# Shared Flashcard Set

## Details

Work & Energy
Concepts and Formulas
56
Physics
10/13/2012

Term
 Work & Energy are vector/scalar quantities
Definition
 scalar
Term
 define work
Definition
 work  is done on an object by a constant force.  This force is constant in both magnitutiude & direction.
Term
 Work defintion in terms of energy
Definition
 work is change in mechanical energy
Term
 units for work
Definition
 the joule (J)  = 1 N • m
Term
 If motion and force are in the same direction so that : θ = 0 cos θ = 0 what will work be?
Definition
 W= Fd For ex you push a cart 50 m with 30 N of force your work will be   W= 30 x 50 =1500 J
Term
 T or F: Can force can be exerted on an object & do no work?
Definition
 True, ex you carry a grocery bag at rest  OR if you carry it horizontally across the room at a constant velocity   (this b/c there is horzontial force required to MOVE the bag at a constant velocity)   therefore theta is 90 and cos 90 =0 b/c displacment is perpendicular to force
Term
 Energy is a (conserved/nonconserved) quantity
Definition
 conserved, energy is not lossed or gained
Term
 kinetic energy means
Definition
 energy in motion
Term
 If gravity & normal force are perpendicular (90 angle) _____ work is done
Definition
 no work is done
Term
 translational kinetic energy is defined by what quanity?
Definition
 KE= 1/2mv2   aka work is force x displacment
Term
 The work energy principle defined
Definition
 the net work done on object is equal to the change in the object's kinetic energy.
Term
 Give the formula for work energy principle
Definition
 Wnet= ΔKE   OR Wnet= KE2 - KE1
Term
 Work done by forces that ____ the direction of motion such as _____ will be negative
Definition
 oppose; friction
Term
 Do centripetal forces do work?
Definition
 No, they are always perpendicular to motion
Term
 Unit for energy
Definition
 J (joules)
Term
 if mass is doubled then kinetic energy is
Definition
 doubled
Term
 if velocity is doubled then kinetic energy
Definition
 4x times greater = is capable of doing 4x more work
Term
 If the net work  (W) is positive then KE ___ If the net work (W) is negative then KE ___- If the net work is (W) is zero then KE
Definition
 1. Increases 2. Decreases 3.same
Term
 Potenital Energy is defined as
Definition
 energy associated with forces that depend on the position or configuration of an object/s REALTIVE to surroundings
Term
 examples of potential energy include
Definition
 wound-up spring(waiting to be released) stretched elastic band (waiting to be released) an object at some height above the ground (like a cup on a counter)
Term
 give formula that combines friction and Work by a nonconservative force
Definition
 -Ffrd = 1/2mv22 - 1/2mv12 -mgy2-mgy1   friction= (KE2-KE1)-(PE2-PE1)
Term
 Recall the previous example:  object at some height above the ground (like a cup on a counter) is an example of what specific potential energy?
Definition
 potential gravitational energy  (b/c of its position realtive to Earth)
Term
 Potential Energy: In raising a mass 'm' to a height 'h', the work  done by the external force is expressed by what equation:
Definition
 Wext= Fext d cos 0 = mgh Wext=mg (y2-y1) Wext = PE2-PE1 =ΔPE
Term
 Official formula for gravitational potential energy
Definition
 PEgrav= mgy m= object mass in kg g= 9.8 m/s y= height in m
Term
 Gravitational Potential Energy can become ______ if the object is dropped.
Definition
 kinectic energy
Term
 Potential energy is property part of
Definition
 system NOT whole b/c it depends on external forces
Term
 Gravitational Potential Energy: The change in potential energy in terms of the work done  gravity itself can be written as:
Definition
 WG= -mg (y2- y1) WG= - (PE2 - PE1) WG= - Δ PE   therefore the work done by gravity on an object is NEGATIVE
Term
 Gravitational P.E. depends on
Definition
 the VERTICAL height of the object above some reference level.
Term
 The force to compress  or stretch a spring is represented by what formula?
Definition
 Fs = - kx where k= spring constant x= distance
Term
 the further the spring is compressed or stretched causes  the force is _____
Definition
 increased
Term
 Give the formula to find the potential energy in a stretched/compressed string measured from its equilibrium positon
Definition
 elastic PE= 1/2 kx2 scalar value where x is the amount of spring stretched/compressed (meters) units: N
Term
 Is there a single formula for potential energy?
Definition
 no, it depends on the force involved. note: kinetic energy does, 1/2mv2
Term
 Nonconservative force def
Definition
 work that is done by a force that depends on : path start point end point   ex: friction
Term
 conservative force defined
Definition
 work  depends on intial + end position NOT PATH ex. gravity
Term
 gravity is a ___ force
Definition
 conservative
Term
 elastic is a _____ force
Definition
 conservative
Term
 electric is a ____ force
Definition
 conservative
Term
 friction is a _____ force
Definition
 nonconservative
Term
 tension in a cord is a ____ force
Definition
 nonconservative
Term
 push or pull by a person is a ____force
Definition
 nonconservative
Term
 Work done by a nonconservative force is equal to
Definition
 the total change in kinetic & potential energies
Term
 Give the formula for work by a nonconservative force
Definition
 WNC = ΔKE +ΔPE 1/2mv12 + mgy1 = 1/2mv22 +mgy2   * remember friction, tension are ex
Term
 When can ΔKE +ΔPE =0 ?
Definition
 when there are NO nonconservative forces   * the kinetic & potential energy changes are equal but opposite in sign  (-5 and + 5)
Term
 Define the principle of total mechanical energy
Definition
 if onlt conservative forces are acting, the total mechanical energy of a system neither decreases or increases in ANY process . (aka it is conserved).
Term
 Give the formula for the total mechanical energy
Definition
 E= KE + PE E= 1/2 mv2 + mgy   recall this is  energy is constant E1 =E2  energy before= energy after
Term
 The rollercoaster problem: If there is no friction, then the speed of the rollercoaster will depend on
Definition
 current height compared to its starting height
Term
 if there is a nonconservative force like friction, where does the KE +PE go?
Definition
 become heat & temp of materials can be calculated
Term
 Power is def
Definition
 rate at which work is done
Term
 unit for power (P) capital P
Definition
 W (watts) 1W= 1 J/s
Term
 P (average power) = (in words)
Definition
 work/time = energy transformed
Term
 P (average power) =  in terms of force & average velocity
Definition
 P   = W /t = Fd/t = F v
Term
 P  = W /t give another formula for average power in terms of potential energy
Definition
 = mgyf-mgyi t
Term
 air resistance is a ____ force
Definition
 nonconservative
Term
 motor/rocket propulsion is a _______ force
Definition
 nonconservative force
Term
 elastic PE energy formula : where there is kinetic energy such as firing  a spring loaded gun
Definition
 1/2mv12 + 1/2 kx12  = 1/2 mv2 2 + 1/2 kx22
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