ll6 - work, kinetic, potential energy · 2019-09-19 · microsoft powerpoint - ll6 - work, kinetic,...
TRANSCRIPT
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PHYSICS
Unit 3 Kinetic and Potential EnergyConservation of Mechanical Energy
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PORTFOLIO 2 CONSERVATION OF MECHANICAL ENERGYDUE 9/26/19
If/Then Thesis Statement
Required for full credit for
this lab
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This info in included in your Intro
section
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This info in included in
your Materials and Methods
section
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THIS INFO IN INCLUDED IN YOUR DATA AND ANALYSISSECTION
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CONCLUSION SECTION
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FINAL CHECK LIST
DO NOT NEED TO COMPLETE HYPOTHETICAL
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AS USUAL – VIRTUAL LAB = IDEAL CONDITIONS
•Real spring will not oscillate forever
•Friction acts on the spring and will cause heat
•Mechanical energy is also not conserved
• Therefore kinetic energy that is lost is due to heat loss from friction
•This is why the potential and kinetic energy are not the same in real life
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WORK
• Work = force x displacement
• Work can be positive or negative
• Unit = Joules (J) or kilojoule (kJ)
• displacement needs to be in meters
• force needs to be in N (kg ◦ m/s2)
• Student holding chair has completed no work!
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• There is a trick – if no displacement then NO PHYSICAL WORK is done
• Also work is done only when component of force is parallel to displacement
• Even though it is hard “work” holding huge gear bag even when you move = no work
• only work is moving gear bag onto back and again when lowering to ground
• Climber has displacement up while gravity force downward = negative work
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•A good samaritan stops to help a person with their broken
car. A 400N force is applied to the move the 1325kg car
the 25.0 m to get off the road. How much work was
completed?
• W = F x d
•= 400N x 25.0m
•= 1.00 x 104 J
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•A child pushes his toy truck with a 68.0N force at a 42.0°
angle downward from the horizontal for .0025km
• When we have an angle
•W = F cos x d
•= .0025km = ?m
• x = 2.5m
•= (68 cos 42°) 2.5
•=126 J
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•A neighbor pushes a lawnmower 4 times as far as you do
but exerts only ½ of the force, who does more work and
by how much?
•Wneighbor = F x 4d
•Wyou = ½F x d
•Easiest way to figure it out is to choose a value and
calculate
•Wn = 2 x 4(5) = 40J Wy = ½2 x 5= 5J
•Neighbor will do 8x more work
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•Kinetic Energy – energy of an object due to the objects motion
•KE=
•Unit = J
•
•
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•What is the kinetic energy of a .0012 kg golf ball hit at 62 m/s ?
•KE=
•=
•2.3 J
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•Work-Kinetic Energy Theorem
•allows us to think of kinetic energy as energy stored in the motion
of an object
• even when object changes speed
•Wnet =
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•Dash, who is 21kg runs with a speed of 1.8 m/s till he crosses the
finish line. The coefficient of friction of the new age track is .22.
How far is his race?
• W = F cos x d
• We sub in using the Fk for force with kinetic friction
• Wnet =
• We also know
• Wnet = 𝑖2
• Set each equation equal to each other and then solve for displacement
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• = 1.8 m/s .22 m= 21kg g = 9.81 m/s2
• =
•= Running on horizontal so
•
•d =
•=
= .075.08 meters
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• Potential Energy – energy due to position
•Can be thought of as “stored” energy
• PEg = mgh units = Joules
• In ideal conditions energy is conserved so all potential energy
becomes kinetic energy
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•What is the kinetic energy of a bowling ball dropped from the
third story of a building just before it reaches the ground? Each
story of the building is 3.8m and the ball has a mass of 5.0kg.
• PEg = mgh
• h = 3.8x3
•=11.4m
• PEg = 5.0 (9.81)(11.4)
•= 559.17
•= 560 J
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• Elastic Potential Energy = PEelastic
•Depends on the distance an object is compressed or stretched
• PEelastic = ½ kx2
• k = spring constant x = distance compressed or stretched
• Total PE in a system PEtot takes into account PEg due to position
and elastic potential energy if there is a compressed or stretched
component
• PEtot = PEg + PEelastic
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•Deriving equations
•As we build our equation bank we will need to substitute in and
solve for specific variables more frequently!
• This course is cumulative so you don’t get to forget as we move
forward
•Keep resources organized
•Use organized equation sheets
•Know what all components mean and labeled clearly
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•On U3 Quiz 1 you will be required to derive an equation using
your knowledge of these 3 given equations:
• Power = work/change in time P=
•W= and v =
•Don’t overthink and psych yourself out!