Ch 14

Work, Power, and Machines

• Work – transfer of energy

• through motion• a. Force must be

exerted through a distance

Ch 14 WORK AND POWER

• b. Amount of Work depends on:

–1. Amount of applied Force

–2. Distance over which the force is applied

Work

F d

• c. W=Fd

(Force multiplied by distance)

• d. Unit is the Joule (1J = 1Nm)

Work

F d

• e. In order for work to be done:

–1. Something has to move

–2. Motion must be in the direction of the force

• How much work is done when a car is pushed with 600 N of force for a distance of 50m?

• How far will a ball roll if 17J of work was done to push it with 2 N of force?

Power – rate of doing work

a. To increase Power:

–1. increase work done

– 2. do work in less time

b. P=W/t • (Work divided by time)

c. SI unit is Watt

(1W=1J/s)

d. common unit is horsepower (1hp=746W)

Work

P t

• How much power is needed to do 40 Joules of work in 5 seconds?

• How long will it take to do generate 70 Watts of power if I do 4900 Joules of work?

• How much power will I need if I want to do the same amount of work in half the time?– Twice as much

Reference table check

Work

F d

Work

P t

Fd

P t

• You exert a vertical force of 72 N to lift a box a height of 1 meter in a time of 2 seconds. How much power is used to lift the box?

Star questions

• True or False

–Decreasing the amount of time it takes to do work, increases the power

–True

–Motion must occur in order for work to be done

–True

Machines

• Make work easier

• Device that changes a force

How do machines make work easier?• Increasing force

–Rising a car using a jack• Increasing distance

–Using oars to row a boat• Changing direction

–Pulling back on the

oars to row a boat

More details later-this is an overview :o)

Work Input (on) vs Work Output (by)

• Work done on a machine as the input force acts through the input distance

• Work done by a machine as the output force acts through the output distance

You cannot get more work out of a machine than you put into it!

Mechanical advantage

• # of times the machine increases an input force

• Actual (AMA) = output force( FR)

input force (FE)

• FR = resistance (output) force

• FE = effort (input) force

• You test a machine and find it exerts a force of 5N for each 1N of force you exert operating the machine. What is the Actual Mechanical Advantage of the machine?

• 5N/1N = 5

• If the output force is 100 N and the input force is 15 N, what is the AMA of the simple machine?

• Output/input=100N/15N=

• Ideal (IMA) = MA in absence of friction

• IMA = input distance( dE)

Output distance (dR)

• dE = effort (input) distance

• dR = resistance (output) distance

• bc friction is always present,

AMA is always less (<) than IMA

• If x = 3 cm and y = 15 cm, what is the ideal mechanical advantage of the pliers?

x y

• 15cm / 3cm = 5

• A student working in a grocery store after school pushes several carts together along a ramp. The ramp is 3 m long and rises 0.5 m. What is the mechanical advantage?

• Input/output=3m/0.5m=6

• What is the MA?

• 12 / 3=4

12 m

3 m

• What is the MA of the pulley?1

2

no machine can be 100% efficient, why not?

Efficiency

work outputEfficiency = ------------------------- X 100%

work input

Thanks Cambridge Physics Outlet for amazing graphics!

• You have just designed a machine that uses 1000J of work from a motor for 800J of useful work the machine supplies. What is the efficiency of your machine?

800J / 1000J X 100 = 80%

STAR Questions

• What three things make machines work easier

• Increasing force• Increasing distance• Changing direction

• What two things does the amount of work depend on? – Force & distance

• To increase Power I must do work in ________ time – less

Types of simple machines

1. Lever – rigid bar that is free to move around a fixed point

• Fulcrum=the fixed point the bar rotates around

• Input arm (effort) = distance between the input force and fulcrum

• Output arm (load) = distance between the output force and the fulcrum

• Position of fulcrum identifies class of lever

First class levers

• Fulcrum located between the input force (effort) and output force (load)

Second class levers• Output force

(effort) is located between the input force (load) and fulcrum

• MA is >1

• Increases force

The input distance your hands move to lift the wheelbarrow is larger than the output distance the wheelbarrow moves to lift its load

Third class levers• Input force is

located between the fulcrum and the output force

• MA <1

• Increases distance

The output distance over which the lever exerts its force is always larger than the input distance you move the lever through

2. Wheel and Axle

• Variation of a lever

• Consists of two disks (cylinders) each one with a different radius

3. Pulley

• Variation of a lever

• Consists of a rope that is different in size, direction, or both from that of the input force

3 types of pulleys

1. Fixed pulley = changes only the direction of the input force

2. Movable pulley = changes both direction and the size of the input force

3. Pulley System= made up of both fixed & movable pulleys

4. Inclined Plane

• Slanted surface along which a force moves an object to a different elevation

5. wedge• Variation of inclined plane

• A V-shaped object whose sides are two inclined planes sloped toward each other

* A thin wedge has a greater IMA than a thick wedge of the same length

6. screw• Variation of inclined plane

• Wrapped around a cylinder

*Screws w/ threads that are closer together have a greater IMA

Identify which class lever goes with each picture

STAR Questions

Complex (Compound) Machines

• Combination of 2or more simple machines to operate together

• device, or apparatus is a deliberately over-engineered machine that performs a very simple task in a very complex fashion, usually including a chain reaction.

• The expression is named after American cartoonist & inventor Rube Goldberg.

FYI: Rube Goldberg machine

STAR Questions

Wheel and axle

Pulley

Lever

Inclined Plane

Wedge

Screw