p. 1/7 PRELAB: FORCES AND MOTION 1. Skech Predicion 1-1 on he axes a he righ. Explain he reasoning behind your acceleraion graph. Predicion 1-1 velociy + acceleraion - scalke reading 2. Draw he free body diagrams decribed in Sep 1 of Invesigaion 2. 3. Skech Predicion 3-2 on he axes a he righ. Explain he reasoning behind your predicion. Predicion 3-2 acceleraion + velociy - posiion
p. 2/7 Topic: Newon s 1 s and 2 nd laws Overview: FORCES AND MOTION You will be invesigaing he relaionship beween forces and moion in a series of hree shor invesigaions. The invesigaions can be performed and analyzed in any order. Wriing i up: Throughou his handou, you will be asked o answer quesions, skech graphs and diagrams, and do calculaions. Wrie hese hings in your lab noebook as you go hrough he experimen. Label each answer/graph/calculaion/diagram so ha you (or your lab TA) can find hings quickly. If you have any compuer prinous (such as graphs), remember o affix hem o your lab noebook. Afer lab, wrie a shor (<3 words) conclusion of he experimen ha summarizes wha you did and he major findings of he experimen. Invesigaion 1: The elevaor In his shor experimen, you will place a mass on a scale and observe wha happens o he scale s reading as you ride he elevaor. Predicion 1-1 (Going up): Skech your predicion of velociy, acceleraion and scale reading as a funcion of ime for a ride from he 1 s floor o he 2 nd floor from he ime he elevaor sars moving unil i is a res on he second floor on he axes provided. Label imporan evens (such as elevaor sars moving, elevaor sops speeding up ) on your graph. Arrange he hree ses of axes verically, so ha he ime axes line up (as shown in he prelab). Explain he reasoning behind your predicions for acceleraion and scale reading. 1. There is no way (using he equipmen provided) o direcly generae velociy and acceleraion graphs for he moion. Insead, check your predicion concerning he velociy and acceleraion graphs wih your parner. Resolve any inconsisencies. 2. Tes your predicion concerning he scale s reading. Record he scale s readings. Compare your daa o your predicion. If your observaions do no mach your predicions, figure ou why. Q1-1: Is he scale s reading equal o he ne force on he objec a any poin during he ride? If so, when? Explain your answer. Q1-2: Is he scale s reading equal o he weigh of he objec a any poin during he ride? If so, when? Explain your answers. (Noe: The downward graviaional force he earh exers on an objec is referred o as weigh). Q1-3: Wha force does he scale reading equal? Predicion 1-2 (Going down): Skech predicion graphs of velociy, acceleraion and scale reading for a downward ride from he 2 nd floor o he 1 s floor. 3. Check Predicion 1-2.
p. 3/7 4. Skech a free body diagram for he mass siing on he scale during each phase of he upward ride. (There should be hree diagrams). Label each force compleely. Each force label should clearly indicae he objec being pushed (or pulled) and he objec doing he pushing (or pulling). For example, he graviaional force he earh exers on he mass would be wrien as Fearh Æ mass (see example below). Each arrow should indicae he direcion of he force. The lengh of he arrow should indicae he srengh of he force. Keep he same scale for all hree force diagrams; wo forces of equal magniude should be drawn he same lengh, even if he wo arrows appear on differen diagrams. Complee he force diagrams below. A res Speeding up Consan speed Slowing down F scale mass F Earh mass 5. Skech free body diagrams for each phase of he downward ride. 6. Use your daa o esimae he acceleraion of he elevaor during each phase of he wo rides. Q1-4: Are he algebraic signs of he acceleraion values consisen wih your predicions? Are he values larger or smaller han you expeced? If your observaions do no mach your predicions, figure ou why. Q1-5: You jus used he scale s readings o esimae he elevaor s acceleraion during various phases of he ride. Can you use he scale readings o esimae he cruising speed of he elevaor? Explain. Invesigaion 2: Falling coffee filer In his shor experimen, you will invesigae he fligh of a falling coffee filer. 1. Skech a free body diagram for he single coffee falling hrough he air a hree differen insans: immediaely afer release afer release, bu before he filer sops speeding up, and afer he filer sops speeding up. Clearly label each force by indicaing he objec being pushed/pulled and he objec doing he pushing/pulling (e.g. F Æ ). Make sure each vecor has a lengh ha earh filer corresponds o he srengh of he force. Use he same scale for your vecors on all hree diagrams.
