Unformatted text preview: UCSD Physics 10 Work, Energy, Power, Momentum
Measures of Effort & Motion; Conservation Laws UCSD Physics 10 Work, defined Work carries a specific meaning in physics Simple form: work = force distance W=Fd 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 UCSD Physics 10 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 =
Force Distance Distance UCSD Physics 10 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 UCSD Physics 10 Gravitational Potential Energy Gravitational Potential Energy near the surface of the Earth:
Work = Force Distance m W = mg h h m UCSD Physics 10 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 Nm 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 UCSD Physics 10 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 Nm, 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 UCSD Physics 10 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 UCSD Physics 10 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 UCSD Physics 10 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 UCSD Physics 10 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 UCSD Physics 10 Kinetic Energy The kinetic energy for a mass in motion is
K.E. = mv2 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 mv2 = mgh h = gt2 v = gt v2 = g2t2 mgh = mg (gt2) = mg2t2 = mv2 sure enough Ball has converted its available gravitational potential energy into kinetic energy: the energy of motion UCSD Physics 10 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 UCSD Physics 10 Energy Conversion/Conservation Example
10 m P.E. = 98 J K.E. = 0 J P.E. = 73.5 J K.E. = 24.5 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 mv2 = 49 J v2 = 98 m2/s2 v 10 m/s 8m 6m P.E. = 49 J K.E. = 49 J 4m P.E. = 24.5 J K.E. = 73.5 J P.E. = 0 J K.E. = 98 J at floor (0 m), all potential energy is given up to kinetic energy mv2 = 98 J v2 = 196 m2/s2 v = 14 m/s 2m 0m UCSD Physics 10 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 r higher than top of loop to stay on track, ignoring frictional losses
r r UCSD Physics 10 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 UCSD Physics 10 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! UCSD Physics 10 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% (~6) 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 UCSD Physics 10 Momentum Often misused word, though most have the right idea Momentum, denoted p, is mass times velocity
p = mv 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 kgm/s (6 kg)(2 m/s) UCSD Physics 10 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) UCSD Physics 10 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: mv2cue = mv2cue, after + mv28-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. UCSD Physics 10 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 UCSD Physics 10 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 = m1v2initial Energy after = (m1 + m2)v2final + heat energy UCSD Physics 10 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? UCSD Physics 10 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 UCSD Physics 10 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 UCSD Physics 10 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 UCSD Physics 10 Angular Momentum Angular momentum is proportional to rotation speed () times rotational inertia (I) Rotational inertia characterized by (mass) (radius)2 distribution in object UCSD Physics 10 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 UCSD Physics 10 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 UCSD Physics 10 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_activity.h UCSD Physics 10 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. 125128, 138140, 143146 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 ...
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