work, energy, power
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Work, Energy, Power. What’s the difference?. Force is the agent of change Energy is a measure of change Work is a way of transferring energy from one system to another. What is work?. Work= force*displac. W=Fd - PowerPoint PPT PresentationTRANSCRIPT
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Work, Energy, Power
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What’s the difference?
• Force is the agent of change• Energy is a measure of change• Work is a way of transferring energy from
one system to another
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What is work?• Work= force*displac.• W=Fd• Only work if there is
motion- if you push against a brick wall and it doesn’t move, you might be tired but you have done no work
• Unit=Joule (unit of energy)
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Are they work?• Teacher pushes wall and
becomes exhausted• Book falls off table to floor• Waiter carries large tray
across restaurant at constant v
• Starship Enterprise accelerates through space
• No- no displacement
• Yes- force=g and displacement=fall
• No-why?
• Yes- force from engines
So what’s with the waiter???????
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Work = 0 • Work = 0 if:–No force–No displacement–force is perpendicular to
displacement
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Power• Power= rate at which work
gets done= work over time• P=W/t• Since W=Fd then P=Fd/t and
d/t=v• P=Fv• Unit= J/s=watt (W) • Careful not to confuse unit W
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Ex: Power
• A mover pushes a large crate mass=75kg across the truck bed for a total distance of 6m. He exerts a steady force of 300N for 20s. What is his power output?
• P=W/t P=Fd/t=(300N)(6m)/20s=90W
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Kinetic Energy
• Energy of MOTION• K=1/2mv2
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Example…• Determine the kinetic
energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s
• KE = (1/2)*m*v2
• KE = (0.5) * (625 kg) * (18.3 m/s)2
• KE = 1.05 x105 Joules
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Potential Energy=U
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Energy an object has due to its position or configuration- stored energy that can be retrieved.
Ex- height on a wave gives U, pulling back the string on a bow gives it U, compressing or stretching a spring gives U.
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Potential Energy: Energy of POSITION
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Gravitational Potential
Energy: Ug
• Potential energy due to position relative to surface of the earth
• Ug=mgh• Unit = Joule
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Gravitational Potential Energy: Ug
and Work done by gravity
• Gravity can do + or - work depending on motion
• Path independent- depends on height, not path taken
• Wg=mgΔh • Where h is height
above arbitrary 0 pt
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Examples
• Physicsman (mass=60kg) scales a 40m tall rock face. What is his potential energy (relative to the ground)?
• Ug=mgh=(60kg)(10N/kg)(40m)=24000J
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Mechanical Energy: U and K:Energy because of position or
motion
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Total Mechanical Energy is CONSERVED
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PE
ONLY
PE +
KEKE
ONLY
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Take it 1 step further…
• If physicsman (60kg) were to jump of the cliff (remember his U=24000J), what would his velocity be when he hits the ground? Think…U is transformed to K
• At the top he has all U=24000J• At the bottom he has all K=1/2mv2
• Utop=Kbottom
• 24000J=1/2(60kg)v2
• V=28m/s
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Work by Conservative vs. Nonconservative Forces
• Conservative forces are path independent–Ex: gravity
• Nonconservative forces depend on path–Ex: kinetic friction-
longer path means more work
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Work and Energy
• E=K+U• E=1/2mv2+mgh
– Object’s mechanical energy is sum of kinetic and potential energies
– Since U is relative to position, so is E
• Wnc=ΔK+ΔU– Work done by nonconservative forces is sum of changes in K
and U
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Conservation of Energy• Since E=K+U, if no nonconservative forces (friction
for example) act on a system then mechanical energy is conserved
• Ei=Ef • Ki+Ui=Kf+Uf
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Ex: conservation of energy
• A ball of mass 2kg is gently pushed off the lab table, 5.0m above the floor. Find the speed of the ball as it strikes the floor
• Ei=Ef or Ki+Ui=Kf+Uf
• 0+mgh=1/2mv2+0