Write a list of 10 examples your idea of work. • What do all of these have in common? • What do they require? • How do you assess if work is done? • What is work? • What is the formula? • What are the SI units for work and what do they represent? DO NOWs
Work is done only when a force moves an object • A force acting on an object and causing it to move a distance is work • Not every force is work..if you push against the wall it does not move..that is not work! • Work = force X distance • Work is measured in Joules • What is a joule exactly? (derive it) • If you pick up a bag of groceries and walk across the room the work is picking up the groceries not the walking. What is work?
The object must move some distance as a result of your force • The force you exert must be in the same direction as the objects motion. • Ie: the groceries • You walk
What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?
What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • (Already in her arms) A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 kg grocery bag to her car? No
Work can be determined by calculating • Force used x distance moved = amount of work • Therefore what is the formula for work? HOW CAN WE CALCULATE WORk??
Work = force x distance Joule – is the SI unit for work. Newton = force Meters = distance Therefore if you exert: 1 Newton of force for 1 meter of distance = 1 joule of work or 1N/m Calculations
Work is done when a force is exerted through a distance. A student lifts a bag of books that weighs 135 N. If the bag is lifted .75 m, how much work does the student do? F = 135 N d = .75 m W = Fd W = (135 N)( .75 m) W = 101.25 J
COMPLETE PRACTICE PROBLEMS IN BOOK PAGE 285 (1-5) COMPLETE MORE PRACTICE PROBLEMS HANDOUT
Power tells you how fast something is happening..how fast the work is being done • Power = work/time or Power = Force X Distance Time • Power is measured in watts (W) • One watt is equal to 1 joule per second of work divide joules/seconds Power
Power – the rate at which energy is transferred. P = W P = power Watts t W = work Joules t = time seconds 1 Watt (W) = 1 J/s
How much power must a motor have to operate a pump that raises 1500 kg of water every minute a distance of 12 m?
1. WDIK- m = 1500 kg t = 60 s d = 12 m 2. Equations- P = W/t W = Fd F = mg 3. Plug and chug- F = mg = (1500 kg)(9.8 m/s2) = 14,700 N W = Fd = (14,700 N)(12 m) = 1.76 x 105 J P = W/t = (1.76 x 105 J)/(60 s) = 2940 W
Demo: Use a ramp and 4 books and a spring scale and measure distance to move the 200g mass up vertically and horizontallyon a ramp Create a data table use books as height w/ 200g hanging mass 1st Write a hypothesis –more –less- the same-work 2nd calculate the work for 1. vertically-straight up 2. up the ramp Mechanical advantage -
Mechanical Advantage – when you increase distance you decrease force but the work remains the same. • Machines – • Multiply force • redirect force- ie: pull down rope –lifts sail • work equation- • force x distance = work MECHANICAL ADVANTAGE
Machines do not increase the amount of work. They spread out the distance so you don’t have to use the same amount of force to receive the same amount of work. Prove it: Work 32 J = work 32 J Force x distance force x distance 8N x 4 m 4 N x 8 m
Ideal Mechanical advantage = ratio between output force and input force or output distance and input distance without friction If you have force information use: Output force /Input force = MA If you have distance information use: Input distance/output distance = MA
Mechanical advantage – multiplying force if you need 3200 N to lift a piano then use a ramp to exert 1600 N of force.
OF 3200N = 2 the ramp doubled your IF 1600N force. Your output force is 2x your input force. MA- is 2 no units
Mechanical Advantage – multiplying distance -you use a ramp that is 6 meters long to raise a piano 3 meters
ID- 6 meters = 2 the ramp doubled OD 3 meters the distance mechanical advantage of two
Write a paragraph on what you now know and did it differ from what you knew before,
Mechanical advantage to machines problem set /answers http://library.thinkquest.org/CR0210120/Mechanical%20Advantage.html
An instrument that makes work easier is called a machine • Machines do not have to be complex electrical or gas powered deviced. Even simple objects can be a machine. • A pair of pliers would make it easier to take out a bolt so the pliers would be a machine Machines
There are two types of work involved in using a machines: • Work that goes into the machine (input) • Work done by the machine (output) Work that comes out of the machine is NEVER greater than the force that is applied to the machine or work that goes into the machine Machines Cont.
Machines make work easier because they change either the size or the direction of the force put into the machine. • Machines multiply either the force or distance to make work easier, but never both! • The comparison of the work output to the work input is called efficiency. • The closer the amount of output is to the amount of input the more efficient the machine is. Machines Cont.
Efficiency is measured in percent and is never more than 100%. This is because the output can never be more than the input • The lower the friction of the machine the more efficient it will be. Keeping a car engine oiled makes it work better and more efficient Efficiency cont.
Efficiency – a measure of how much work that is put into a machine is changed to useful work; answer will be a percentage. efficiency = Wout x 100% Win Win = work put into the machine Wout = work put out by the machine
Wout = Frdr Fr = resistance/output force dr = resistance/output distance Win = Fede Fe = effort/input force de = effort/input distance For an ideal machine: Win = Wout Fede = Frdr
A worker applies an effort force of 20 N to pry open a window with a resistance force of 500 N. Find the mechanical advantage of the crowbar.
Fe = 20 N Fr = 500 N MA = ? MA = Fr = 500 N Fe 20 N MA = 25
Find the effort force needed to lift a 2000 N rock, using a jack with a mechanical advantage of 10.
Fr = 2000 N MA = 10 Fe = ? MA = Fr / Fe Fe = Fr/ MA Fe = (2000 N)/(10) Fe = 200 N
There are six types of simple machines: • Inclined plane • Wedge • Screw • Lever • Pulley • Wheel and axle Simple and Compound Machines
A ramp is an example of an inclined plane Simply put in inclined plane is a flat slanted surface A wedge is an inclined plane that moves and is usually made up of 2 inclined planes The screw is an inclined plane wrapped around a center bar Inclined Plane, Wedge, Screw
A lever is a rigid bar that pivots or moves around a fixed point. A seesaw is an example Fulcrum is the fixed point of a lever A pulley is a rope, belt or chain wrapped around a grooved wheel A pulley can change the direction of a force or the amount of a force When you use a pulley you change the direction of the force you are applying. Lever and Pulley
A wheel and axle is a simple machine made up of two circular objects of different sizes The wheel is the larger object the axle is the smaller one Bicycle is an example of a wheel and axle.. The bike wheel is the large while and the sprocket the chain wraps around is the axle Wheel and Axle
www.phs.d211.org/Science/okeefenm/Okeefe/Okeefe/PhySci233/EnergyMachines/Mechanical%20Advantage.ppt – www.cwcboe.org/gcms/teachers/apanagiotakis/Notes/Work%20&%20Power/Mechanical%20Advantage%20and%20Efficiency.ppt - Similar pages education.jlab.org/jsat/powerpoint/0708_simple_machines_8.ppt - Sources used