Quiz practice

For the quiz on Wednesday (E) and Thursday (A):

1.  Consider the earth revolving around the Sun.  The average distance between earth and Sun is about 150 million km.

A.  What is the Earth's average speed around the Sun?

B.  Convert this to ft/second and show the conversion process.

2.  From Park School, you travel 2 km east, 4 km south and an additional 3 km east.  Find:

A. Your displacement, including angle from start.

B.  Your average speed and velocity, if the trip takes 25 minutes to mountain bike.

Quiz upcoming

A - next Thursday (5/1)

E - next Wednesday (4/30)

Yay!

Velocity Lab

Formal Lab – The Measurement of Velocity

In this experiment, we will determine the velocity of a cart by 2 methods:

·              Photogate timer

·              Ticker tape timer

Each method can be quite accurate, though what is actually being measured by each is worth some discussion. 

Recall that velocity is calculated by knowing the displacement and the amount of time required to traverse it:

v = d / t

Strictly speaking, this is average velocity.  In theory, the average velocity is a mathematical average of all (if that were possible) instantaneous velocity points throughout the trip.

Instantaneous velocity is the type of velocity you receive from a speedometer – it is the velocity at that instant.  In the case where the object moves at a constant rate, the instantaneous velocity (at all points) is equal to the average velocity.  That should be the case (approximately) for this lab.  We will determine the extent to which this idea is true in this lab.  In this lab, you may work in cm OR m – be consistent.

Procedure

1.      Set up a path for your car to travel – 1 meter should be long enough.  Place your motorized car on it.

2.      Attach a piece of timer tape to the card and ready the cart for motion.

3.      Place a photogate timer at some point along the cart’s path.  Place a flag on the cart – it must break the light gate fully.  Ready the photogate for timing.  Measure the width of the flag for future reference.

4.      Set the tape timer and note the frequency of operation.  Turn on the tape timer.

5.      Turn on the car and allow it to run the length of the path.

6.      Remove the tape and write the time value from the photogate on the tape for future reference.

7.      Repeat for 2 different cart trials, using new tapes each time.

Analysis I – the Ticker Tape Timer

·              Examine the ticker tape.  If the car is traveling at a uniform velocity, how should the dots appear?  Verify that this does occur. 

·              Starting with the first clear dot, measure the distance that each consecutive dot is from the first dot.  Recall the frequency of the timer – this determines the time intervals.  For example, if it is set at 10 Hz, the time between each dot is 1/10 of a second.  With this in mind, write down the first 30 or so total displacements from the first point.  The corresponding times (for 10 Hz) are 1/10, 2/10, 3/10, and so on.

·              Plot total displacement versus time on a graph.  What type of relationship is it?  Does this seem correct?

·              Find the slope of the graph.  What does this represent?

·              What would a (displacement vs. time) graph of an accelerating car look like?  How about a decelerating car?  How about a car moving backwards with constant velocity?  Draw these in your lab.

Analysis II – the Photogate Timer

·              Calculate the instantaneous velocity of the car using the time and width of flag.

·              Compare, by means of % difference, the velocities from both methods.  Percent difference is found by taking one value minus the other value, divided by the average of the two values, and multiplied by 100.

In your conclusion, discuss the relative accuracy of the two methods and give methods for improving the lab.

HW for A

1.  Draw the following graphs (roughly) for a car that starts from rest and speeds up consistently for 10 seconds, then holds its speed constant for 10 seconds, then hits the brakes and comes to a halt in 5 seconds:

a.  displacement vs. time
b.  velocity vs. time

2.  Think about the lab - you'll be comparing the velocity measurement of a toy car using 2 techniques:  photogate timer and ticker-tape device (getting the slope of the graph).  Which might you expect to be more "accurate"?

3.  Find the definitions (mathematical and conceptual) of acceleration.  What are the units?

HW reminder for E

1.  Make sure that you have your graph of d vs. t for class tomorrow - they should be individual graphs, NOT group graphs.  Make sure to have the program find the slope, too.

2.  Also, be sure to find the speed using the photogate data.

Lab questions will be forthcoming after tomorrow's class.

HW for E

1.  Draw the following graphs (roughly) for a car that starts from rest and speeds up consistently for 10 seconds, then holds its speed constant for 10 seconds, then hits the brakes and comes to a halt in 5 seconds:

a.  displacement vs. time
b.  velocity vs. time

2.  Think about the lab - you'll be comparing the velocity measurement of a toy car using 2 techniques:  photogate timer and ticker-tape device (getting the slope of the graph).  Which might you expect to be more "accurate"?

