Nae: Anwer Key Date: Regent Phyic Tet # 9 Review Energy 1. Ue GUESS ethod and indicate all vector direction.. Ter to know: work, power, energy, conervation of energy, work-energy theore, elatic potential energy, gravitational potential energy, kinetic energy, reference (bae) level, pring contant, Hooke Law, watt, joule, echanical energy, pendulu, iple haronic otion. 3. What are the two type of echanical energy? Potential & Kinetic 4. Why doe the aount of gravitational potential energy an object ha depend on the reference level? Becaue PE i baed on how uch energy i required to lift the object a certain ditance, thi depend on the height above a certain object or the reference level. 5. In which ituation i a peron doing work? Why or why not? a) lifting a box b) carrying a box c) holding a box d) dragging a box up a hill e) walking up tair 6. Which are vector (if any) and which are calar (if any): work - vector, power - calar, energy -calar i. ( + or direction) 7. A the tie it take to lift an object at a contant peed decreae, what happen to: a) the work done in lifting the object? Nothing force x ditance b) the power exerted by the peron lifting it? Increae W/t 8. What are the unit and alternate unit for: work, power, energy? Work: N- = Joule (J) Power: J/ = Watt (W) Energy: Joule (J) 9. Decribe the energy tranforation and tranferal in the following yte: a) a planet orbiting the Sun. Nuclear, PE, and EM fro the Sun tranferred to the Earth b) a pendulu winging back and forth. Potential at the top, kinetic at botto, potential at top c) a ball bouncing off the floor. Potential at the top, kinetic on way down, back to PE on way up d) an arrow hot up into the air. Kinetic to potential on way up, back to kinetic on way down e) a car bouncing up and down on it hock aborber Kinetic to potential on way up, back to kinetic on way down f) a roller coater ride. Potential to kinetic and back again a it goe up and down hill 146
Direction: Read each quetion carefully and record your anwer in the pace provided. Be ure to how all work! Anwer hould be in ignificant figure. You will be graded on proper ue of the GUESS ethod. Thee will be the ae direction on the tet. Practice the GUESS ethod now. 10. A 160. N box i puhed 10.0 along a frictionle plank by Anne A. Bollick. She exert 36. N of force. Deterine the aount of work Anne did in oving the box. WFappd (36. N)(10.0 ) 36J 11. A 50.0 kg child running at 6.0 eter per econd jup onto a tationary 10.0 kg led. The led i on a level frictionle urface. a. Calculate the peed of the led with the child after he jup onto the led. Before Pbefore = p1 + p = 1v1 + v = (50.0 kg)(+6.0 /) + (10.0 kg)(0) = 300 kg / = 5.0 / = Pafter After p1, (1 + ) vf = (50.0 kg + 10.0 kg)( vf) = (60.0 kg) vf vf Sticky Two eparate object before, one cobined object after b. Calculate the kinetic energy of the led with the child after he jup on it. KE v (60.0 kg )(5.0 ) 750 J 1 1 Ue the cobined a becaue they ove together c. After a hort tie, the oving led with the child aboard reache a rough level urface that exert a contant frictional force of 54 newton on the led. How uch work ut be done by friction on the led to bring it to a halt? WE KE KE 0J 750 J 750 J T f i KEf = 0 J becaue they top 1. A tudent drag a phyic teacher for 5 on the end of a rope that ake a 40.0 0 angle with the ground. The force on the rope i 650 N. How uch work i done? WFd( Fco ) d(650 Nco40.0)(5 ) 1,000 J x Moveent i horizontal, o need the horizontal coponent of the force 13. A 0568 Watt otor applied a contant force of 104 N on a vehicle. What i the vehicle velocity? v P 0568W 198 F 104 N Velocity need direction, the quetion i vague o ue a + or forward 147
14. Claire D. Steer, whoe a i 7 kg, decided to try the power lab. She wa able to clib the tair in 3.00 econd. If the height of the tair cae i 3.6 eter, how uch power did he develop in clibing the tair? (7 kg )(9.81 )(3.6 ) P Fd gd 850W t t 3.00 The applied force i equal in agnitude to her weight 15. A 4.0 kg rock i dropped fro a height of 3.0 eter. Jut before it land, it i going. /. a. How uch PE doe the rock have before it i dropped? PE gh (4.0 kg )(9.81 )(3.0 ) 7530 J b. How uch KE doe the rock have right before it land? [Hint: Actually ue the peed to calculate] KE v (4.0 kg )(. ) 5910 J 1 1 c. What i the difference between the PE at the top and the KE at the botto? What could have caued thi difference? E lot E E KE PE 5910 J7530 J160 J T f i botto top Overcoing air reitance The internal energy increaed due to friction with the air d. How uch work ut be done to the rock to lift it back to the original height? (4.0 )(9.81 WFd gd kg )(3.0 ) 7530 J 16. It take an 84 newton force to hold a pring tretched a ditance of 9 centieter. a. What i the pring contant of thi pring? k F 84N 90 x 0.9 N Convert centieter to eter b. What i the elatic potential energy of the pring in thi poition? 1 1 90 N (0.9 ) elatic 1 PE kx J 148
