Ch04 Work, Energy and Power What is work in physics?

Transcription

1 Eunl Won Dept o Physcs, Korea Unversty 1 Ch04 Work, Energy and Power What s work n physcs?

2 Eunl Won Dept o Physcs, Korea Unversty

3 Eunl Won Dept o Physcs, Korea Unversty 3 Work W F d W Fd cosφ W Fd ο cos 90 0

4 Eunl Won Dept o Physcs, Korea Unversty 4 W Fd W > 0 d 0 W Fd W 0 d 0, F d W Fd W 0

5 Eunl Won Dept o Physcs, Korea Unversty 5 Example 4.1 Calculate the work done by an ambulance attendant who pushes a patent on a gurney wth a horzontal orce o 5 N or a dstance o 3.0 m W Fd ( 5 N)(3.0 m) 75 N m 75J

6 Eunl Won Dept o Physcs, Korea Unversty 6 Example 4. How much work s done n ltng a 10-kg sack o groceres 1. m rom the loor to a table, as n the gure below? W Fd ( mg) d [(10 kg)( kg m J m/s /s )](1. m)

7 Eunl Won Dept o Physcs, Korea Unversty 7 Example 4.3 I the proud ather n the gure exerts a orce o 10 N at an angle o 30 o below the horzontal, how much work does he do n pushng the baby carrage a dstance o 5.0 m? W F d h (8.66N)(5.0m) 43.3J

8 Eunl Won Dept o Physcs, Korea Unversty 8 Energy: ablty to do work Knetc Energy W Knetc Energy: work done gong nto causng moton x x x x x x F( x) dx m dv dt mvdv dx 1 x x x x mv madx mdv 1 dx dt mv Knetc Energy K W K 1 mv Work-Energy theorem K K 1N m 1J( joule)

9 Eunl Won Dept o Physcs, Korea Unversty 9 Example 4.4 How much energy s requred to accelerate a 1000-kg car rom rest to 30 m/sec, assumng no rcton? KE mv 1 1 (1000 kg)(30 m/s) kg m J /s

10 Eunl Won Dept o Physcs, Korea Unversty 10 Example 4.5 A pcture throwng a astball steps orward. Calculate the speed o the baseball when t leaves hs hand the pcture exerts an average orce o 00 N or a dstance o.50 m on a 0.5-kg baseball. v W W K K Fd mv Fd m 1/ K 1 (00N)(.50m) 0.500kg 1/ m/s ~158 km/h

11 Eunl Won Dept o Physcs, Korea Unversty 11 Knetc Energy : The Energy o Moton Potental Energy : Energy due to ts shape or poston Gravtatonal potental energy Nuclear Energy Electromagnetc Energy Mass Energy : MassEnergy E mc Internal Energy: Knetc and potental energy o molecules (has close relaton wth the thermal energy)

12 Eunl Won Dept o Physcs, Korea Unversty 1 Gravtatonal Potental Energy U ( x) x 0 h 0 mgh F( x) dx ( mg) dx E K + U ( x) 1 mv + mgh const.

13 Eunl Won Dept o Physcs, Korea Unversty 13 Example 4.6 Calculate the work done by a 50-kg nurse n ltng a 5-kg chld 0.50 m onto a table W PE mgh (75 kg)(9.8 m/s 368kg m /s 368 J )(0.50 m) 368Nm

14 Eunl Won Dept o Physcs, Korea Unversty 14 Chemcal Energy: det? 1 kcal 4186 J

15 Eunl Won Dept o Physcs, Korea Unversty 15 Example 4.7 An ambulance attendant pushes horzontally on a gurney wth a orce o 100 N. He starts rom rest and pushes or a dstance o 5.0 m. There s rcton n the wheels. The nal speed s 1.0 m/s and the mass o gurney and patent s 80 kg. Calculate the amount o work done aganst rcton and the orce o rcton W Fd mv (100)(5) 460J 1 1 (80)(1.0) W d 460J W t KE + W 460J d 460J 0.5m 9N

