Chapter 4: The laws of motion. Newton s first law

Transcription

1 Chapte 4: The laws of motion gavitational Electic magnetic Newton s fist law If the net foce exeted on an object is zeo, the object continues in its oiginal state of motion: - an object at est, emains at est; - an object moving with some velocity continues with that same velocity. Net foce: the sum of all extenal foces i =

2 Mass and inetia Inetia is the tendency of an object to continue in its oiginal motion in the absence of a foce Mass is the esistance of an object to changes in its motion due to a foce Newton s second law The acceleation of an object is diectly popotional to the net foce acting on it and invesely popotional to its mass a = Units m 1N 1Kgm / s i 2 ix iy = ma = ma x y 2

3 Mass, Acceleation and oce oce and mass Tue (a) o false (b)? 1. If an object is at est, no extenal foces act on it 2. If one single foce acts on an object, the object acceleates 3. If an object expeiences an acceleation, at least one foce acts on it 4. If an object expeiences no acceleation, no extenal foce acts on it 5. If the net foce acting on an object is in the x diection, the object moves in the x diection 3

4 Newton s law of univesal gavitation m1m g = G 2 2 Weight The magnitude of the gavitational foce acting on an object of mass m nea the Eath s suface is called the weight. w = G M E m 2 How would you modify this fomula fo an object on the moon? 4

5 Elastic oce el = K x Action and Reaction 5

6 Newton s thid law If two objects inteact, the foce exeted by object 1 on object 2 is equal in magnitude but opposite diection to the foce exeted by object 2 on 1 Action = -Reaction ee body diagam To study the cate s motion, we should only conside the foces acting on the cate 6

7 ee body diagam quiz A hose pulls on a sled with a hoizontal foce to get the sled in motion. Newton s 3 d law says that the sled exets a foce of equal magnitude and opposite diection on the hose. Which statement is tue? a) These 2 foces cancel so the sled cannot acceleate. b) Newton s 3 d law is not applicable. c) None of the above Equilibium make a sketch daw a fee-body diagam fo the object unde consideation and label ALL foces esolve all foces in x and y components use the equilibium equations (keep tack of signs) solve the simultaneous equations fo the unknowns in tems of the known quantities ix iy = 0 = 0 7

8 Equilibium: example A taffic light weighing 100 N hangs fom the vetical cable tied to two othe cables that ae fastened to a suppot. ind the tension in each of the thee cables Equilibium: example A taffic light weighing 100 N hangs fom the vetical cable tied to two othe cables that ae fastened to a suppot. ind the tension in each of the thee cables 8

9 Newton s second law: stategy daw a diagam of the system isolate the object of inteest that is being analyzed; daw a fee-body diagam fo this object showing all extenal foces (if moe than one object, daw seveal fee-body diagams) establish convenient coodinate systems fo each object and find the components of the foces along each axis (one of the axes should be paallel to the diection of motion) solve the component equations fo the unknowns ix iy = ma = ma x y Newton s laws: application The combined weight of the cate and the dolly is 300 N. If the peson pulls on the ope with a constant foce of 20.0 N, a) what is the acceleation of the cate and how fa will it move in 2.00 s? Assume the system stats fom est and thee ae no fiction foces; b) What is the eaction foce fom the floo on the cate? 9

10 Newton s laws: application A ca of mass m is on an icy diveway inclined at an angle of 20.0 deg. Detemine the acceleation of the ca, assuming thee is no fiction. Newton s laws: application A peson weighs a fish of mass m on a sping scale attached to the ceiling of an elevato. The value ead when the elevato is still is 100 N. What is the value ead if the elevato acceleates upwad a=2m/s 2? And if it acceleates downwad? 10

11 oces of fiction: static and kinetic Coefficient of static fiction f f s k µ N s = µ N k Coefficient of kinetic fiction oces of fiction: example You ae playing with you younge siste in the snow. Which case a) o b) would be easie to push he? 11

12 oces of fiction: example The hockey puck is given an initial speed of 20.0 m/s on a fozen pond. The puck emains on ice and slide 120 m befoe it comes to est. Detemine the coefficient of kinetic fiction puck and ice. Newton s laws: moe poblems Two packing cates of masses 10.0 kg and 5.0 kg ae connected by a light sting that passes ove a fictionless pulley. The 5.00 kg cate lies on a smooth incline of angle ind the acceleation of the 5.00 kg cate and the tension in the sting Repeat the poblem consideing a fiction coefficient of 0.2! 12

13 Equilibium: moe poblems The leg and cast weigh 220 N (w 1 ). Detemine the weight w 2 and the angle needed so that no foce is exeted on the hip joint by the leg plus the cast. Newton s laws: Moe poblems The pelvis has a mass of 30 kg. Detemine its acceleation, consideing the typical foces exeted on it. 13