PHYS 221 Study Guide - Quiz Guide: Energy Economics

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puceraven738

30 Apr 2020

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If wnc=0, the initial and final energy of the system is the same. Ei=ef: energy of the system is constant, energy of the system is conserved, total mechanical energy= k+u, energy balance equation: ki+ui+wnc=kf+uf. Kt (translational kinetic energy): energy due to motion of the center of mass. Kr (rotational kinetic energy): energy due to the rotation of an object about its center of mass. Ug (gravitational potential energy): work done by f g. Us (elastic potential energy): work done by f s.

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Two blocks of masses m and 3m are placed on a frictionless,horizontal surface. A light spring is attached to the more massiveblock, and the blocks are pushed together with the spring betweenthem (Fig. P9.9). A cord initially holding the blocks together isburned; after that happens, the block of mass 3mmoves to the rightwith a speed of 2.00 m/s. (a) What is the velocity of the block ofmass m? (b) Find the systemâs original elastic potential energy,taking m = 0.350 kg. (c) Is the original energy in the spring or inthe cord? (d) Explain your answer to part (c). (e) Is the momentumof the system conserved in the bursting-apart process? Explain howthat is possible considering (f) there are large forces acting and(g) there is no motion beforehand and plenty of motionafterward?

Please check my work and if incorrect please provide detailedresponse.
a.) conservation of energy
3m*2+m*v=0
v=-6(m/s) (left)

b.) Energy is conserved
K=((0.350kg)(6^2))/2+((3)(0.350kg)(2^2))/2
K=8.4j
c.) The cord has stored no potential energy in the spring.
d.) There is no potential energy in the spring because thepotential energy in the spring has transferred to kinetic energy ofthe two masses.

e/f/g.)Yes. The two blocks of masses m and 3m are in one isolatedsystem (attached to one another with a spring and pushed together)and the mass of the spring is less than both blocks and is onlyused for storing potential energy.

A charge q that moves between two points A and B, and has anelectric force acting on it ( and perhaps several other forces) haswork W done on it by the electric force. The electrostatic force isconservative and so we can define a change in electric potentialenergy, Delta U = -W. We defined electric field using the relationF^^\-> = q E^^\->, and in a similar way we define electricpotential commonly called voltage as
Delta V = V_B - V_A = -W/q=(Delta U)/q or equivalently q Delta V =q (V_B - V_A) = -W=Delta U

A ball has mass 3.0 Âµg and charge -6 nC moves from point A to pointB. It has the same speed at points A and B, and the work done bythe electric field during the move is -1.5 mJ.
(a) What is the change in electric potential energy of the ball?1
(b) What is the change in voltage between points A and B? 2
(c) At which point does the ball have a higher electric potentialenergy?
(d) Which point has the higher voltage?
(e) Has the kinetic energy of the ball changed?

To practice Problem-Solving Strategy 21.1 Conservation of energy incharge interactions.

An alpha particle (), which is the same as a helium-4 nucleus, ismomentarily at rest in a region of space occupied by an electricfield. The particle then begins to move. Find the speed of thealpha particle after it has moved through a potential difference of-3.45Ã10-3 .
The charge and the mass of an alpha particle are = 3.20Ã10-19 and =6.68Ã10-27 , respectively.

PROBLEM-SOLVING STRATEGY Conservation of energy in chargeinteractions

PREPARE Draw a before-and-after visual overview. Define symbolsthat will be used in the problem, list known values, and identifywhat you're trying to find.

SOLVE The mathematical representation is based on the law ofconservation of mechanical energy:

.

Find the electric potential at both the initial and finalpositions. You may need to calculate it from a known expression forthe potential, such as that of a point charge.
and are the total kinetic energies of all moving particles.
Some problems may need additional conservation laws, such asconservation of charge or conservation of momentum.
ASSESS Check that your result has the correct units, is reasonable,and answers the question.

Prepare
Identify the system involving the alpha particle. Then examine theforces acting within this system to determine whether mechanicalenergy is conserved. Make certain to draw a before-and-after visualoverview and to define your symbols.
Part A
Mechanical energy is conserved in the presence of which of thefollowing types of forces?
Select all that apply.
? frictional
? electrostatic
? magnetic
? gravitational

Which of the following quantities are unknown?
? the value of the electric potential at the initial position ofthe alpha particle
? the final speed of the alpha particle
? the charge of the alpha particle
? the mass of the alpha particle
? the difference in potential between the initial and finalpositions of the alpha particle
? the value of the electric potential at the final position of thealpha particle
? the initial speed of the alpha particle

PHYS 221 Study Guide - Quiz Guide: Energy Economics

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