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Chapter 13
WORK & ENERGY
TN Standards
• CLE 3202.4.3 – Demonstrate the
relationship among work, power, and
machines
• CLE 3202.2.6 – Investigate the Law of
Conservation of Energy
Bellwork
• What is Work?
Work, Power, and Machines
• Work is conducted only when an applied (
net ) force results in the change in position of
an object
• Work is measured in Joules ( J )
• Work is calculated by multiplying force and
distance ( over which the force is applied )
–W=Fxd
Work, Power, and Machines
• Power is the amount of work conducted
over a certain time interval
• P = W / Δt
• Power is measured in Watts
Work, Power, and Machines
• Machines help to do work by changing the size
of an input force, the direction of a force, or
both
• Different forces can do the same amount of
work
– Figure 3 ( a box lifted vs a box pushed up a ramp )
Simple Machines
• The six types of simple machines are:
– Simple lever
– Wheel and axle
– Pulley
– Simple inclined plane
– Wedge
– Screw
Compound Machines
• A combination of any of those six simple
machines
• Example – SCISSORS
• Combination of lever and wedge
Bellwork
• What is Energy?
What is Energy?
• Energy is a property of an object due to its
motion or its position
• When work is done, energy is transferred or
transformed from one system to another
– Carry a tennis ball up in the stadium
– You add potential energy to it by carrying it up
What is Energy?
• Energy is measured also in Joules ( J )
• Potential energy – energy of position
– PE = hmg
– h is height ; g is gravity; m is mass
– Height is relative
• Kinetic Energy – energy of motion
– KE = ( ½ ) mv2
– m is mass ; v is velocity
Other Forms of Energy
• Mechanical energy
• Non-mechanical energy
– ( atomic level )
• Chemical reactions involves chemical energy
( a form of potential energy )
– Sun gets energy from nuclear reactions
• Electrical energy ( stored in electric fields )
Conservation of Energy
• Energy easily changes from one form to
another ( battery )
• PE can become KE ( tennis ball dropped
from the top of stadium )
– Vice-versa ( projectile shot / reaches max height )
• Law of Conservation of Energy?
– ( neither created nor destroyed )
Roller Coaster Example
Thermodynamics
• For any system, the net change in energy
equals the energy transferred as work and
heat
• Efficiency ( how much energy/work you get
out of a machine or process compared to how
much is put in )
– Internal Combustion Engine