Physics 10 UCSD Work, Energy, Power, Momentum Measures of Effort

Physics 10 UCSD Work, Energy, Power, Momentum Measures of Effort & Motion; Conservation Laws

Physics 10 UCSD Work, defined Work carries a specific meaning in physics – Simple form: work force distance W F·d Work can be done by you, as well as on you – Are you the pusher or the pushee Work is a measure of expended energy – Work makes you tired Machines make work easy (ramps, levers, etc.) – Apply less force over larger distance for same work Spring 2008 2

Physics 10 UCSD Working at an advantage Often we’re limited by the amount of force we can apply. – Putting “full weight” into wrench is limited by your mg Ramps, levers, pulleys, etc. all allow you to do the same amount of work, but by applying a smaller force over a larger distance Work Force Spring 2008 Force Distance Distance 3

Physics 10 UCSD Ramps Exert a smaller force over a larger distance to achieve the same change in gravitational potential energy (height raised) Larger Force Short Distance Small Force Long Distance M Spring 2008 4

Physics 10 UCSD Gravitational Potential Energy Gravitational Potential Energy near the surface of the Earth: Work Force Distance m W mg h h m Spring 2008 5

Physics 10 UCSD Ramp Example Ramp 10 m long and 1 m high Push 100 kg all the way up ramp Would require mg 980 N (220 lb) of force to lift directly (brute strength) Work done is (980 N) (1 m) 980 N·m in direct lift 1m Extend over 10 m, and only 98 N (22 lb) is needed – Something we can actually provide – Excludes frictional forces/losses Spring 2008 6

Physics 10 UCSD Work Examples “Worked” Out How much work does it take to lift a 30 kg suitcase onto the table, 1 meter high? W (30 kg) (9.8 m/s2) (1 m) 294 J Unit of work (energy) is the N·m, or Joule (J) – One Joule is 0.239 calories, or 0.000239 Calories (food) Pushing a crate 10 m across a floor with a force of 250 N requires 2,500 J (2.5 kJ) of work Gravity does 20 J of work on a 1 kg (10 N) book that it has pulled off a 2 meter shelf Spring 2008 7

Physics 10 UCSD Work is Exchange of Energy Energy is the capacity to do work Two main categories of energy – Kinetic Energy: Energy of motion A moving baseball can do work A falling anvil can do work – Potential Energy: Stored (latent) capacity to do work Gravitational potential energy (perched on cliff) Mechanical potential energy (like in compressed spring) Chemical potential energy (stored in bonds) Nuclear potential energy (in nuclear bonds) Energy can be converted between types Spring 2008 8

Physics 10 UCSD Conversion of Energy Falling object converts gravitational potential energy into kinetic energy Friction converts kinetic energy into vibrational (thermal) energy – makes things hot (rub your hands together) – irretrievable energy Doing work on something changes that object’s energy by amount of work done, transferring energy from the agent doing the work Spring 2008 9

Physics 10 UCSD Energy is Conserved! The total energy (in all forms) in a “closed” system remains constant This is one of nature’s “conservation laws” – Conservation applies to: Energy (includes mass via E mc2) Momentum Angular Momentum Electric Charge Conservation laws are fundamental in physics, and stem from symmetries in our space and time – Emmy Noether formulated this deep connection – cedar.evansville.edu/ ck6/bstud/noether.html Spring 2008 10

Physics 10 UCSD Energy Conservation Demonstrated Roller coaster car lifted to initial height (energy in) Converts gravitational potential energy to motion Fastest at bottom of track Re-converts kinetic energy back into potential as it climbs the next hill Spring 2008 11

Physics 10 UCSD Kinetic Energy The kinetic energy for a mass in motion is K.E. ½mv2 Example: 1 kg at 10 m/s has 50 J of kinetic energy Ball dropped from rest at a height h (P.E. mgh) hits the ground with speed v. Expect ½mv2 mgh – – – – h ½gt2 v gt v2 g2t2 mgh mg (½gt2) ½mg2t2 ½mv2 sure enough Ball has converted its available gravitational potential energy into kinetic energy: the energy of motion Spring 2008 12

