Outline of today's lecture

1
Outline of today's lecture

• Biking problem and discussion
• What is Physics?
• Theories and Models in Physics
• Mathematics and physics
• Units
• Energy
• Conservation of Energy
• Examples of Conservation of Energy

2
Goals for today

• Students Will Be Able to
• Look at the problem and analyze its different
  aspects
• describe in simple terms the general nature of a
  scientific theory and its relationship to the
  experimental data
• explain the difference between a scientific model
  and a scientific theory
• explain why units are necessary for any reported
  physical quantity
• recognize that a clear definition of system
  boundaries is necessary in order to apply
  conservation laws
• explain how a quantity can remain constant even
  when there's a constant influx of that quantity
  to a system

3
Physics as a Science

• Describes matter, energy and their interactions.
• Is this definition too broad?
• Theories, models laws. Are they universally
  correct and unchangeable?
• Newtonian mechanics theory of relativity
• When an approximate theory or even a simple model
  is correct.

4

• A successful theory must be able to describe the
  outcome of experiments correctly and predict the
  results of new experiments performed to test it.
• Example Newtonian Physics predicts the
  existence of Neptune
• If the results of some experiments do not agree
  with the theory it must be either replaced or
  modified. (Perihelion precession of Mercury)
• A scientific theory is generally accepted if it
  agrees with all experiments over many years.
• There is always a possibility that it will be
  modified or expanded in the future.
• Example Newtonian Mechanics - GPS.

5
Models

• What do I have to know about a person to find out
  how fast it will move after falling down 2 m?
• What about after falling down 100 m?

6
Models

• What do I have to know about an animal to
  approximately figure out how much energy it needs
  to keep body temperature at about 36 ºC?

7
Models

• What information I need about a flashlight to
  figure out how long it will operate on 2
  batteries?

8
Models

• According to Knight
• A model is a simplified description of
  realityisolating the essential features, and
  developing a set of equations that provide an
  adequate, although not perfect description of
  reality.
• Physics, in particular, attempts to strip a
  phenomenon down to its barest essentials in order
  to illustrate the basic principles involved.

9
Your friend from Seattle told you that his new
car is doing 20 miles per gallon of gas.

• You know that your Prius uses 5 L of gas per 100
  km.
• Which car is more efficient?

10
What tools did I use solving this problem?Where
did the information came from?
11
Tools we use

• Mathematics provides an extremely powerful tool
  to use theories and models to describe or
  simulate reality and predict outcome of events
• The experimental techniques including the data
  acquisition and analysis give us the ways to test
  theories and collect the data useful in
  technology and sciences

12
Units

• Physical quantities have units. Examples.
• It is very important to use standardized SI
  units m, kg, s, N, J, oK. with appropriate
  prefixes
• We very often see other units such as cm, inches,
  miles, nautical miles, ft, pounds, oC, oF, etc.
• Need to convert units in problem sets.
• Examples
• 0.5 mm 5 x 10-7 m.
• 1 inch 25.4 mm .0254 m
• Always check that the units are correct.
• Try to make an order-of-magnitude estimate and
  compare with your calculated result.

13
Dimensional Analysis

• A very powerful tool to check the correctness of
  formulas or even recall some parts of a formula.

14
Energy

• Definition?

15
Lets talk about money first

• Cash flow
• Income
• Spending
• Steady state

Household Dad Mom Kids
Spending
Income
16
Whats Energy?

• Knight Its like natural money
• Ability to do (mechanical) work or change the
  thermal energy of an object (heat up or cool down
  an object) or a system.
• Energy can be transferred to a system or an
  object and stored (like saving money).
• Energy from system can be transferred to an
  object or another system and do work or heat up
  environment (like spending money).
• So energy is a quantity that is stored in a
  system or belongs to an object while work and
  heat are due to interactions with other objects
  or the environment.