Physics 11 Chapter 4: Forces and Newton s Laws of Motion. Problem Solving

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1 Physics 11 Chapte 4: Foces and Newton s Laws of Motion Thee is nothing eithe good o bad, but thinking makes it so. William Shakespeae It s not what happens to you that detemines how fa you will go in life; it is how you handle what happens to you. Zig Zigla Reading: pages Outline: Newton s 1 st law inetia and mass foces what is a foce? diffeent kinds of foces identifying foces Newton s 2 nd law units of foce fee-body diagams (FBDs) Newton s 3 d law Poblem Solving Definite pocedues has been devised fo identifying the foces acting on an object and then dawing a fee-body diagam. These pocedues ae given in Tactics Box 4.2 on page 111 and Tactics Box 4.3 on page 118. The list below gives a simplified vesion of these pocedues. 1. Identify the object to be consideed. It is usually the object on which the given foces act o about which a question is posed. 2. Daw a sketch of the situation that includes the object of inteest and all othe objects (opes, sufaces, ) that touch it. Identify all contact and long-ange foces that act on the object. The had pat is getting all the foces. If appopiate, don't foget to include the gavitational foce on the object, the nomal foce of a suface on the object, any fictional foces, and the foces of any stings o ods attached to the object. (Some students eoneously include foces that ae not acting on the object. Fo each foce you include you should be able to point to something in the envionment that is exeting the foce. This simple pocedue should pevent you fom eoneously including a nomal foce, fo example, when the object you ae consideing is not in contact with a suface.) 3. Daw a coodinate system and epesent the object by a dot at the oigin of the coodinate axes. 4. Daw vectos that epesent each of the identified foces. Place the tail of each foce vecto on the object, which is epesented as a dot. Be sue to label each foce vecto. 5. Daw and label the net foce vecto F net next to the fee-body diagam. Check that F net points in the same diection as the acceleation vecto a, o, if appopiate, wite F net = 0.

2 SUMMARY The goal of Chapte 4 has been to establish a connection between foce and motion. GENERAL PRINCIPLES Newton s Fist Law Conside an object with no foce acting on it. If it is at est, it will emain at est. If it is in motion, then it will continue to move in a staight line at a constant speed. v F 5 0 v v v v a 5 0 The fist law tells us that no cause is needed fo motion. Unifom motion is the natual state of an object. Newton s Second Law An object with mass m will undego acceleation a B = FB net m whee the net foce F B 3 + Á F B net = F B 1 + F B 2 + is the vecto sum of all the individual foces acting on the object. v v F net v a v The second law tells us that a net foce causes an object to acceleate. This is the connection between foce and motion. The acceleation points in the diection of F B net. v Newton s Thid Law Evey foce occus as one membe of an action/eaction pai of foces. The two membes of an action/eaction pai: act on two diffeent objects. point in opposite diections and ae equal in magnitude: F B on A F B A on B =-F B B on A A B Action/eaction pai A B F A on B IMPORTANT CONCEPTS Foce is a push o pull on an object. Foce is a vecto, with a magnitude and a diection. A foce equies an agent. A foce is eithe a contact foce o a long-ange foce. The SI unit of foce is the newton (N). A 1 N foce will cause a 1 kg mass to acceleate at 1 m/s 2. Net foce is the vecto sum of all the foces acting on an object. F 1 F 3 F 2 5 F net 5 F 1 1 F 2 1 F 3 Mass is the popety of an object that detemines its esistance to acceleation. If the same foce is applied to objects A and B, then the atio of thei acceleations is elated to the atio of thei masses as a A a B = m B m A The mass of objects can be detemined in tems of thei acceleations. APPLICATIONS Identifying Foces Foces ae identified by locating the points whee othe objects touch the object of inteest. These ae points whee contact foces ae exeted. In addition, objects feel a long-ange weight foce. Thust foce F thust Weight w Nomal foce n Fee-Body Diagams A fee-body diagam epesents the object as a paticle at the oigin of a coodinate system. Foce vectos ae dawn with thei tails on the paticle. The net foce vecto is dawn beside the diagam. y n w F thust F net x

3 Questions and Example Poblems fom Chapte 4 Question 1 Why do you lung fowad when you ca suddenly comes to a halt? Why ae you pessed backwad against the seat when you ca apidly acceleates? Question 2 A peson sits on a sloped hillside. Is it eve possible to have the static fiction foce on this peson point down the hill? Explain. Question 3 Two foces act on an object that is on a fictionless suface, as shown below. Rank these situations fom geatest acceleation to least acceleation. (Note: All vectos diected to the ight ae positive, and those to the left ae negative. Also, 0 m/s 2 > -10 m/s 2.) Geatest Least

4 Poblem 1 A baseball playe is sliding into second base. Identify the foces on the baseball playe. Poblem 2 A constant foce is applied to an object, causing the object to acceleate at 8.0 m/s 2. What will the acceleation be if a) The foce is doubled? b) The object's mass is doubled? c) The foce and the object's mass ae both doubled? d) The foce is doubled and the object's mass is halved? Poblem 3 Two childen fight ove a 200 g stuffed bea. The 25 kg boy pulls to the ight with a 15 N foce and the 20 kg gil pulls to the left with a 17 N foce. Ignoe all othe foces on the bea (such as its weight). a) At this instant, can you say what the velocity of the bea is? If so, what ae the magnitude and diection of the velocity? b) At this instant, can you say what the acceleation of the bea is? If so, what ae the magnitude and diection of the acceleation?

5 Poblem 4 In the figues below, one foce is missing. Use the given diection of acceleation to detemine the missing foce and daw it on object. Poblem 5 Vey small foces can have temendous effects on the motion of vey small objects. Conside a single electon, with a mass of kg, subject to a single foce equal to the weight of a penny, N. What is the acceleation of the electon?

6 Poblem 6 When a 58-g tennis ball is seved, it acceleates fom est to a speed of 45 m/s. The impact with the acket gives the ball a constant acceleation ove a distance of 44 cm. What is the magnitude of the net foce acting on the ball? Poblem 7 A student daws the flawed fee-body diagam shown in the figue below to epesent the foces acting on a golf ball that is taveling upwad and to the ight a vey shot time afte being hit off the tee. Ai esistance is assumed to be elevant. Identify the eos in the diagam, then daw a coect fee-body diagam fo this situation.

7 Poblem 8 Fo each situation, daw a sketch of the situation, a motion diagam, and a FBD. a) An elevato, suspended by a single cable, has just left the tenth floo and is speeding up as it descends towad the gound floo. b) A heavy box is in the back of a tuck. The tuck is acceleating to the ight. Apply you analysis to the box. Poblem 9 Fo each situation, daw a sketch of the situation, a motion diagam, and a FBD. a) A ocket is being launched staight up. Ai esistance is not negligible. b) You've slammed on the bakes and you ca is skidding to a stop while going down a 20 hill.