Physics 10 UCSD Kinetic Energy, cont. Kinetic energy is proportional to v2 Watch out for fast things! – Damage to car in collision is proportional to v2 – Trauma to head from falling anvil is proportional to v2, or to mgh (how high it started from) – Hurricane with 120 m.p.h. packs four times the punch of gale with 60 m.p.h. winds Spring 2008 13

Physics 10 UCSD Energy Conversion/Conservation Example 10 m 8m P.E. 98 J K.E. 0 J P.E. 73.5 J K.E. 24.5 J 6m P.E. 49 J K.E. 49 J 4m 2m 0m P.E. 24.5 J K.E. 73.5 J Drop 1 kg ball from 10 m – starts out with mgh (1 kg) (9.8 m/s2) (10 m) 98 J of gravitational potential energy – halfway down (5 m from floor), has given up half its potential energy (49 J) to kinetic energy ½mv2 49 J v2 98 m2/s2 v 10 m/s – at floor (0 m), all potential energy is given up to kinetic energy ½mv2 98 J v2 196 m2/s2 v 14 m/s P.E. 0 J K.E. 98 J Spring 2008 14

Physics 10 UCSD Loop-the-Loop In the loop-the-loop (like in a roller coaster), the velocity at the top of the loop must be enough to keep the train on the track: v2/r g Works out that train must start ½r higher than top of loop to stay on track, ignoring frictional losses ½r r Spring 2008 15

Physics 10 UCSD Heat: Energy Lost? Heat is a form of energy – really just randomized kinetic energy on micro scale – lattice vibrations in solids, faster motions in liquids/gases Heat is a viable (and common) path for energy flow – Product of friction, many chemical, electrical processes Hard to make heat energy do anything for you – Kinetic energy of hammer can drive nail – Potential energy in compressed spring can produce motion – Heat is too disordered to extract useful work, generally notable exceptions: steam turbine found in most power plants Solar core : heat is important in enabling thermo-nuclear fusion Spring 2008 16

Physics 10 UCSD Power Power is simply energy exchanged per unit time, or how fast you get work done (Watts Joules/sec) One horsepower 745 W Perform 100 J of work in 1 s, and call it 100 W Run upstairs, raising your 70 kg (700 N) mass 3 m (2,100 J) in 3 seconds 700 W output! Shuttle puts out a few GW (gigawatts, or 109 W) of power! Spring 2008 17

Physics 10 UCSD More Power Examples Hydroelectric plant – Drops water 20 m, with flow rate of 2,000 m3/s – 1 m3 of water is 1,000 kg, or 9,800 N of weight (force) – Every second, drop 19,600,000 N down 20 m, giving 392,000,000 J/s 400 MW of power Car on freeway: 30 m/s, A 3 m2 Fdrag 1800 N – In each second, car goes 30 m W 1800 30 54 kJ – So power work per second is 54 kW (72 horsepower) Bicycling up 10% ( 6º) slope at 5 m/s (11 m.p.h.) – raise your 80 kg self bike 0.5 m every second – mgh 80 9.8 0.5 400 J 400 W expended Spring 2008 18

Physics 10 UCSD Momentum Often misused word, though most have the right idea Momentum, denoted p, is mass times velocity p m·v Momentum is a conserved quantity (and a vector) – Often relevant in collisions (watch out for linebackers!) News headline: Wad of Clay Hits Unsuspecting Sled – 1 kg clay ball strikes 5 kg sled at 12 m/s and sticks – Momentum before collision: (1 kg)(12 m/s) (5 kg)(0 m/s) – Momentum after 12 kg·m/s (6 kg)·(2 m/s) Spring 2008 19

Physics 10 UCSD Collisions Two types of collisions – Elastic: Energy not dissipated out of kinetic energy Bouncy – Inelastic: Some energy dissipated to other forms Sticky Perfect elasticity unattainable (perpetual motion) Spring 2008 20

