CHAPTER 5 Newton s Laws of Motion

Transcription

1 CHAPTER 5 Newton s Lws of Motion We ve been lerning kinetics; describing otion without understnding wht the cuse of the otion ws. Now we re going to lern dynics!! Nno otor 103 PHYS - 1 Isc Newton ( ) Lws of Motion Lw of Grvity Clculus Nture of Light 103 PHYS - 2 Dr. Abdllh M. Azzeer 1

2 ORCEs re wht cuse n object to ove Cn soeone tell e wht ORCE is? The bove stteent is not entirely correct. Why? Becuse when n object is oving with constnt velocity no force is exerted on the object!!! Usully push or pull Vector Either contct or field force orces 103 PHYS - 3 Mesuring forces - orces re often esured by deterining the elongtion of clibrted spring. - orces re vectors!! Reeber vector ddition. - To clculte net force on n object you ust use vector ddition. 103 PHYS - 4 Dr. Abdllh M. Azzeer 2

3 TYPE O ORCES 103 PHYS Newton s irst Lw of Motion: An object continues in stte of rest or in stte of unifor otion t constnt speed long stright line unless copelled to chnge tht stte by net force. In other words; If the net force exerted on n object is zero the object continues in its originl stte of otion. Tht is, if = 0, n object t rest reins t rest nd n object oving with soe velocity continues with the se velocity. Why? Becuse objects hve inerti L:\103 Phys LECTURES SLIDES_T2Y3839\CH5lsh 103 PHYS - 6 Dr. Abdllh M. Azzeer 3

4 Inerti: The tendency tht Newton observed for objects t rest to stytrestndobjectsinotiontostyinuniforotion in stright line. How do we esure inerti? MASS A esure of the resistnce of n object to chnges in its otion due to force Sclr SI units re kg Don t confuse ss nd weight 103 PHYS - 7 L:\103 Phys LECTURES SLIDES_T2Y3839\CH5lsh 103 PHYS - 8 Dr. Abdllh M. Azzeer 4

5 Blnced forces: We sy tht the NET force is zero! Accelertion: Reeber tht the word ccelertion denotes n increse in velocity OR decrese in velocity OR chnge in the direction of velocity. 103 PHYS Newton s 2 nd Lw of Motion (very, very iportnt) The ount of ccelertion () produced by force () depends on the ss () of the object being ccelerted. Mtheticlly: Alterntively: = =/ L:\103 Phys LECTURES SLIDES_T2Y3839\CH5lsh 103 PHYS - 10 Dr. Abdllh M. Azzeer 5

6 Dr. Abdllh M. Azzeer PHYS - 11 or 1 Newton s 2 nd Lw of Motion 103 PHYS - 12 The ccelertion of n object is directly proportionl to the net force cting on it nd inversely proportionl to its ss. x x y y z z Newton s 2 nd Lw of Motion

7 Units of orce = = [kg] /s 2 ] [N] Stndrd Unit: Newton US Custory unit is pound (lb) 1 N = lb One Newton: The force required to ccelerte 1 kg ss by 1 /s PHYS - 13 Exple 5.1 Deterine the gnitude nd direction of ccelertion of the puck whose ss is 0.30kg nd is being pulled by two forces, 1 nd 2, s shown in the picture, whose gnitudes of the forces re 8.0 N nd 5.0 N, respectively. Coponents of 1 1x 1 y o N cos 8.0 cos 0 1 o N sin 8.0 sin 9 1 Coponents of 2 Coponents of totl force 2 x 2 y x y o N 2 cos cos 7 o N 2 sin sin N 1 x 2 x 7 x 1 y 2 y N y 103 PHYS - 14 Dr. Abdllh M. Azzeer 7

8 Mgnitude nd direction of ccelertion x x y 29 / s / s 0.3 y / s x 29 1 y 1 tn tn 30 Accelertion Vector x iy j 29 i 17 j / s PHYS - 15 The force of grvity nd weight Cobining Lw of grvity with Newton s 2 nd Lwofotion,wecn derive n expression for the ccelertion due to grvity. Objects re ttrcted to the Erth. This ttrctive force is the force of grvity g. g g The gnitude of this force is clled the weight of the object. The weight of n object is, thus g. The weight of n object cn very with loction (less weight on the oon thn on erth, since g is sller). The ss of n object does not vry. 103 PHYS - 16 Dr. Abdllh M. Azzeer 8

9 5.6 Newton s 3 rd Lw of Motion Whenever one body exerts force on second body, the second body exerts n oppositely directed force of equl gnitude on the first body. or every ction there is n equl nd opposite rection. If two objects interct, the force 12 exerted by object 1 on object 2 is equl in gnitude nd opposite in direction to the force 21 exerted by object 2 on object 1: Action nd rection forces lwys ct on different objects. 103 PHYS - 17 Where is the ction nd rection force? Action-Rection Pirs: Act On Different Bodies orces exerted BY body DO NOT (directly) influence its otion!! orces exerted ON body (BY soe other body) DO influence its otion!! When discussing forces, use the words BY nd ON crefully. L:\103 Phys LECTURES SLIDES_T2Y3839\CH5lsh 103 PHYS - 18 Dr. Abdllh M. Azzeer 9

10 103 PHYS - 19 The Norl orce: The norl force, N, is one coponent of the force tht surfce exerts on n object with which it is in contct, nely, the coponent perpendiculr to the surfce. The Norl orce: How to Mesure The gnitude of the norl force is esure of how hrd two objects push ginst ech other. The direction is perpendiculr to the surfce. 103 PHYS - 20 Dr. Abdllh M. Azzeer 10

11 Where does the norl force coe fro? ro the other body!!! Does the norl force ALWAYS equl the weight? NO!!! 103 PHYS - 21 Tension 103 PHYS - 22 Dr. Abdllh M. Azzeer 11

12 Soe coents on strings/cords/cbles/ropes Cn be used to pull fro distnce. Tension (T) t certin position in string is the gnitude of the force cting cross cross-section of the string t tht position. The force you would feel if you cut the string nd grbbed the ends. An ction-rection pir. T cut T T 103 PHYS - 23 More on cords/strings/ropes/cbles. Consider horizontl segent of string hving ss : Drw free-body digr (ignore grvity s string is lost ssless T 1 T 2 Using Newton s 2nd lw (in x direction): x NET = T 2 -T 1 = So if = 0 (i.e. the string is light) then T 1 = T PHYS - 24 Dr. Abdllh M. Azzeer 12