16 Eunl Won Dept o Physcs, Korea Unversty 16 Example 4.8 I a person who normally requres an average o 3000 kcal o ood energy per day nstead consumes 4000 kcal per day, he wll steadly gan weght. How much bcyclng per day s requred to work o ths extra 1000 kcal? tme energy energy/tme 1000kcal 5.7kcal/mn 175 mn 3hr

17 Eunl Won Dept o Physcs, Korea Unversty 17 Example 4.9 Calculate the amount o energy obtaned rom convertng 1.0 g o mass nto energy and compare t to the 3000 kcal o ood used by a normal person n one day E mc ( kg m J kg)( /s 8 m/s) J 1kcal 4186J kcal kcal kcal 0,000 years

18 Eunl Won Dept o Physcs, Korea Unversty 18 Conservaton o Total Energy Total energy n a closed system s always conserved Mechancal Energy KE + PE + OE cons tan t

19 Eunl Won Dept o Physcs, Korea Unversty 19 Example 4.10/11 0 kg 100 m KE + PE KE mgh v Velocty o sled at the bottom? (no rcton) (gh ) 1 1/ mv [(9.8)(100)] + 0 1/ PE 44.3m/s Thermal Energy by rcton velocty at the bottom s 30 m/s? 0 + mgh TE mgh KE + PE + OE KE + PE mv mv 19600J 9000J 10600J 0(9.8)(100) TE 1 (0)(30) OE

20 Eunl Won Dept o Physcs, Korea Unversty 0 Power and Ecency The tme rate o dong Work P t W [ 1W 1J/s] E out n Work E W Energy E out E / out E / n E Power E P out / P n

21 Eunl Won Dept o Physcs, Korea Unversty 1 Example 4.1 a) Calculate the amount o power generated by a 60-kg person clmbng a.0-m-hgh lght o stars n 7.0 s b) Repeat the calculaton or the same person runnng up the stars n.0 s P W mgh t 7.0 (60)(9.8)(.0) 168W 7.0 (168) (1hp/746W) 0.5hp W mgh P t W.0 (588) (1hp/746W) 0.788hp

22 Eunl Won Dept o Physcs, Korea Unversty Example 4.13/4.14 The ecency o the human body n convertng ood energy to work ~ 0%. What s the ood energy needed to clmb one lght o stars n Example 4.1? E W / E E n out n W mgh (60)(9.8)(.0) 0. (5880J)(1kcal/4186J) 1.40kcal 0. a) Thermal energy by the person clmbng the stars? b) Rate o thermal energy when the person walks up? c) Rate o thermal energy when the person runs up? PE + FE PE + TE FE mgh 5880J 1176J P P TE t TE t 4704J 7.0s 4704J.0s TE 4704J 67W 350W

23 Eunl Won Dept o Physcs, Korea Unversty 3 Example 4.15 A large number nuclear power plant has an ecency o 35% and generates 1000 MW o electrc power. What s the plant s rate o producton waste thermal energy n megawatts? E Pout/ Pn Pn Pout / E (1000 MW)/ MW Pn Pout + Pwaste P P P waste n out 857 MW MW 1857 MW

24 Eunl Won Dept o Physcs, Korea Unversty 4 Example 4.16 The cost o electrc energy vares around the country, but 50 won per klowatt-hour s typcal. What s the cost o runnng a 500-W color TV set 6.0 hr per day or 30 days? E Pt (500 W)(6.0 h/day)(30days) 90,000 Wh 90 kwh 50wons cost (90 kw h) kw h 4,500wons

25 Eunl Won Dept o Physcs, Korea Unversty 5 Basal Metabolc Rate (BMR) personal BMR thyrod ( ) actvty Consumpton o Oxygen 1 lter 4.9 kcal

26 Eunl Won Dept o Physcs, Korea Unversty 6 Warm-blooded anmal BMR vs. mass

27 Eunl Won Dept o Physcs, Korea Unversty 7 Example 4.17 How many grams o at wll an dle person gan n a day by consumng 500 kcal o ood? (1.kcal/mn)(60mn/hr)(4hr/day) 1757kcal/day 500kcal-1757kcal 743kcal 1.0g at Mass gan 743kcal 9.3kcal 80g at 9kg/year