p. 4/7 Predicion 2-1: Skech your predicion of ne force, acceleraion and velociy as a funcion of ime for he filer, from he ime is released unil he filer is a res on he ground. Label imporan evens on your graph. Explain he reasoning behind your predicion for acceleraion. 2. To check your predicions, you will use an ulrasonic posiion deecor conneced o a compuer. Double click on he file coffee.ds o se up he compuer. Posiion he coffee filer under he deecor and click he green sar buon. Release he coffee filer. Click on he sop buon afer he filer his he ground. The compuer will produce a posiion-ime graph for he moion of he filer. Record he graph in your noes or aach a prinou o your lab noebook. Idenify imporan feaures on he graph. 3. To check he daa agains your predicion, you will need o use he posiion-ime graph o infer he shape of velociy-ime and acceleraion-ime graphs. Q2-1: Compare your inferred acceleraion-ime and velociy-ime graphs wih your predicions. Resolve any discrepancies. Q2-2: Does he acceleraion increase or decrease wih ime? Is his resul consisen wih he free body diagrams you drew in Sep 1? Explain. Q2-3: Which direcion does he acceleraion vecor poin? Explain how you can deduce his from he changes in he velociy vecor. Noes abou he ulrasonic deecor: The deecor works by emiing a sound and lisening for he reflecion. The posiion repored by he deecor is simply he disance from he objec o he deecor. The deecor is no reliable for disances less ha abou 15 cm or for disances beyond abou 2 m.
p. 5/7 Invesigaion 3: On he air rack In his invesigaion, you will invesigae he moion of a glider ha rides on an almos fricionless surface. You will observe how forces acing on he glider affec is moion. Noes abou using he air rack: Do no slide he glider along he air rack when he air source is off (o avoid damage o he air rack). Avoid ouching he air rack surface or he underside of he glider (o avoid damage o he air rack). Minimize he amoun of ime he air source is on (o minimize noise and hea). Before you ake daa: Check o see if he air rack is horizonal. Place he glider on he rack and urn on he air source. If he rack is no level, he car will move in one direcion or he oher when he air is urned on. Try a few differen spos. If he glider goes consisenly one direcion of he oher, your air rack is no level. If your air rack is way ou of level, consul your TA. Lay he spring scale on a horizonal surface. Make sure he scale reads zero when he hook is no being pulled. (You may need o adjus he spring scale o make his happen). Aciviy 3-1: Pull and release In his secion, you will observe wha happens you give he glider a brief pull o he righ and hen release i. Predicion 3-1: Suppose you gave he glider a brief pull o he righ and released i. Describe he velociy, applied force and acceleraion of he glider in words. 1. Se up he glider and spring scale as shown above. The body of he spring scale should res agains he meal plae aached o he fron of he glider, as shown above. (This ensures ha he glider ges pulled along by your hand). 2. Tes Predicion 3-1 qualiaively. Turn on he air source while holding he hook of he spring scale. Give he glider a brief pull, release i and observe he moion of he glider and he reading on he scale. Sop he glider and urn off he air source. Skech qualiaive graphs for he observed velociy, scale reading and acceleraion as funcions of ime. Q3-1: Is he scale s reading equal o he ne force on he glider during each phase of he moion? Explain.
