Physical SciencePhysical ScienceChapter 4Chapter 4

Work & MachinesWork & Machines

Section 4-1: What is Section 4-1: What is Work?Work? WorkWork is is forceforce exerted on an object that exerted on an object that

causes the object to move some causes the object to move some distancedistance Force without moving a distance yields NO Force without moving a distance yields NO

WORK!!WORK!!

Work = Force x DistanceSI Unit for work is the Joule1 Joule = 1Newton x 1 Meter

Word ProblemsWord Problems Word problems can be confusing; but w/ some practice Word problems can be confusing; but w/ some practice

they’re not that bad. Here are a few hints to make them they’re not that bad. Here are a few hints to make them easiereasier 1. Be sure you remember the “Need-to-Know” formulas1. Be sure you remember the “Need-to-Know” formulas

S =d/t ; S =d/t ; A = VA = Vff – V – Vii ; F = MA ; W=FxD; Power = Work/Time ; F = MA ; W=FxD; Power = Work/Time

Time Time In the word problem be sure you know the units for each of the In the word problem be sure you know the units for each of the

variables in the particular formula being discussed.variables in the particular formula being discussed. Distance – Meter; Force – Newton; Volume - cmDistance – Meter; Force – Newton; Volume - cm33 or Liter or Liter

2. In the word problem, all but one of the variables is told to you in 2. In the word problem, all but one of the variables is told to you in one way or another. Identify what variable is being asked to solve, one way or another. Identify what variable is being asked to solve, then plug in the remaining variables to the formulathen plug in the remaining variables to the formula

Solve it!! Make sure you also keep track of the unitsSolve it!! Make sure you also keep track of the units

How much work How much work performed:performed: How much work is performed if you apply How much work is performed if you apply 85 newtons 85 newtons of force on a box of force on a box

causing it to move causing it to move 3 meters3 meters::

W = F x DW = F x D

W = 85N x 3m = 255 NmW = 85N x 3m = 255 Nm

255 J = 255 Nm255 J = 255 Nm How much work is performed if you apply How much work is performed if you apply 37 newtons 37 newtons of force and move a of force and move a

wagon wagon 4.3 meters4.3 meters??

W = F x DW = F x D

W = 37N x 4.3m = 159.1 NmW = 37N x 4.3m = 159.1 Nm

159.1 J = 159.1 Nm159.1 J = 159.1 Nm How much work is performed if you apply How much work is performed if you apply 118 newtons 118 newtons of force on a car that of force on a car that

is stuck in the mud and is stuck in the mud and doesn’t movedoesn’t move?:?:

W = F x DW = F x D

W = 118N x 0m = 0 NmW = 118N x 0m = 0 Nm

0J =0Nm 0J =0Nm You might be tired from pushing but no work was done!!You might be tired from pushing but no work was done!!

How much force How much force required:required: How much force was required to move How much force was required to move

an object 3 meters if 75 Joules of work an object 3 meters if 75 Joules of work were expended?were expended? Formula: Formula: Work = Force x DistanceWork = Force x Distance Need to solve for Force, w= 75 J & Need to solve for Force, w= 75 J &

D=3MD=3M

75 J = F x 3M75 J = F x 3M

75 NM / 3M = F75 NM / 3M = F

75 NM / 3M = F75 NM / 3M = F

25N = F25N = F

What is a What is a MachineMachine?? A device that makes A device that makes work easier work easier or or more more

effectiveeffective A machine makes work easier by changing the A machine makes work easier by changing the

amount of forceamount of force, the , the distance covered distance covered or by or by changing the changing the direction of the forcedirection of the force

Section 4-2 Mechanical Section 4-2 Mechanical AdvantageAdvantage A machine’s A machine’s mechanical advantage mechanical advantage is the number of is the number of

times a force exerted on a machine is times a force exerted on a machine is multiplied.multiplied. Ideal Mechanical Advantage has no units ( they Ideal Mechanical Advantage has no units ( they

cancel each other out when doing the math cancel each other out when doing the math problemproblem

IMA = output force / input forceIMA = output force / input force

Section 4-2 Section 4-2 Efficiency of a Efficiency of a MachineMachine The amount of work obtained from a machine is The amount of work obtained from a machine is

always less than the amount of work put into it. always less than the amount of work put into it. This is because work is lost to friction.This is because work is lost to friction.

