Presentation on theme: "Work Work = Force x Distance The force must be in the direction"— Presentation transcript:
1Work Work = Force x Distance The force must be in the direction No WorkWork = Force x DistanceThe force must be in the directionof the motion, or no work is done.WorkWFD
2The unit of work is the Joule Force x Distance = Work1 Newton x 1 meter = 1 Newton meter1 Newton meter = 1 JouleJames Prescott Joule
3Example 1:A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high?Answer: W = F x dW = 700N x 2.0 m= 1400 nm= 1400 Joules
4No work is done if there is no distance! The statue of liberty has been holding up her torch for an awfully long time. How much work has she done?Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done.
5Which does more work? Frictionless The net work is the force (weight) of the cart X the vertical distance. This is the same in all three cases. In the first, the force is less, but distance is greater to reach the same vertical height.
6Example 2A force of 200N is required to push a lawn mower. If 4000 J of work is performed on the lawnmower, how far does it move?Answer4000Nm ÷ 200N = D= 20mWFD4000JD200N
7RememberFor work to happen the force has to be applied in the SAME direction as the motion.
8Power = Force x distance Work done per unit of timePower = WorkTimePower = Force x distancePT
9PowerAn object that has more power than another object does more work in the same time.You need more power torun up the stairs than to walkbecause it takes you less timeto do the same work.
10The SI unit of power is the WATT (W) Power = WorkTime1 watt = Joule1 Second
11A Watt is small! Usually measured in kilowatts(kW)= 1,000W Another unit of power:Horsepower (not an SI unit)1horsepower= 746 watts
12Example 3A crane lifts a car into a junk pile in 10 seconds. What is the crane’s power if 120,000 J of work are performed?Answer: Power = WorkTime120,000J =10 sec12,000 Watts
13Power = Force x distance Example 4A 750 N diver does a somersault off a 10m platform. It takes her 1.5 seconds to hit the water. What is her power?Since work is Force x distance, the power formula can be written asPower = Force x distanceTime
14Answer: Power = 750N x 10m Power = 7500N m Time 1.5 sec = 5000 Watts Power = Force x distanceTimePower = 750N x 10m1.5 secPower = N m= Watts
15Other units of power 1 kilowatt = 1000 watts 1 horsepower = 746 watts In the previous example, how many kilowatts power was generated? How much horsepower?Answer:5000 watts ÷ 1000 kw/w = 5 kilowatts5000 watts ÷ 746 hp/w = 6.7 horsepower
16What is a Machine?A device that allows you to do work in a way that is easier.It changes at least one of 3 factors:The amount of force you exertThe distance over which you exert your forceThe direction in which you exert your force.
17Input & Output Force Input force- The force you exert on the machine. The input force moves the machine a certain distance, called the input distance.Output force- The force the machine exerts on an object.The machine does work by exerting a force over another distance, called the output distance.
18Input & Output Work Input work= input force x input distance Output work= Output force x Output distanceThe amount of output work can never be greater than the amount of input work.
19Changing ForceIn some machines, the output force is greater than the input forceThis is possible by increasing the distance.What kind of machine allows you to exert a smaller input force?Ramp, faucet knob.
20Changing DistanceIn some machines, the output force is less than the input forceWhy would you use a machine like this?Hockey stick, chopsticks, riding a bike on a high gear.
21Changing DirectionSome machines don’t change either the force of the distance.What is the advantage of using a machine like this?It is easier to pull down then to push up.
23Mechanical AdvantageA machine's mechanical advantage is the number of times a machine increases a force.Formula:mechanical advantage (MA) = Output ForceInput ForceThere is no unit for MA.
24Mechanical Advantage(IMA) Ideal MA: This is the MA of a machine in a world with no friction, and no force is lost anywhere.(AMA) Actual MA: This is simply the MA of a machine in the world as we know it.- Force is lost due to friction.- It is transferred into thermal energy.Can we have an ideal machine?
25Increasing ForceWhen the output force is greater than the input force, the MA is greater than 1.Suppose you exerted a force of 10 N on a hand-held can opener and the can-opener exerts a force of 30 N on the can. What is the MA of the can opener?MA= Output forceInput ForceMA= 30N10NMA= 3
26Increasing DistanceFor a machine to increase distance, the output force is less than the input force. The MA is less than 1Suppose you applied a force of 20N and the machine applied a force of 10 N. What is the MA?MA= Output forceInput ForceMA= 10N20NMA= 0.5The MA is less but the machine would exert the force over a longer distance.
27Changing DirectionWhat can you predict about the MA of a machine that changes the direction of the force?If ONLY the direction is change the input force will be the same as the output force so the MA will always be 1
28EfficiencyIn real world situations, the output work is always less than the input work because some work is wasted overcoming friction.Efficiency compares the output work to the input work.Efficiency= Output work x100%Input workEfficiency is expressed as a percent
29Try it outYou do 20 J of work while using a hammer. The hammer does 18J of work on the nail. What is the efficiency of the hammer?Efficiency= Output work x100%Input workEfficiency= 18J x100%20JEfficiency= .9 x100% = 90%
30You do 250,000J of work to cut a lawn with a hand mower You do 250,000J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000J, what is the efficiency of the lawn mower?Efficiency= Output work x100%Input workEfficiency= 200,000J x100%250,000JEfficiency= .8 x100% = 80%
31Is there a machine with 100% efficiency? Unfortunately, there is no such machine. In all machines some work is transferred into thermal energy due to friction.Ideal vs. ActualAll machines have an efficiency of less than 100%