Chapter 6 Work and Machines - PowerPoint Presentation

Chapter 6 Work and Machines

A. Work Work transfer of energy through motionforce exerted through a distance W = Fd Distance must be in direction of force! W : work (J) F : force (N) d : distance (m) 1 J = 1 N·m

B. Work Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W = ?WORK: W = F·d W = (30 N)(1.5 m) W = 45 J F W d

B. Work A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - afterW = ? WORK: W = F·d F = m·a F =(40kg)(9.8m/s 2 )=392 N W = (392 N)(1.4 m) W = 549 J during lift No work after lift. “d” is not in the direction of the force. F W d

B. Power Power Amount of work done in a certain amount of time. (aka the “rate” work is done) SI Units are in watts (W) P = W/t

B. Power You do 900 J of work in pushing a sofa. If it took 5 seconds to move the sofa, what was your power? GIVEN: W = 900 J t = 5 sP = ? WORK: P = W/t P = 900 J/ 5s P = 180 J/s or watts (W)

B. Power Power and Energy Doing work is a way of transferring energy from one object to another Work = Energy transferred P = Etransferred/t

B. Power A color TV uses 120 W of power. How much energy does the TV use in 1 hour? GIVEN: P = 120 Jt = 1 hrE = ? WORK: P = E/t or E = Pt t = 1 hr(60 min/1 hr)(60 sec/1 min) t = 3600 s E = 120 J/s (3600s) E = 432,000 J

Bell Work A 600 kg great white shark is lurking below an observation cage. His movement is being studied from a series of motion sensors below the boat. The shark leisurely charges the cage, strikes it and makes the cage rattle. Later, the shark returns for another run at the cage. The shark strikes the cage with 12,800 N of force. The shark traveled for 10 m in making its run. The cage is designed to withstand 124,500 N-m (J) of “work” before it breaks. Should the man in the cage be worried? Explain.

Section 2

A. Machines Machine device that makes work easier

There are four ways that a machine helps us to do work. Transfers our effort force from one place to another. Ex: seesaw Multiply your effort force. Ex: crowbarMagnify speed and distance. Ex: baseball batChanging the direction of the force. Ex: pulley on the flagpole

Mechanical Advantage The number of times a machine multiplies your effort force. Example: If you push on the handle of a car jack with a force of 30 lbs and the jack lifts a 3000 lb car, what is the jack’s mechanical advantage? The jack multiplies your effort force by 100 times. Link

There are 2 types of mechanical advantage. IMA – Ideal mechanical advantage. This is the number of times a machine is designed to multiply your effort force.Ignores frictionAMA – Actual mechanical advantageThis is the number of times the machine actually multiplies your effort force.Includes the effects of friction IMA is always larger than AMA.

B. Force Input Force ( Fi)force applied to the machine“what you do”Output Force (Fo)force applied by the machine“what the machine does”

C. Work Work Input (W in )work done on a machineWork Output (Wout) work done by a machine W in = F in × din W out = Fout × d0ut

C. Work Conservation of Energy can never get more work out than you put intrade-off between force and distance Win = Wout Fin × d in = F out × dout

C. Work In an ideal machine (no friction)... But in the real world… some energy is transferred to heat due to friction Win = W out W in > W out

Work A hammer claw moves a distance of 1 cm to remove a nail. If an output force of 1,500 N is exerted by the claw and you move the handle 5 cm, what is your input force? GIVEN: F i = ?F o = 1,500 N D in = 5 cm D out = 1 cm WORK :W in = Wout Find in = Foutd outFin (0.05 m) = (1,500 N) (0.01 m) Fin (0.05 m) = 15 NmFin = 300 N

D. Mechanical Advantage Mechanical Advantage (MA) number of times a machine multiplies the effort force MA > 1 : force is increased MA < 1 : distance is increased MA = 1 : only direction is changed

D. Mechanical Advantage A worker applies an effort (input)force of 20 N to open a window with a resistance (output) force of 500 N. What is the crowbar’s MA? GIVEN:F i = 20 N F o = 500 N MA = ? WORK : MA = Fo ÷ FiMA = (500 N) ÷ (20 N) MA = 25 MA F o F i

D. Mechanical Advantage Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: F i = ?F o = 2000 N MA = 10 WORK : F i = Fo ÷ MA Fi = (2000 N) ÷ (10) Fi = 200 N MA F o F i

Efficiency Efficiency – measure of how much of the work put into a machine is changed into useful output work by the machine High efficiency = less thermal energy from friction and more energy converted to useful output work. (ideally Wout = Win 100% efficient)How would you increase a machine’s efficiency? Grease, lubricant

There are 3 Classes of Levers Depends on the location of 3 items: 1. Fulcrum – fixed point on a lever 2. Effort Arm – the part of the lever that exerts the effort force. 3. Resistance Arm – the part of the lever that exerts the resistance force.  EA  RA

1 st Class Lever Changes the direction of the forceMultiplies effort force Magnifies speed and distanceEx: seesaw, crowbar, scissors

2 nd Class Lever Multiply effort force Mechanical advantage is always greater than 1.Ex: bottle opener, boat oars, wheel barrow

3 rd Class Lever Magnifies speed and distanceMechanical Advantage always less than 1 Ex: baseball bat, golf club, broom, shovel

1. What is the IMA of this pulley system? 2. Ignoring friction, if you want to lift the resistance 3 meters what will the effort distance be? 3. Ignoring friction how much effort force will be necessary to lift a load of 15 newtons? 4. How much work is done?