Efficiency = output work / input work x 100Efficiency = output work / input work x 100

Remember that work = force x distance

Section 4-3 Simple MachinesSection 4-3 Simple Machines

Inclined PlaneInclined Plane A plane is a flat surface. When that plane is inclined, or slanted, it can A plane is a flat surface. When that plane is inclined, or slanted, it can

help you move objects across distances. And, that's work! A common help you move objects across distances. And, that's work! A common inclined plane is a ramp. Lifting a heavy box onto a loading dock is much inclined plane is a ramp. Lifting a heavy box onto a loading dock is much easier if you slide the box up a ramp--a simple machine.easier if you slide the box up a ramp--a simple machine.

IMA = length of incline / height of incline

WedgeWedge you can use the edge of an inclined plane to push things apart. Then, the you can use the edge of an inclined plane to push things apart. Then, the

inclined plane is a wedge. So, a wedge is actually a kind of inclined plane. inclined plane is a wedge. So, a wedge is actually a kind of inclined plane. An axe blade is a wedge. Think of the edge of the blade. It's the edge of a An axe blade is a wedge. Think of the edge of the blade. It's the edge of a smooth slanted surface. smooth slanted surface.

ScrewScrew an inclined plane wrapped

around a cylinder A screw can convert a rotational

force (torque) to a linear force and vice versa.

LeverLever Any tool that pries something loose is a Any tool that pries something loose is a

lever. A lever is a rigid bar that "pivots" (or lever. A lever is a rigid bar that "pivots" (or turns) against a "fulcrum" (or a fixed point). turns) against a "fulcrum" (or a fixed point).

IMA = Distance from input force to fulcrum / distance from output force to fulcrum

11stst Class Levers Class Levers Notice howNotice how

The input & output forces are in opposite directions The input & output forces are in opposite directions The fulcrum is between the input & output forcesThe fulcrum is between the input & output forces

Examples include nail remover, paint can opener scissors, seesawExamples include nail remover, paint can opener scissors, seesaw

22ndnd Class Levers Class Levers Notice how:Notice how:

The input & output forces The input & output forces are in the same directionare in the same direction

Input force is farther Input force is farther away from the fulcrum away from the fulcrum than the output forcethan the output force

Examples include: wheel Examples include: wheel barrow, door, nutcrackerbarrow, door, nutcracker

33rdrd Class Lever Class Lever Notice how:Notice how:

The input & output The input & output forces are in the same forces are in the same directiondirection

The input force is closer The input force is closer to the fulcrum than the to the fulcrum than the output forceoutput force

Examples include rake, Examples include rake, shovel, baseball bat shovel, baseball bat and fishing poleand fishing pole

What Class of Lever?What Class of Lever?

1

76

45

3

2

8

1. _______ 2. _______ 3. _______ 4. _______

5. _______ 6. _______ 7. _______ 8. _______

1. 3rd Class 2. 1st Class 3. 1st Class 4. 2nd Class

5. 2nd Class 6. 3rd Class 7. 1st Class 8. 2nd Class

Wheel and AxleWheel and Axle two circular objects attached two circular objects attached

together about a common axistogether about a common axis Wheel is the large cylinderWheel is the large cylinder Axle is the small cylinderAxle is the small cylinder

IMA = Radius of the wheel / Radius of the axle

PulleyPulley In a pulley, a cord wraps around a wheel. As the wheel In a pulley, a cord wraps around a wheel. As the wheel

rotates, the cord moves in either direction. Now, attach rotates, the cord moves in either direction. Now, attach a hook to the cord, and you can use the wheel's a hook to the cord, and you can use the wheel's rotation to raise and lower objects.rotation to raise and lower objects.

IMA of a pulley system = the number of ropes that IMA of a pulley system = the number of ropes that support the weight of the objectsupport the weight of the object