9/17/20101I’ll be in the help room:• Friday: 1:30-2:30Today-2) conservation of energy- pushing file cabinets, andblocks of stone across ground and up ramps.3) Human energy. Work whenexercising, calories burned.4) motional energy (kinetic energy).Force is the same if book stationary or moving upwith constant velocity (= weight =mg)What about the amount of work needed?Get tired more quickly lifting book (or weight)up and down or just holding it up?a. same, b. more tired if lifting. work = force x distance, only do work on weights if moving through a distance by lifting.To transfer energy to other objects requires workConfusing Terminology: “WORK”In “Everyday life language” : we are doing work if it takes our energy to do it! In “Physics”: work = force applied times distance through “work” transfer energy to the object it is acting on or the surrounding world.To lift cart straight up at constant speed, force up needs to balance force of gravity down. So force up is mg. So work is mg x (change in height “h”). So amount of work to move something up without friction is always mgh!! Steeper ramp, bigger force, shorter distance, same work.Longer ramp, smaller force, longer distance, same work.I can only push with a force of 10 N, but I want to movea 3 kg weight up a height of 0.5 m. How steep a rampshould I make (keeping it as short as possible)?a. can’t do it, takes 29.4 N to raise a 3 kg mass.b. a 50% grade (2m forward for 1 m up)c. 3 m forward and 1 m up?d. 5 m forward and 1 m up?e. 5 m forward and .5 m up?9/17/20102I can only push with a force of 10 N, but I want to movea 3 kg weight up a height of 0.5 m. How steep a rampshould I make (keeping it as short as possible)?a. can’t do it, takes 29.4 N to raise a 3 kg mass.b. a 50% grade (2m forward for 1 m up)c. 3 m forward and 1 m up?d. 5 m forward and 1 m up?e. 5 m forward and .5 m up?ans. cWhat could be a good first step to solve this problem?a. Ask the professorb. Remember that the work to push something up a ramp to 0.5 m is the same as to lift it straight up 0.5 m.c. Draw a picture with the forces labeled.d. Remember that W= Fxd so multiply 10N x 0.5m = 5Je. All of the aboveI can only push with a force of 10 N, but I want to movea 3 kg weight up a height of 0.5 m. How steep a rampshould I make (keeping it as short as possible)? How to start?a. Ask the professorb. Remember that the work to push something up a ramp to height h is the same as to lift it straight up height h.c. Draw a picture with the forces labeled.d. Remember that W= Fxd so multiply 10N x 0.5m = 50Ne. All of the above•Wstraight up= F x d = mgh = 3 kg x 9.8 x 0.5 •Wup ramp=10 N x d and Wstraight up= Wup ramp •so d = 30 x .5/10 = 1.5 m. •1.5 m long for 0.5 m up is same as 3 m for 1 up.h = 0.5mFhand = 10NFgravity = Weight = 30NPush weight (file cabinet, stone block) across board.Weight of “file cabinet” = 19.5 NMeasure force needed. F= weight x ? measure Ffriction= 5.8 N= 0.3 x weightslide it 0.5 m. (work) Force x distance done =? a. 5.8 J, b. 19.5 J, C. 2.9 J., d. 11.6 Janswer: C. = 5.8 N X 0.5 m = 2.9 J0. 5 mA note about friction• Friction force varies with lots of things … roughness of material, little things sticking up on surface, etc. • But we can find an average friction force for a particular type of surface. Many fairly smooth surfaces near friction force of 0.3 x weight..25 mNow push it (weight = 19.5 N) up a hill so it goes up 0.25 m in moving 0.5 m. How much work will it take to push it up?a. 2.9 J. b. 4.9 J, c. 9.8 J, d. 19.5 J, e. 7.8 Jans. is e: 7.8 J. Total work = work against friction nearly the same (= 2.9 J)+ work to lift it up against gravity (= mg*change in height = 19.5 N x 0.25 m = 4.9 J= 7.8 JSo now, how muchforce is required topush it up the board?a. 9.75 N, b. 15.6 N, c. 19.5 N, d. 5.8 N, e. impossible to tellans. is b: 15.6 N. work=force x distance = 7.8 J from above = F x 0.5 m.so F= 7.8 J/0.5 m = 15.6 N. .25 mPush it up same hill so it goes up 0.25 m in moving 0.5 m.Friction force was 0.3 weight. Now add some gritty sandso friction larger, 0.5 weight. How much is the energy that turns into heat though friction compared to energy in grav.potential energy? (heat J)/(grav. energy).a. They are the same, b. Heat energy greater than grav. pot. energyc. Grav pot energy greater than heat energy, d. can’t tell whichWgrav. = mgh = 19.5 N x 0.25 m = 4.9 JWfrict.= 0.5 mg x dist = 0.5 x 19.5 N x 0.5 m = 4.9 JThey are the same! Note on homework- bunch of calculations.“Expert” approach- sees how concepts apply. Actuallysame problems over again. Different form but applying sameconcepts- easy. “Novice” new form= new problem, startfrom scratch each time-hard.What about without the gritty sand?Wfrict.less than Wgrav.9/17/20103Human energy. 1. What the heck is a Joule??= force(N) x distance(m) = 0.75 foot pounds (lift 1 pound up 1ft)lift 20 pound barbell up 7 feet = 140 foot pounds = 187 Joules.I jump 0.5 m. How many Joules of energy?power= energy/second. 1 watt = 1J/s. 100 watt lightbulbmeans 100 Joules electrical energy/sec.Human eats 2500 calories per day (and burns it up, not to fat)means 2500/24x 60 x 60 s = 0.29 cal/sec =0.29 cal/s x 4184 cal/J = 121watts. 100 watts of heat just sitting here! 1 food calorie = 4184 J. How much of food goes into work?How much force x distance can person do?easy to measure with stationary bicycle. elite women bicyclist can produce 300 W power,typical tour de France rider 400 W of power, [~1/2 horsepower]and Lance Armstrong can hit 500 W of power. Athletic man sustain ~150 watts (nonheat!) for 10 hour day= 150 J/s x 10 x 60 x 60 s = 5,400,000 J of work/day.How many calories is this?a. 2140 cal, b. 1290 cal, c. 150 cal d. 12000 cal b. 5.4E6 J/4184J/cal =1291 cal.eats ~ 5800 cal/day1 food calorie = 4184 J. where does the rest of the energy (5800-1291=4500 cal) go?a. chemical, b. sweating, c. light, d. heat, e. odord. heat.(80%) 755 cal in 1 hour on stair stepper = 3.6 million J!=2,237,000 ft pounds. 1 step = 1 ft, F x dist = 170 ft lbs, x 3600 stepsper hour= 1ft x 170 lb x 3600 = 612,000 foot-pounds of F x dist. 2,237,000 ft-lb chem. energy from