Physics 10 UCSD Elastic Collision: Billiard Balls Whack stationary ball with identical ball moving at velocity vcue 8 To conserve both energy and momentum, cue ball stops dead, and 8-ball takes off with vcue 8 Momentum conservation: mvcue mvcue, after mv8-ball Energy conservation: ½mv2cue ½mv2cue, after ½mv28-ball The only way v0 v1 v2 and v20 v21 v22 is if either v1 or v2 is 0. Since cue ball can’t move through 8-ball, cue ball gets stopped. Spring 2008 21

Physics 10 UCSD Desk Toy Physics The same principle applies to the suspended-ball desk toy, which eerily “knows” how many balls you let go Only way to simultaneously satisfy energy and momentum conservation Relies on balls to all have same mass Spring 2008 22

Physics 10 UCSD Inelastic Collision Energy not conserved (absorbed into other paths) Non-bouncy: hacky sack, velcro ball, ball of clay Momentum before m1vinitial Momentum after (m1 m2)vfinal m1vinitial (because conserved) Energy before ½m1v2initial Energy after ½ (m1 m2)v2final heat energy Spring 2008 23

Physics 10 UCSD Questions Twin trouble-makers rig a pair of swings to hang from the same hooks, facing each other. They get friends to pull them back (the same distance from the bottom of the swing) and let them go. When they collide in the center, which way do they swing (as a heap), if any? What if Fred was pulled higher than George before release? A 100 kg ogre clobbers a dainty 50 kg figure skater while trying to learn to ice-skate. If the ogre is moving at 6 m/s before the collision, at what speed will the tangled pile be sliding afterwards? Spring 2008 24

Physics 10 UCSD Real-World Collisions Is a superball elastic or inelastic? – It bounces, so it’s not completely inelastic – It doesn’t return to original height after bounce, so some energy must be lost Superball often bounces 80% original height – Golf ball 65% – Tennis ball 55% – Baseball 30% Depends also on surface, which can absorb some of the ball’s energy – down comforter/mattress or thick mud would absorb Spring 2008 25

Physics 10 UCSD Superball Physics During bounce, if force on/from floor is purely vertical, expect constant horizontal velocity – constant velocity in absence of forces – like in picture to upper right BUT, superballs often behave contrary to intuition – back-and-forth motion – boomerang effect Spring 2008 26

Physics 10 UCSD Angular Momentum Another conserved quantity is angular momentum, relating to rotational inertia: Spinning wheel wants to keep on spinning, stationary wheel wants to keep still (unless acted upon by an external rotational force, or torque) Newton’s laws for linear (straight-line) motion have direct analogs in rotational motion Spring 2008 27

Physics 10 UCSD Angular Momentum Angular momentum is proportional to rotation speed ( ) times rotational inertia (I) Rotational inertia characterized by (mass) (radius)2 distribution in object Spring 2008 28

Physics 10 UCSD Angular Momentum Conservation Speed up rotation by tucking in Slow down rotation by stretching out Seen in diving all the time Figure skaters demonstrate impressively Effect amplified by moving large masses to vastly different radii Spring 2008 29

Physics 10 UCSD Do cats violate physical law? Cats can quickly flip themselves to land on their feet If not rotating before, where do they get their angular momentum? There are ways to accomplish this, by a combination of contortion and varying rotational inertia Spring 2008 30

Physics 10 UCSD For more on falling cats: Websites: – www.pbs.org/wnet/nature/cats/html/body falling.html play quicktime movie – www.exploratorium.edu/skateboarding/trick midair ac tivity.html Spring 2008 31

Physics 10 UCSD Announcements/Assignments Midterm review next week (Thu. evening?) Exam Study Guide online by this weekend Should have read Hewitt 2, 3, 4, 5, 6, 7 assignments by now Read Hewitt chap. 8: pp. 125–128, 138–140, 143–146 HW #3 due 4/25: – Hewitt 2.E.22, 2.E.29, 2.E.33, 3.E.27, 3.P.3, 3.P.4, 3.P.10, 4.E.1, 4.E.6, 4.E.10, 4.E.30, 4.E.44, 4.P.1, 5.E.17, 5.P.2, 7.R.(4&7) (count as one), 7.R.16, 7.E.40, 7.P.2, 7.P.4 Next Question/Observation (#2) due Friday 4/25 Spring 2008 32