p. 6/7 Q3-2: Once he glider is moving, how much force is required o keep he glider moving a consan velociy? Aciviy 3-2: A seady pull In his secion, you will observe wha happens as he glider is pulled o he righ by a consan force. 1. Se up he glider, spring scale, sring and hanging mass as shown above. Selec a hanging mass ha is large enough ha he scale can be easily read, bu is small enough ha he moion is slow enough o observe. The body of he spring scale should res agains he meal plae aached o he fron of he glider. (This ensures ha he glider ges pulled along by he sring). Adjus he posiion of he pulley so ha he sring is horizonal. 2. Turn on he air source while holding he glider. Noe he scale s reading. (If he force value recorded by he scale is oo small o read easily, increase he hanging mass). Release he glider and observe wha happens o he scale s reading. Sop he glider before he hanging mass can hi he floor. (You may need o pracice his a few imes o see when changes in he scale s reading occur). Shu off he air source. Skech a graph of he scale s reading vs. ime in your noes. Indicae he momen ha you released he glider on your graph. Record he value of he hanging mass. Q3-3: Is he scale s reading equal o he ne force on he glider before you release he glider? Explain. Q3-4: Is he scale s reading equal o he ne force on he car afer you release he glider? Explain. Q3-5: Use Newon s 2 nd law o esimae he acceleraion of he glider afer i is released. (Hin: You will need o measure he mass of he suff being pulled o he righ by he sring.) Predicion 3-2: Skech predicion graphs for acceleraion-ime, velociy-ime and posiion-ime for he moion of he glider you jus observed. 3. Use he spark imer o check Predicion 3-3. To save ime, use Excel o calculae he velociy values and o plo he posiion and velociy graphs. Do no boher o consruc an acceleraion graph from he spark imer daa. Use he velociy graph o deermine he acceleraion value (boh magniude and direcion). Q3-6: Do he shapes of he posiion and velociy graphs mach your predicions? If no, resolve he inconsisencies. Q3-7: Would you expec he magniude of he acceleraion ha you measured using he spark imer o be larger han, smaller han, or he same as he acceleraion he glider had when he spark ape was no aached o he glider? Explain.
p. 7/7 Compare he acceleraion value you jus found from he spark daa wih he value you prediced using Newon s Law in Q3-5. Is his wha you expec? Aciviy 3-3: There and back again Again he glider is pulled o he righ by he sring, bu his ime he glider sars a he righ end of he rack. You will give he glider a brief push up he ramp, so ha he glider slows down, reverses direcion and reurns o your hand. Predicion 3-3: Skech predicion graphs for scale reading-ime, acceleraion-ime and velociy-ime for he moion of glider, from he momen you release he glider unil jus before you sop i. 1. Place he glider a he righ end of he air rack as shown above. (Use he same hanging mass as before. You should also be able o use he same sring as before). Hold he glider and urn on he air source. Noe he scale s reading. Turn on he air source. Give he glider a brief (bu srong) push up he ramp and observe wha happens o he scale s reading. The glider should slow down, reverse direcion, and reurn o your hand. Sop he glider before he hanging mass can hi he floor. Shu off he air source. Skech a graph of scale reading versus ime. Indicae he momen you released he car and he momen he car reversed direcion on your graph. Q3-8: Is he scale s reading equal o he ne force on he car for each phase of he moion afer you release he glider? Explain. Q3-9: In which direcion does he ne force vecor poin before he glider reverses direcion? In wha direcion does i poin afer he glider reverses direcion? Explain your observaions suppor your answers. Q3-1: Based on your answer o he previous quesion, deermine he direcion of he acceleraion vecor during each phase of he moion. Q3-11: Does he magniude of he acceleraion vecor increase, decrease or remain unchanged during he moion? Explain how your answer is suppored by your observaions. Q3-12: Based on your observaions, deermine he direcion of he velociy vecor during each phase of he moion. Q3-13: Based on your observaions, describe how he magniude of he velociy vecor changes during he moion. Q3-14: Based on your analysis so far, draw velociy and acceleraion graphs of he moion. Indicae he momen ha he glider reverses direcion on each graph. Make sure ha he algebraic signs you use are consisen wih he direcions of he vecors. Q3-15: Does he slope of your velociy graph change a any momen during he moion? Should i? Explain.