Chapter 5Work and

Machines

Section 1: Work• Work—the transfer of energy that occurs

when a force makes an object move.–For work to be done, an object must move– If you push against the desk and nothing

moves, you haven’t done any work.

• You must have 2 conditions in order for work to be done.–The object has to move – The motion must be in the direction of the

applied force.

Work and Energy• How are work and energy

related?–When work is done, a transfer of

energy always occurs.–You can think of energy as the

ability to do work.

• Example: when carrying a heavy box up a flight of stairs you transfer energy from your muscles to the box and increase its potential energy by increasing the height.

Calculating Work• The amount of work depends on the

amount of force exerted and the distance the force is applied.

• You do more work when you apply more force and move an object a greater distance.

• Work = force x distance• W = F x d–The unit for work is the joule (J), force is

the newton (N) and distance is the meter (m)

Power• Power—the amount of work done

in a certain amount of time.– It is the rate at which work is done.

• You can calculate the amount of power it takes to push a box up a ramp.

• The equation for power is–Power = work / time–P = W/t

• The unit for power is watts – (named after James Watt)

Power & Energy• Anytime energy is transferred from

one object to another, power can be calculated.–Power = energy / time–P = E/t

• You can change energy by doing work. • When you lift a book off your desk

you increase its potential energy. • You transferred energy to the book

by doing work on it.

Section 2: Using Machines

• Machine—a device that makes doing work easier.–Machines can be complicated:

something with many moving parts like an engine.–Machines can be simple: powered by

a person like knives, scissors, and doorknobs.

• Machines make work easier by increasing the force applied to an object or increasing the distance that a force is applied.

Increasing Force & Distance

• You can do the same amount of work by applying a small amount of force over a long distance as you can applying a large force over a short distance.

• Examples: – A car jack is a machine that multiplies your

force to lift a car. The jack applies a large amount of force over a short distance.

– A ramp is a machine that decreases the amount of force needed because it increases the distance.

Work Done by Machines• 2 forces are involved when a

machine is used to do work.–Effort Force (Fe)—the force

you apply to the machine–Resistance Force (Fr)—the

force applied by the machine.• 2 different forces mean 2

different types of work.– Input work (Win)—done by you

on the machine.–Output work (Wout)—done by

the machine.

Conservation of Energy• Because of the conservation of energy, the effort force

should equal the resistance force and Win = Wout.

• Usually when a machine is used, friction changes some of the energy into thermal energy.

• In a perfect machine, no energy would be converted to heat.

• Equations for an ideal machine:

– Win = Wout

– Fe x de = Fr x dr

• Fe = effort force, de = distance of effort force,

• Fr =resistance force, dr = distance of resistance force.

Efficiency• Efficiency—a measure of

how much of the work put into a machine is changed into useful output work by the machine.

• Calculating Efficiency:•An ideal machine would have 100% efficiency.•The efficiency of a real machine is always less than 100%.•The more efficient the machine, the better.

Simple Machines Project• Simple Machines Rubric• Neatness, Organization • and Creativity ____/4 pts• Accuracy ____/5 pts• Information from content • Is accurate• Content ___/41 pts• Title ______/1• All 6 Machines and 3 levers• A. Name____/8• B. Picture____/8• C. Definition____/8• D. 2 Examples ____/8• E. MA if given in book____/8• Total Points_________/50 pts

• ***Starts on pg 138. • ***Compound Machine= Extra Credit!!!!

Simple Machines Game

• http://www.edheads.org/activities/simple-machines/

• Complete and Record Scores for all 5 activities!!!– House: Bathroom, Kitchen, Garage,Bedroom– Tool Shed: Compound Machines

Rube Goldberg Simulation

• Google: Zoom Rube Goldberg• Click on Goldberger to go. Click Run Now.• Objective: Deliver the cheeseburger!!!!!• The arrow will turn into a wrench and you will

be able to adjust the different simple machines. • Type in the google search bar: – Zoom Rube goldberg.

• Have FUN!!!!!!