Physics 101: Lecure 03 Kinemaics Today s lecure will coer Texbook Secions 3.1-3.3 (and some Ch. 4) Physics 101: Lecure 3, Pg 1
A Refresher: Deermine he force exered by he hand o suspend he 45 kg mass as shown in he picure. A) 220 N B) 440 N C) 660 N Sep 1 Draw! Sep 2 Forces! D) 880 N E) 1100 N Sep 3 Newon s 2 nd! F Ne = ma = 0 equilibrium! T + T - W = 0 Remember he magniude of he ension is he same eerywhere along he rope! T +y -y T W Physics 101: Lecure 3, Pg 2
Force a Angle Example A person is pushing a 15 kg block across a floor wih m k = 0.4 a a consan speed (a=0). If she is pushing down a an angle of 25 degrees, wha is he magniude of her force on he block? x- direcion: F Ne, x = ma x P x f = 0 P cos(q) m k N = 0 N = P cos(q) / m k y- direcion: F Ne, y = ma y N W P y = 0 N mg P sin(q) = 0 q Sep 1 Draw! Sep 2 Forces! Sep 3 Newon s 2 nd! Combine: (P cos(q) / m k ) - mg P sin(q) = 0 P [cos(q) / m k - sin(q)] = mg P = mg / [cos(q)/m k sin(q)] P = 80 N +y -x +x fricion P y -y Normal q Weigh P y = P sin q P x Pushing Physics 101: Lecure 3, Pg 3 Pushing P x = P cos q
Today: An Oeriew Wha happens when a 0? Kinemaics: Descripion of Moion Posiion and Displacemen Velociy» aerage» insananeous Acceleraion» aerage» insananeous Relaie elociy (firs pass) Physics 101: Lecure 3, Pg 4
Posiion s Time Plos DISPLACEMENT is change in posiion, Dx = x f x 0. VELOCITY is rae of change of posiion, = Dx/D = slope Slope beween any wo poins gies aerage elociy a ha poin Slope of angen line a any poin gies insananeous elociy a ha poin. Displacemen beween =5 and =1: Dx = x f x 0 Dx = 1.0 m - 2.0 m = -1.0 m Aerage elociy beween =5 and =1. = Dx/D -1 m / 4 s = -0.25 m/s x (m) 3-3 x (m) 3-3 4 Consan elociy 4 Changing elociy Physics 101: Lecure 3, Pg 5
Velociy s Time Plos Gies elociy a any ime. Area gies displacemen = Dx/D => Dx = D Slope a any poin gies insananeous acceleraion. elociy a =2, (2) = 3 m/s Displacemen beween =0 and =3: Dx= =0 o =1: ½ (3m/s) (1 s) = 1.5 m =1 o =3: (3m/s) (2 s) = 6 m Aerage elociy beween =0 and =3? = Change in beween =5 and =3. D = 7.5 m 3 (m/s) Aerage acceleraion beween =5 and =3: a = -5 m/s / (2 s) = -2.5 m/s 2 1.5-3 6 4 7.5 m / 3s = 2.5 m/s -2 m/s 3 m/s = -5 m/s Physics 101: Lecure 3, Pg 6
Acceleraion s Time Plos Gies acceleraion a any ime. a = D/D Area gies change in elociy a = D/D => D = a D Acceleraion a =4, a(4) = -2 m/s 2 Change in beween =4 and =1. D = =1-3: D = (3m/s 2 )(2s) = 6 m/s +4 m/s 3 a (m/s 2 ) 6 2 4 =3-4: D = (-2m/s 2 )(1s) = -2 m/s -3 Physics 101: Lecure 3, Pg 7
Acceleraion Checkpoins Is i possible for an objec o hae a posiie elociy a he same ime as i has a negaie acceleraion? 1 - Yes 2 - No If he elociy of some objec is no zero, can is acceleraion eer be zero? 1 - Yes 2 - No Physics 101: Lecure 3, Pg 8
Velociy ACT If he aerage elociy of a car during a rip along a sraigh road is posiie, is i possible for he insananeous elociy a some ime during he rip o be negaie? A - Yes B - No Drie norh 5 miles, pu car in reerse and drie souh 2 miles. Aerage elociy is posiie. Physics 101: Lecure 3, Pg 9
Draw y s Dropped Ball A ball is dropped from a heigh of wo meers aboe he ground. 9 A 9 B 9 C y x -6 0.5 9 D -6 0.5 9 E -6 0.5-6 0.5-6 0.5 Physics 101: Lecure 3, Pg 10
Dropped Ball A ball is dropped for a heigh of wo meers aboe he ground. y Draw s Draw y s Draw a s a Physics 101: Lecure 3, Pg 11
Tossed Ball A ball is ossed from he ground up a heigh of wo meers aboe he ground. And falls back down y 9 Draw s A 9 B 9 C x -6 1 9 D -6 1 9 E -6 1 1-6 -6 1 Physics 101: Lecure 3, Pg 12
Tossed Ball A ball is ossed from he ground up a heigh of wo meers aboe he ground. And falls back down y Draw s Draw y s Draw a s a Physics 101: Lecure 3, Pg 13
ACT A ball is hrown sraigh up in he air and reurns o is iniial posiion. During he ime he ball is in he air, which of he following saemens is rue? A - Boh aerage acceleraion and aerage elociy are zero. B - Aerage acceleraion is zero bu aerage elociy is no zero. C - Aerage elociy is zero bu aerage acceleraion is no zero. D - Neiher aerage acceleraion nor aerage elociy are zero. Physics 101: Lecure 3, Pg 14
Relaie Velociy (firs pass) You are on a rain raeling 40 mph Norh. If you walk 5 mph oward he fron of he rain, wha is your speed relaie o he ground? A) 45 mph B) 40 mph C) 35 mph Physics 101: Lecure 3, Pg 15
Relaie Velociy You are on a rain raeling 40 mph Norh. If you walk 5 mph oward he rear of he rain, wha is your speed relaie o he ground? A) 45 mph B) 40 mph C) 35 mph Physics 101: Lecure 3, Pg 16
Relaie Velociy You are on a rain raeling 40 mph Norh. If you walk 5 mph sideways across he car, wha is your speed relaie o he ground? A) < 40 mph B) 40 mph C) >40 mph Physics 101: Lecure 3, Pg 17
Relaie Velociy Someimes your elociy is known relaie o a reference frame ha is moing relaie o he earh. Example 1: A person moing relaie o a rain, which is moing relaie o he ground. Example 2: a plane moing relaie o air, which is hen moing relaie o he ground. These elociies are relaed by ecor addiion: ac ab bc» ac is he elociy of he objec relaie o he ground» ab is he elociy of he objec relaie o a moing reference frame» bc is he elociy of he moing reference frame relaie o he ground Physics 101: Lecure 3, Pg 18
Tracor Demo 1 Which direcion should I poin he racor o ge i across he able fases? A) 30 degrees lef B) Sraigh across C) 30 degrees righ 1 2 3 Physics 101: Lecure 3, Pg 19
Tracor Demo (moing able) Which direcion should I poin he racor o ge i across he able fases? A) 30 degrees lef B) Sraigh across C) 30 degrees righ 1 2 3 Physics 101: Lecure 3, Pg 20
Summary of Conceps kinemaics: A descripion of moion posiion: your coordinaes displacemen: Dx = change of posiion elociy: rae of change of posiion aerage : Dx/D insananeous: slope of x s. acceleraion: rae of change of elociy aerage: D/D insananeous: slope of s. relaie elociy: ac = ab + bc Physics 101: Lecure 3, Pg 21