Transcript ME101 Lecture 10

Power, Efficiency, and Potential Energy
Class Lectures 11: 3.6 & 7
Today’s Objective
1. Power and Efficiency
2. Potential Energy
3. Conservation of Energy Method
Power
• Power is defined as the rate at which work is done,
Units of power are: Watts and
HP
Efficiency
• Efficiency is defined as the ratio of work output to the work
input:
• If there are energy losses due to electrical and thermal energy,
the resultant efficiency is the product of all.
Problem 3/131
Given: WA = 1000 kg, vA = 3 m/s upward
Required: Power required for the motor, if the efficiency e = 0.8
Potential Energy
Two types of potential Energy:
1.
2.
Gravitational potential energy
Elastic potential energy
Gravitational potential Energy
• Potential energy is measured from a datum
plane
• Datum can be selected arbitrarily
• If the Particle’s position is above the
Datum, it has a positive potential energy
and if its position is below the datum
it has a negative potential energy
Potential Energy
• Potential energy can be converted to work
• Potential energy = work done by the weight
• As the potential energy is used up, it decreases in value and produces
positive work, thus
• The increase in potential energy will produce a negative work, and vice
versa.
Potential Energy
Elastic potential energy
• When a spring is extended or compressed, it stores a positive
potential energy
• The stored potential energy = potential work by the spring force
• x1 and x2 are the initial and final deflections in the spring.
Work and Energy Equation
• The equation for work energy with the potential energy
included can be written as,
• Expanding it, we can write,
Work energy equation
Conservation of Energy
• If the forces are conservative, i.e., independent of the path
travelled (non-conservative forces: forces produced by friction,
heat, sound, etc.), then,
conservation of energy equation
Problem 3/139
• Given: 2-lb collar released from rest at A. No friction. k = 1.6
lb/ft, upstretched length = 15 in.
• Required: Velocity at point B
Problem 3/141
Given: released from rest at position A. Neglect friction
Required: Velocity at B, Maximum deflection in the spring
Problem 3/152
Given: Released from rest. Neglect friction
Required: Speed of mass A and B after B moved 1 m.
Midterm Exam Review
Date and Time:
Time:
Thursday, March 5, 2015
4:30 – 5:45
Material Covered: Chapters 1, 2, and 3
Number of Problems: 8 – 10
How graded:
1. No partial credit given for solutions based on wrong concepts.
2. Partial credit given only if there is a math error.