2 Work and PowerWork – occurs when a force causes an object to move in the direction of the force.How do you know if work is being done?The object moves as a force is applied.The direction of the object’s motion is the same as the direction of the force being applied.
3 Example:You exert an upward force on the suitcase. But the motion of the suitcase is forward. Therefore you are not doing “work” on the suitcase.
4 Question:If you pulled a wheeled suitcase instead of carrying it, would you be doing work on the suitcase? Why or why not?
5 Answer:Pulling a wheeled suitcase is doing work because the force applied and the motion of the suitcase are in the same direction.
6 Calculating WORK Work = Force x Distance Remember: Force in measured in NewtonsThe SI unit for distance are metersThe units for work are newton-meters or joules (J)
7 Work depends on force and distance The force needed to lift an object is equal to the gravitational force on the object – in other words, the objects weight.
8 Cont’dIncreasing the amount of force increases the amount of work done.
9 Cont’dIncreasing the distance also increases the amount of work done.
10 POWER Def: the rate at which work is done. Power= Work time Remember: Is increased as more work is done in a given amount of time.Power= WorktimeRemember:Unit for work =JouleUnit for time= secondJ/s = Watts (W)
11 What is a machine?Def: a device that helps make work easier by changing the size or direction of a force.
12 Input and Output Work Input – the work that you do to a machine. Work Output – the work done by the machine.OUTPUT FORCEINPUT FORCE
13 Do machines save work?NO! Machines make work easier but do not get rid of the work to be done.
14 Mechanical AdvantageDef: tells you how many times the machine multiplies force.Compares the input and output forcesMechanical Advantage (MA)= output forceinput force
15 Example: MA = 500 N 50 N MA = 10 OUTPUT FORCE = 500 N INPUT FORCE
16 Cont’dBecause the MA of the hand truck is 10, that means that the output force is 10 times bigger than the input force.The larger the MA , the easier a machine makes your work.BUT as your MA increases, the distance that the output force moves the object decreases.
17 Mechanical Efficiency Def: a comparison of a machine’s work output with the work input.Mechanical Efficiency= work outputwork input*the 100 means that the answer is a percentage.X 100
18 REVIEWWork is done on a ball when a pitcher throws it. Is the pitcher still doing work on the ball as it flies through the air? Explain.No, the pitcher is no longer doing work on the ball as it flies through the air because he is no longer exerting a force on it.However, work is being done on the ball by the Earth, which exerts a force on the ball and pulls it back toward the ground.
19 Review: 2. Explain the difference between work and power? Work occurs when a force causes an object to move in the direction of the force.Power is the rate at which work is done.*the more work you do the more power you get.
20 Review:3. You lift a chair that weighs 50 N to a height of 0.5 m and carry it 10 m across the room.*How much work do you do on the chair and when does the work occur?Work is done on the chair only when it is picked up, not when it is carried across the room.W=50 N x 0.5 mW= 25 J
21 Review 4. Explain how using a ramp makes work easier Using a ramp makes work easier because it allows you to apply a smaller input force than you would have to apply when lifting a load straight up.However, the smaller force has to be exerted over a longer distance.
22 Review: 5. Why can’t a machine be 100% efficient? A machine can’t be 100% efficient because some of the work input is used to overcome friction.Therefore, work input is always greater than work output.
23 Review:6. Suppose you exert 15 N on a machine, and the machine exerts 300 N on another object. What is the machine’s mechanical advantage? MA = 300 N 15 N MA = 20
24 The Types of Simple Machines There are 6 types of simple machines:LeversInclined PlanesWedgesScrewsWheel and AxlePulleys
25 LEVERSDef: a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum.Levers are used to apply force to a load.
26 First Class LeversThe fulcrum is between the input force and the load.Ex: hammer pulling a nail out, seesaw
27 Second Class LeversThe load is between the fulcrum and the input force.Ex: wheelbarrow, bottle cap opener
28 Third Class LeversThe input force is between the fulcrum and the load.Ex: using dumb bell weights, Using a hammer
29 Mechanical Advantage of Levers MA = length of input arm / length of output arm
30 Inclined PlanesDef: a simple machine that is a straight, slanted surface.Ex: a ramp, a door stop
31 Mechanical Advantage of Inclined Planes MA = length of slope / height of slope
32 Wedges Def: a double inclined plane that moves. Ex: knives, an axe blade
33 ScrewsDef: an inclined plane that is wrapped in a spiral.Ex. screw
34 Wheel & AxleDef: a simple machine consisting of two circular objects of different sizes.Ex: Ferris wheels, the steering wheel on a car
35 Mechanical Advantage of Wheel and Axle MA = radius of wheel / radius of axle
36 PulleysDef: a simple machine consisting of a grooved wheel that holds a cable or rope.Two types:MovableFixed*Many combined pulleys are called a block and tackle