Chapter 5

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Chapter 5 - PowerPoint PPT Presentation

Chapter 5. Work and Machines. 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.

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Chapter 5

Work and Machines

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.

Section 1: Work
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 rateat 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