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Work, Energy and Power
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Is the student doing work in pushing against the wall?
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Is the girl doing work in pushing the cart?
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Is the man doing work in carrying the load across the street?
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Is the lady doing work while holding the weights above her head?
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Is work done in lifting the box?
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Is work done in putting down the box?
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Work If a constant force F acts on an object as it
undergoes a displacement d, the work done by the force on the object during the displacement is
W = Fdcos
where W = work done in Joules (J)
F = force in N
d = displacement in m
= angle (180 or less) between the
direction of F and the direction of
d
Note: 1 J = 1 Nm
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Work
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Requirements in order for work to be done
1. Force need to be exerted
2. There must be a displacement
3. The force must be exerted in such a way that it has a component that is in the same direction or opposite to the direction of the displacement.
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Energy
The capacity of a physical system to do work.
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Kinetic Energy (KE)The energy an object has because of
its motion.
where KE = kinetic energy in J
m = mass in kg
v = speed in m/sThe kinetic energy of an object
changes when its speed changes from vi to vf.
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Gravitational Potential Energy (PEg)
The energy an object has because of its vertical separation from the Earth’s surface.
where PEg = gravitational potential
energy in J
m = mass in kg
g = acceleration due to gravity (9.8 m/s)
h = height of the location of the object in m The gravitational potential energy changes when the
vertical location of the mass changes from hi to hf
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Elastic Potential Energy (PEs)
The energy of an object such as a spring has because it is compressed or stretched from its equilibrium position.
where PEs = potential energy in J
k = force constant in N/m
x = displacement of the spring in m The elastic potential energy of a spring
changes when its displacement changes from xi to xf.
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Work-Energy Calculation
When a system gains or loses energy from its environment because of work done on the system by forces origination in the environment, then the change in the system’s energy is
W = Ef – Ei
Rearranging and substituting for the different types of energy results to
KEi + PEgi + PEsi + W = KEf + PEgf + PEsf
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Law of conservation of energy
In a closed, isolated system, energy is not created or destroyed, but rather, is conserved.
KEi + PEgi + PEsi = KEf + PEgf + Pesf
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Power
The rate of doing work or the rate of energy conversion from one form to another.
where P = power in Watts (W)
W = work done in J
E = energy in J
t = time in s