Announcements. 30 o. The pumpkin is on the left and the watermelon is on the right. The picture on page 138 is better.

Transcription

1 Annuncements Em 1 is ne eek. Ems frm revius semesters hve been sted n the ebsite. HITT quiz slutins re sted n ebsite. Td e ill finish Chter 4 nd begin Chter 5. ill st Em 1 brekdn nd revie mteril. Lk fr it in the lectures link. Other lectures ill be sted n lectures link. Hefull, ill return lecture quiz net time. Net Tuesd ill be revie. D u nt t meet this eekend? Prblem-Slving Strteg fr Netn s Secnd L (ge 113) Decide ht bject ill hve Netn s secnd l lied t it. Identif ll the eternl frces cting n the bject. Dr n FBD t sh ll the frces cting n the bject. Chse crdinte sstem. If the directin f the net frce is knn, chse es s tht the net frce (nd the ccelertin) re lng ne f the es, Find the net frce b dding the frces s vectrs. Use Netn s secnd l t relte the net frce t the ccelertin. Relte the ccelertin t the chnge in the velcit vectr during time intervl f interest. Prblem 4.9 A 10.0-kg termeln nd 7.00-kg umkin re ttched t ech ther vi crd tht rs ver ulle. Frictin is negligible everhere in this sstem. () Find the ccelertin f the umkin nd the termeln. Secif mgnitude nd directin. (b) If the sstem is relesed frm rest, h fr lng the incline ill the umkin trvel in 0.30 s? (c) ht is the seed f the termeln fter 0.0 s? The umkin is n the left nd the termeln is n the right. The icture n ge 138 is better. Decide ht bject ill hve Netn s secnd l lied t it. There re t bjects, the termeln nd the umkin. Identif ll the eternl frces cting n the bject. On the umkin, the frces re: its eight, the nrml frce due t the left incline, nd the tensin in the crd. On the termeln, the frces re: the eight f the termeln, the nrml frce frm the right incline, nd the tensin in the crd.

2 The mst generl rblem uld include frictinl frces, but e re tld tht frictin is negligible. Dr n FBD t sh ll the frces cting n the bject. T bjects, t free-bd digrms. Chse crdinte sstem. If the directin f the net frce is knn, chse es s tht the net frce (nd the ccelertin) re lng ne f the es, T different crdinte sstems re needed since there re t bjects. In bth cses, set the -is lng the inclines. Find the net frce b dding the frces s vectrs. Frm the FBD, find the net frce in the nd directins fr ech bject. Here is h u del ith inclined lnes. Dr the figure s tht the incline hs s shll ngle. θ θ Ntice tht the eight mkes n ngle ith the negtive -is equl t the ngle f the incline. This mens = sinθ = csθ

3 nd the -cmnent is sscited ith the sine functin nd the -cmnent is sscited ith the csine functin. h? The ngle is mesured frm the verticl. The frces fr the umkin: nd fr the termeln: F = T nd F = N F = T nd F = N Use Netn s secnd l t relte the net frce t the ccelertin. Fr the umkin: Fr the termeln: T m g sin 53 N m T F N g cs 53 = 0 m g sin 30 N m F T F T N g cs 30 = 0 F Relte the ccelertin t the chnge in the velcit vectr during time intervl f interest. But the gurds re tied tgether. (h?) This mens tht =

4 Fr generl urse, let s cll the ccelertin. The mtin in the -directin is given b Frm the first equtin, T m g sin 53 nd mg sin 30 T T + m g sin 53 Substituting int the secnd equtin m g sin 30 ( m + m g sin 53 ) m g sin 30 m g sin 30 T m g sin 53 = ( m + m m = ) sin 30 m (10 kg)sin 30 (7 kg)sin 53 = 7 kg + 10 kg = m/s m + m sin 53 g (9.8 m/s ) ht des the negtive sign men? (b) Distnce trveled in 0.30 s is = v = 0 + i 1 t + = m 1 ( t) ( m/s )(0.30 s) ht if the gurds re nt frictinless? ht is the cefficient f frictin needed t kee them frm mving? All rblems fll this frmt. Arent eight Elevtr r rller cster effect. Air Resistnce T hrd fr us t rk ith quntittivel Fundmentl Frces Grvit

5 Electrmgnetism Strng ek Chter 5 Circulr Mtin Describing unifrm circulr mtin Rttin Angulr dislcement Rdin mesure Angulr velcit Angulr ccelertin