W = Fd. KE = 1 2 mv2

Transcription

1 Ch 10 Energy, Work and Simple Machines work: moving an object in the direction of the force exerted upon it (Joules) work W = Fd force (Newtons) (meters) distance object is displaced in the direction of the force kinetic energy: energy of motion (Joules) KE = 1 2 mv2 mass (kg) (m/s) velocity work-energy theorem: work done on or by an object is equal to the change of KE of the object W = KE W = KE final KE initial ket. 21 3:40 PM "Issues" Susie pushes the 5.0 kg carcass of her dead puppy across the level grass yard under her treehouse with a force of 23.2 N. Old Yeller moves a distance of 9.5 meters before he falls into the grave she has previously dug for him. How much work, if any, was done on Old Yeller by Susie? ket. 21 4:20 PM 1

2 How far could Old Yeller have been displaced if Susie exerted the same 23.2 N of force, but only did 169 J of work? ket. 21 4:27 PM When a club head strikes a kg golf ball, the ball picks up 43 J of kinetic energy. A constant force of 2300 N is applied to the ball while the club head and the ball are in contact. Over what distance is the club head in contact with the ball? ket. 21 4:34 PM 2

3 How much kinetic energy is in a kg semi truck that is traveling at 15.9 m/s? What is the mass of a boulder that has J of KE, moving at 1.75 m/s? How fast is a kg object moving if it has J of kinetic energy?. ket. 21 4:59 PM friendly reminder: the weight of an object is a force in the vertical dimension So...when you are moving something vertically at a constant velocity, the force you exert is equal to the weight of the object EX: an elevator lifts a total mass of 1.1 x 10 3 kg a distance of 40.0m. How much work is done on the mass by the elevator? ket. 21 4:46 PM 3

4 Calculating Work W = Fd W = 0 when object moves in direction of force when object moves perpendicular to the force example: planet orbiting the sun velocity is tangential (motion is tangential) acceleration is center seeking and perpendicular so the force is also center seeking and perpendicular to the motion since force is perpendicular to motion, no work is done on the planet if work = 0, KE = 0. This means the planet's speed doesn't change. ket. 21 3:56 PM What if the force is exerted at an angle? break the force into its x and y components N = what direction will the box move?f which component is in that same dimension? the other component is to motion, so does NO work on the box so how much work is done on the box if it is moved 4.0 meters to the right? in this case, W = Fd cos ket. 21 4:04 PM 4

5 How much work is done in pushing a tall box 15.0 meters horizontally with a force of 4.0 x 10 2 N that is applied slightly upward at an angle of 10.0 degrees from the horizontal? ket. 21 4:23 PM What work is done if this crate of tigers is moved 10.0 m along the slope of the hill? 12.0 N 10m 37.5 what direction will the box move? which component is in that same dimension? the other component is to motion, so does NO work on the box ket. 21 5:05 PM 5

6 other forces acting on the box? F = 149 N 35 gravity normal force friction friction is in direction opposite of the motion so it does NEGATIVE work on the box positive work increases the KE of an object, negative work decreases the KE of an object ket. 21 4:12 PM How much work is done by the force of friction and by a person when the person pushes a box 10.0 m at a constant speed across a level floor? The mass of the box is 40.0 kg and the coefficient of sliding friction is ket. 21 4:18 PM 6

7 POWER Power the rate at which work is done work (Joules) P = W (Watts) t time (seconds) could substitute Fd in for work P = Fd t ket. 21 5:12 PM If it takes 2.5 seconds for you to perform 5.0 Joules of work, what is your power? A glass weighs 5.0 N. Calculate the powere if you lift it 0.5 meters above the table in 1.5 seconds. ket. 24 6:31 AM 7

8 10.2 Machines 1) Machines make tasks easier...how? 2) What does it mean if mechanical advantage (MA) is greater than 1? 3) Ideal machines have an efficiency of 1, or 100%. Why do you think real machines have efficiencies of less than 1, or less than 100%? ker. 3 12:12 PM Six simple machines: lever ( 3 classes) pulley wheel and axle inclined plane screw wedge ker. 3 12:12 PM 8

9 Ch 10.notebook 10.2 Machines Machines make work easier by: changing the magnitude or the direction of the effort force resistance force: force exerted by the machine force exerted by a person on the machine ex: bottle opener You lift the handle (you moved it a distance by exerting a force upon it) = input work output work = work done by the machine output work cannot exceed input work ker. 3 12:12 PM Mechanical advantage MA = resistance force effort force basically, it's how many times the machine multiplies the effort force remember: WORK is not multiplied or increased, but force can be picture a screwdriver opening a paint can work input work output d F F d work out = work in Fresistancedresistance = Feffortdeffort 3 N ( 0.01m) 1N (0.03m) but resistance force is greater than effort force because resistance distance is less than effort distance ker. 3 12:27 PM 9

10 Mechanical advantage ideal mechanical advantage MA = resistance force effort force IMA = effort distance resistance distance ker. 3 12:42 PM theoretically efficiency work in = work out but in reality, machines have moving parts so some work input is converted to thermal energy due to friction therefore = no machine can have 100% or greater efficiency ker. 3 12:47 PM 10

11 two equations to find efficiency: W out W in e = x 100 % MA e = x 100 % IMA ker. 3 12:49 PM Compound Machines when two or more simple machines are linked so that the resistance force of one becomes the effort force for another The mechanical advantage of a compound machine is found by multiplying all of the MA's of the simple machines that make it up ker. 3 12:54 PM 11

12 Bob raises a N piano a distance of 5.00 m using a set of pulleys. He pulls in 20.0 m of rope. a) How much effort force would Bob apply if this were an ideal machine? b) What force is used to balance the friction force if the actual effort is 340 N? c) What is the output work? d) What is the input work ( using actual effort force of 340 N )? e) What is the mechanical advantage? f) What is the efficiency of his pulley set? ker. 3 4:08 PM Because there is very little friction, the lever is an extremely efficient simple machine. Using a 90.0 percent efficient lever, what input work is required to lift an 18.0 kg mass through a distance of 0.50 m? ker. 3 4:14 PM 12

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