Physics 131: Lecture 16. Today s Agenda

Transcription

1 Physcs 131: Lecture 16 Today s Agenda Intro to Conseraton o Energy Intro to some derent knds o energy Knetc Potental Denton t o Mechancal Energy Conseraton o Mechancal Energy Conserate orces Examples Pendulum duu Physcs 01: Lecture 1, Pg 1

2 Energy Very mportant dea n physcs Knght splts s ths nto two chapters: Ch 10: How to use conseraton o energy Ch 11: How to thnk about t conceptually Physcs 01: Lecture 1, Pg

3 Energy s Consered Energy s Consered meanng t can not be created nor destroyed Can change orm Can be transerred Total Energy does not change wth tme. Ths s a BIG deal! Physcs 01: Lecture 1, Pg 3

4 Knetc Energy An object n moton has energy Energy o moton s called Knetc energy True or any object wth elocty and mass m. K 1 m Unts: Joule Physcs 01: Lecture 1, Pg 4

5 Clcker Queston 1: Ater a bomb at rest explodes nto two unequal ragments, the more masse ragment has (a) the same knetc energy than the less masse ragment. (b) less knetc energy than the less masse ragment. (c) more knetc energy than the less masse ragment. Physcs 01: Lecture 1, Pg 5

6 Potental energy Potental energy: Stored energy Energy depends on the poston or conguraton o an object. Examples Potental energy due to graty Potental energy o a sprng. U = mgy Physcs 01: Lecture 1, Pg 6

7 Gratatonal Potental Energy or Free Fall The gure shows a beore-and-ater representaton o an object n ree all. One o the knematcs equatons rom Chapter, wth a y = g, s: gy g y y Rearrangng: gy gy Multplyng both sdes by ½m: 1 m mgy 1 m We can thnk o ths n terms o energy! mgy Physcs 01: Lecture 1, Pg 7

8 Gratatonal Potental Energy on a Frctonless Surace on a Frctonless Surace s a a g sn g sn a sn sn y g y y g y y g y y g gy gy s y H O sn 1 1 mgy m mgy m Physcs 01: Lecture 1, Pg 8 sn y s Ths works or any rctonless surace!

9 Conseraton o Mechancal Energy When graty s the only orce transerrng energy nto or out o the system mechancal energy s consered E mech E mech K U g K U g 1 1 m mgy m mgy Mechancal energy s not always consered!!!!! But Energy s. Physcs 01: Lecture 1, Pg 9

10 Gratatonal Potental Energy on a Frctonless Surace Can approxmate any surace as a bunch o small nclnes! Physcs 01: Lecture 1, Pg 10

11 Clcker Queston : Three balls are thrown rom a cl wth the same speed but at derent angles. Whch ball has the greatest speed just beore t hts the ground? A. Ball A. B. Ball B. C. Ball C. D. All balls hae the same speed. Physcs 01: Lecture 1, Pg 11

12 Clcker Queston 3: A hockey puck sldng on smooth ce at 4 m/s comes to a 1-m-hgh hll. Wll t make t to the top o the hll? A. Yes. B. No. C. Can t answer wthout knowng the mass o the puck. D. Can t say wthout knowng the angle o the hll. Physcs 01: Lecture 1, Pg 1

13 F SPRING = -k(s) Force due to a sprng s s dstance sprng s compressed or stretched rom ts relaxed length k s the sprng constant (how strong the sprng s) F SPRING F HAND F SPRING = -k(-s) = +ks s F HAND F SPRING F SPRING = -k(+s) = -ks s Physcs 01: Lecture 1, Pg 13

14 Sprng orce Restorng orce: Always works to restore the orgnal poston. The k constant t depends d on many actors (materal, how many rngs, etc) Only good or a partcular sprng Measure o stness Large k real strong (shock absorber) Small k not strong (slnky) Physcs 01: Lecture 1, Pg 14

15 Potental energy due to a sprng A compressed sprng has potental energy. Energy o conguraton U = S ½k(s) s s dstance sprng s compressed or extended. relaxed poston s 0 s D k s constant that depends on sprng. Physcs 01: Lecture 1, Pg 15

16 New Conseraton o Mechancal Energy When graty or a sprng-lke orce are the only orces transerrng energy nto or out o a system, mechancal energy s consered E = E K + U =K + U ½kx + ½m + mgh = ½kx + ½m + mgh Physcs 01: Lecture 1, Pg 16

17 Clcker Queston 4: Asprng-loaded gun shoots a plastc ball wth a launch speed o.0 m/s. I the sprng s then compressed twce the dstance t was on the rst shot. The ball s new launch speed wll be A. 10m/s 1.0 m/s. B..0 m/s. C. 8m/s.8 m/s. D. 4.0 m/s. E m/s. Physcs 01: Lecture 1, Pg 17

18 Clcker Queston 5: A sprng s compressed by a ball an amount x = 0.5 m. The system s released and the ball s shot out down a rctonless table. At the end o the table the ball encounters a hump that brngs t up to a heght o 0.6 m. I the mass o the ball s 10 kg and the k or the sprng s 600 N/m what s the nal speed o the ball? a).33 m/s b) 4.56 m/s c) 3.45 m/s d) 1.79 m/s? e) 5 m/s Physcs 01: Lecture 1, Pg 18