17. The diagra below how block A, having a and peed v, and B, having a and peed v. 1 A: KE v (1)()(1) B: KE v (1)(1)() 4 1 Copared to the kinetic energy of block A, the kinetic energy of block B i (A) one-half a great (B) twice a great (C) four tie a great (D) the ae 18. The diagra below how a 5.0 kilogra a liding 9.0 eter down an incline fro a height of.0 eter in 3.0 econd. The object gain 90. joule of kinetic energy while liding. E i PE top gh (5.0kg)(-9.81 )(.0) 98J W E E E KE PE 90.J 98J fric t f i botto top How uch work i done againt friction a the a lide the 9.0 eter? (A) 90. J (B) 45 J (C) 0 J (D) 8 J 19. A 1.0 kilogra a gain kinetic energy a it fall freely fro ret a vertical ditance, d. How far would a.0 kilogra a have to fall freely fro ret to gain the ae aount of kinetic energy? (A) d 4 (B) d (C) d (D) 0. A 0.10 kilogra ball dropped vertically fro a height of 1.0 eter above the floor bounce back to a height of 0.80 eter. The echanical energy lot by the ball a it bounce i approxiately (A) 0.78 J (B) 0.0 J (C) 0.080 J (D) 0.30 J 1. A a ball fall freely (without friction) toward the ground, it total echanical energy (A) increae (B) decreae (C) reain the ae. Which graph bet repreent the relationhip between the kinetic energy of a oving object and it velocity? d The KE coe fro the PE = gh Double the a, halve the ditance to get ae value Et lot Ef E i PE bounce PE drop gh gh g(h h ) (0.10kg)( 9.81 )(1.0 0.80) drop bounce drop bounce Total energy tay the ae without external force KE v 1 149
3. A cart of a M on a frictionle track tart fro ret at the top of a hill having height h 1, a hown in the diagra below. The KE coe fro lot PE. EtPE 1 KE Et PE gh gh g(h h ) 1 1 What i the kinetic energy of the cart when it reache the top of the next hill, having height h? (A) M g (h h3) (B) 0 (C) M g h1 (D) M g (h1 h) 4. In the diagra below, 400. joule of work i done raiing a 7 newton weight a vertical ditance of 5.0 eter. Wlift Fd (7N)(5.0) 360J W W W 400.J 360J fric tot lift How uch work i done to overcoe friction a the weight i raied? (A) 400. J (B) 40. J (C) 360 J (D) 760 J 5. Which a ha the greatet potential energy with repect to the floor? (A) 50 kg a reting on the floor PE=gh = (50kg)(-9.81/ )(0) = 0 J (B) kg a 10 eter above the floor PE=gh = (kg)(-9.81/ )(10) = 196 J (C) 10 kg a eter above the floor PE=gh = (10kg)(-9.81/ )() = 196 J (D) 6 kg a 5 eter above the floor PE=gh = (6kg)(-9.81/ )(5) = 94 J 6. A an object fall freely, the kinetic energy of the object (A) decreae (B) increae (C) reain the ae PE becoe KE 7. A 4.0 x 10 3 watt otor applie a force of 8.0 x 10 newton to ove a boat at contant peed. How far doe the boat ove in 16 econd? P Fd t (A) 3 (B) 3. (C) 5.0 (D) 80. 3 d Pt (4.0 10 W)(16) F 8.0 10 N 8. If the tie required for a tudent to wi 500 eter i doubled, the power developed by the tudent will be W 1 (A) quadrupled (B) quartered (C) halved (D) doubled P t 9. A 0.50 kilogra phere at the top of an incline ha a potential energy of 6.0 joule relative to the bae of the incline. Rolling halfway down the incline will caue the phere potential energy to be (A) 3.0 J (B) 6.0 J (C) 1 J (D) 0 J 150 Half the height ean half the PE
Anwer: 10. W = + 36 J 11. a. v = 5.0 / forward b. KE = 750 J c. W = - 750 J 1. W = +1,000 J 13. v = 198 / forward 14. P = 850 W 15. a. PE = 7530 J b. KE = 5910 J c. 160 J d. W = +7530 J 16. a. k = 90 N/ b. PE = 1 J 17. B 18. D 19. D 0. B 1. C. A 3. D 4. B 5. D 6. B 7. D 8. C 9. A 30. B 31. D 3. D 30. In the diagra below, an ideal pendulu releaed fro point A wing freely through point B PEA = KEB KEA = KEA Copared to the pendulu kinetic energy at A, it potential energy at B i (A) half a great (B) the ae (C) twice a great (D) four tie a great 31. The graph below how the relationhip between the elongation of a pring and the force applied to the pring cauing it to tretch. Spring contant i the invere of the lope or you can pick two point ON line and plug into Hooke Law equation k F 5N x 0.50 What i the pring contant of the pring? (A) 0.00 N/ (B).0 N/ (C) 5 N/ (D) 50. N/ 3. The diagra below how a 0.1 kilogra apple attached to a branch of a tree eter above the pring on the ground below. Ei Ef PE g PE gh k gh x pring 1 kx (0.1kg)(9.81 )() (0.1) The apple fall and hit the pring, copreing it 0.1 eter fro it ret poition. If all of the gravitational potential energy of the apple on the tree i tranferred to the pring when it i copreed, what i the pring contant of thi pring? (A) 10 N/ (B) 40 N/ (C) 100 N/ (D) 400 N/ 151