Chapter 2: Work and Energy Flashcards
energy
system’s ability to do work, to make something happen
units if kinetic energy
joules (J)
kinetic energy
K = 1/2 m v^2
it is related to speed, not velocity. An object has the same kinetic energy regardless of the direction of its velocity vector
if the speed doubles, the kineti energy will quadruple
potential energy
there is potential to work, the most common are grvatational potential energy and elastic potential energy
gravitational potential energy
U = mgh
U = potential energy
m = mass in kilograms
g = acceleration due to gravity
objects position with respect to some level identified as the datum (ground)
elastic potential energy
U = 1/2 k x^2
U = potential energy
k = spring constant
x = magnitude of displacement from equilibrium
total mechanical energy
E = U + K
E = total mechanical energy
U = potential energy
K = kineitc energy
conservation of mechanical energy
ΔE = ΔU + ΔK
first law of thermodynamic
energy is never created ir destroyed, it is transfered from one to the other
Conservative forces
those that are path independent and do not dissipate energy
examples: gravity and electrostatic forces
nonconservative forces
W nonconservative = ΔE = ΔU + ΔK
W nonconservative is the work done by the nonconservative forces only. It will be exactly equal to the amount of energy “lost” from the system
Example: air resistance
work formula
W = F . d = Fd cosθ
d = magnitude of the displcement through which the force is applied
θ = angle between the applied force vetor and the displacement vector
work
work is the measurement of energy transfer. The other form of energy transfer is heat
pressure-volume (P-V) curve
the work can be determined by finding the area enclosed by the corresponding pressure-volume curve
when the volume stays contant (isovolumetric) then no work is done because there is no area to calculate
when the pressure if constant (isobaric process), we can calculate work using W=PΔV
when neither volume or pressure is constant then the region I in the graph can be calculated using
A1 = 1/2 ΔVΔP
and area 2 can be calculates using:
AII = P2ΔV
the total work done is:
W= AI + AII
power unit
watt (W) or J/s
power equation
P = W / t = ΔE / t
work-energy theorem
the net work done by forces acting on an object will result in an equal chenge in the objec’s kinetic energy.
If one calculates the change in kinetic energy experienced by an object, then the net work on or by the object is the same
work-energy theorem equation
Wnet = ΔK = Kf -Ki
simple machines
designed to provide mechanical advantage: wedge, wheel and axle, lever, pulley, and screw
mechanical advantages
force exerted on an object by a simple machine (Fout) to the force actually applied on the simple machine (Fin)
mechanical advatage = Fout / Fin
cosθ when θ = 0
= 1
what is the load?
the weight
what is the effort?
only half the force is required to lift the crate
pulleys
to lift an object to a certain height in the air (the load distance), one must pull through a lenght of rope (the effort distance) equal to twice thar displacement
when considering simple machines
load and effort are both forces. The load determines the necessary output force. From the output force and mechanical advantages, we can determine the necessary input force.
efficiency
efficiency = Wout/Win = (load)(load distance) / (effort)(effort distance)
it is often expressed as a percentage by multiplying the efficiency ratio by 100 percent.