3.  Find the definitions (mathematical and conceptual) of acceleration.  What are the units?

HW for A

I mentioned this in class but didn't update it on the blog.

Find out something about the SI kilogram - what is it based on now, and how might this standard be changed over the next few years.

Also - do the homework marked "HW for E and A" below.

HW for E and A

1.  Calculate the values of the average speeds in m/s.

2.  Convert to miles/hour (MPH).

3.  Create a factor for converting from m/s to furlongs per fortnight.

4.  How many seconds is a microfortnight?  Recall that micro means one millionth.  Show the work for this.

5.  How great a distance is a nano-light year?  Work it out.  Recall that nano means one billionth.

6.  If interested, create a similar problem and solve it.

SI Units info

Some comments on standards.

Mass is measured based on a kilogram (kg) standard.
Length (or displacement or position) is based on a meter (m) standard.
Time is based on a second (s) standard.

How do we get these standards?

Length - meter (m)

- originally 1 ten-millionth the distance from north pole (of Earth) to equator
- then a distance between two fine lines engraved on a platinum-iridium bar
- (1960): 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of Kr-86 in a gas discharge tube
- (1983, current standard): the length of path traveled by light during a time interval of 1/299,792,458 seconds

That is, the speed of light is 299,792,458 m/s. This is the fastest speed that exists. Why this is is quite a subtle thing. Short answer: the only things that can travel that fast aren't "things" at all, but rather massless electromagnetic radiation. Low-mass things (particles) can travel in excess of 99% the speed of light.

Long answer: See relativity.

Time - second (s)

- Originally, the time for a pendulum (1-m long) to swing from one side of path to other
- Later, a fraction of mean solar day
- (1967): the time taken by 9,192,631,770 vibrations of a specific wavelength of light emitted by a cesium-133 atom

Mass - kilogram (kg)

- originally based on the mass of a cubic decimeter of water
- standard of mass is now the platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris
- secondary standards are based on this
- 1 u (atomic mass unit, or AMU) = 1.6605402 x 10^-27 kg
- so, the Carbon-12 atom is 12 u in mass

Volume - liter (l)

- volume occupied by a mass of 1 kg of pure water at certain conditions
- 1.000028 decimeters cubed
- ml is approximately 1 cc

Temperature - kelvin (K)

- 1/273.16 of the thermodynamic temperature of the triple point of water (1 K = 1 degree C)
- degrees C + 273.15
- 0 K = absolute zero

For further reading:

http://en.wikipedia.org/wiki/SI_units

http://en.wikipedia.org/wiki/Metric_system#History

>

In addition, we spoke about the spherocity of the Earth and how we know its size. I've written about this previously. Please see the blog entries below:

http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html

http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html

HW

Map out your trip to school, using Google maps, etc.  Print out the map, making sure that there is a legend/scale on it.  You'll need that to find the displacement.

Find your:

- travel time
- total distance
- total displacement
- average speed (distance / time)
- average velocity (displacement / time)

Bring this to class (Wednesday for E block, Thursday for A block)

A block physics - papers due Friday

You can still submit them tomorrow if you wish, but I'd like to chat about e/m induction a bit - and your devices might use this idea, so your paper may benefit.

Pick a device (paper)

Pick one of the following devices to research and write about in a 1-2 page paper.  Make sure to include a helpful diagram or two.  It is ok to talk about the history, but be sure to go into the physics - how does it work?

speaker or headphones/earbuds

telephone (regular, not cell)

guitar pickup (standard electromagnetic, not piezo)

microphone

transformer

MRI

magnetic tape recording
ruining electronics with magnets
hard drive
generator
turbine - wind, steam, water
metal detection
telegraph
phonograph (record player)
maglev trains

If you think of other ideas, let me know so I can tell you if you're in 'over your head'.

This will be due in 2 classes..

A and E hw

1.  Explain how/why the motor works.  It will be important to think about magnetic fields in coils.  In other words, why was it important to wrap the red wire into a coil?  What effect does that have on the magnetic field in the coil?  Is it similar to the electromagnet seen in class?

2.  Real motors are a bit more complicated.  Find images and/or descriptions of real motors, and comment on how they compare to yours.