Work & Work & MachinesMachines

I. Scientific definition of Work: Work is done when a force applied to an object moves the object.

A.Work depends on two things: forceforce and distancedistance.

1)Force is any action that causes a change in motion.

2)Distance measure of the path an object takes from start to finish.

B.Calculating Work:

1) WORKWORK = force X distance

PracticePractice: How much work do you do when you push a box with 3 N of force across the floor for 12 m?

PracticePractice: Kevin pulls a wagon 3 m with a force of 40 N. How much work does he do?

C.C.MachinesMachines: : Make work easier. But they do not make you do less work.

1)Machines make work easier in three ways.a) a) A machine can multiply or

increase the force a person puts into it.

b) b) A machine can change the direction of the force a person puts into it.

c) c) A machine can change the speed of the force a person puts into it.

II.II.MachinesMachines::

A.A.SimpleSimple MachinesMachines are tools with one or two parts.

1)1) LeversLevers make work easier by increasing the force you exert.

a)A lever is a long bar with a support that allows the bar to pivot.

The pivot point is called the fulcrumfulcrum.

Fulcrum

Effort arm

Resistance arm

FORCE APPLIED

B.Mechanical Advantage:{Symbol: M.A} is the number that tells how much a machine increases the applied force.

1.1. MechanicalMechanical AdvantageAdvantage ofof aa LeverLever: : Divide the length of the effort arm by the length of the resistance arm.

a)a) PracticePractice: A crowbar 8 m long is used to pry up some boards. The fulcrum of the crowbar is 1 m from the boards. What is the M.A. of the crowbar?

8 m

1 m

b) Practice: A seesaw 10 m long is placed on a fulcrum that is 2 m from the resistance. What is the M.A. of this seesaw?

c) Practice: A pole 30 m long is used to pry up a stump. The fulcrum is 10 m from the resistance. What is the M.A. of the pole?

15 m Effort 15 m Effort Arm Arm 3 m Resistance 3 m Resistance

ArmArm

F

R

E

FirstFirst ClassClass LeversLevers: : Fulcrum is between the effort and resistance arm. It will multiply the force.

SecondSecond ClassClass LeversLevers: : The resistance is located between the effort arm and the fulcrum. These levers always multiply the force.

F

R E

ThirdThird ClassClass LeversLevers: : The effort arm is located between the resistance arm and the fulcrum. These levers always have an M.A. less then 1.

F

RE

C.C.PulleysPulleys: : is a simple machine consisting of a rope that passes over a grooved wheel.

1)1)Fixed pulley Fixed pulley changes the direction of the force you apply to it.

a)The fixed pulley has an M.A. of one

2)2)MovableMovable PulleyPulley::

a)The movable pulley has a M.A. of 2.

b)The movable pulley multiplies the force applied to it.

50 LBS.

3)3)BlockBlock && TackleTackle: : is a pulley system.

a)Has a M.A. of 4. It multiplies the force.

b)Changes the direction of applied.

LOAD 200 LBS

Effort 50 LBS.

800 N

200 N

D.D.WheelWheel andand AxleAxle::

Wheel

Axle

1. The Wheel is like the effort arm.

2. The axle is like the resistance arm.

3. The bigger the wheel the greater the mechanical advantage.

a)a)PracticePractice:: In this ice cream maker the radius is 20 cm and the axle radius is 5 cm. What is the M.A. of the wheel and axle?

E.E.InclineIncline PlanePlane: : A flat surface with one end higher then the other. The longer the incline plane the greater the M.A.

1)To find the M.A. of an incline plane you divided the length by the height.

A plank is 45 dm long and one A plank is 45 dm long and one end is 5 dm higher then the end is 5 dm higher then the other. What is the M.A. of this other. What is the M.A. of this plank?plank?

45 dm

5 dm

2)2)ScrewScrew = Spiral incline plane.

PitchPitch is the number of threads

ThreadThread ridge.

3)3)WedgeWedge is an incline plane with one or two sloping sides.

Pedals = wheel & axle

Handle = lever

Chain = Pulley

III.III.CompoundCompound MachinesMachines: : Is a combination of two or more simple machines.

A.A. CompoundCompound machinesmachines have a very high M.A.

IV.IV.EfficiencyEfficiency: : Is the amount of useful work obtained compared to the amount of work put in.

A.NO MACHINE IS 100% EFFICIENT.

B. B. To find the efficiency of a machine you divide the Output work by the Input work in and multiply that by 100%.

• Output workOutput work

Input WorkInput WorkX 100%X 100%

1)1)PracticePractice: A machine produces 6 N-m of useful work. It takes 60 N-m of work to run the machine. What is the machines efficiency?

• Useful workUseful work

Work put inWork put inX 100%X 100%

2)2)PracticPractice: A bicycle requires 48 N-m of work. You get 12 N-m of useful work out of the bicycle. What is the bicycles efficiency?