4. D 5. none 0% 0% 0% 0% 0% C D none Two objecs 1 and 2 are ineracing. They are isolaed from oher objecs. Objec 2 has wice he mass of objec 1. The graph on he lef shows he acceleraion of objec 1, ploed agains ime. Which of he graphs o he righ of i correcly shows he acceleraion of objec 2? F on 1 by 2 F on 2 by 1 F on 2 by 1 F on 2 by 1 F on 2 by 1.. C. D C 4. D 5. none 0% 0% 0% 0% 0% C D none Prof W. Loser 13

10/3/2012 In each of he siuaions below, a mover pushes wo craes along a horizonal surface, and he craes move ogeher wih a consan acceleraion.

consan force as i speeds up pulling he wo freigh cars.

In which pair of quaniies below are he wo quaniies unequal in magniude? 1. The acceleraion of car 1 and he acceleraion of car 2 2.

14 10/3/2012 In each of he siuaions below, a mover pushes wo craes along a horizonal surface, and he craes move ogeher wih a consan acceleraion. Which of hese siuaions are possible only if one or more fricional forces are involved? I a = 1 m/s 2 II a = 0 1. I III 2. II 3. III 4. IV 5. II and III 6. II, III and IV 7. none a = 1 m/s 2 a = 1 m/s 2 IV 14% 14% 14% 14% 14% 14% 14% I II III IV II andiii II, III andiv none Tes yourself 302 The engine above exers a consan force as i speeds up pulling he wo freigh cars. Car 1 and is load have a combined mass of 6000 kg; car 2 and is load have a combined mass of 9000 kg. In which pair of quaniies below are he wo quaniies unequal in magniude? 1. The acceleraion of car 1 and he acceleraion of car 2 2. The force ha he engine exers on car 1 and he force ha car 1 exers on car 2 3. The force ha he car 1 exers on car 2 and he force ha car 2 exers on car 1 4. The force ha he engine exers on car 1 and he force ha car 1 exers on he engine 5. The normal force ha he rack exers on he engine and he engine s weigh Theaccelerai 20% 20% 20% 20% 20% Theforceha Theforceha Theforceha Thenormal for Prof W. Loser 14

0 time. 2 Which graph represents the motion of a car that is travelling along a straight road with a uniformly increasing speed?

0 time. 2 Which graph represents the motion of a car that is travelling along a straight road with a uniformly increasing speed? 1 1 The graph relaes o he moion of a falling body. y Which is a correc descripion of he graph? y is disance and air resisance is negligible y is disance and air resisance is no negligible y is speed and