Query 5

#$&*

course PHY 121

6/16 9:30I am worried about all of these formulas and connections that are making my head spin. I can't memorize things the way I used to be able to. I'm hoping there's more practice before a test.

@&

I can tell you a lot about what the years do to our memories.

Fortunately in this course there's not all that much to remember; the main thing is to know the basic definitions and relationships and how to use them. That comes with practice.

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005. `query 5

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Question: `qIntro Prob 6 given init vel, accel, `dt find final vel, dist

If initial velocity is v0, acceleration is a and time interval is `dt, then in terms of

these three symbols what are the final velocity vf and the displacement `ds?

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Your solution:

a = vf-v0/`dt

a`dt=vf-v0

a`dt+v0=vf

vf=a`dt+v0

a=(vf-v0)/`dt

a`dt=vf-v0

a`dt+v0=vf

`ds=(vf+v0)/2*`dt

'ds=[(a`dt+v0) + v0]/2*`dt

`ds=[(a`dt)/2 + 2v0/2]*`dt

`ds=(.5a`dt + v0)*`dt

`ds=.5a`dt^2 + v0`dt

confidence rating #$&*: 2

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Given Solution:

`a**You would use accel. and `dt to find `dv:

a * `dt = `dv.

Adding `dv to initial vel. v0 you get final vel.

Then average initial vel. and final vel. to get ave. vel.:

(v0 + vf) / 2 = ave. vel.

You would then multiply ave. vel. and `dt together to get the displacement

For example if a = 3 m/s^2, `dt = 5 s and v0 = 3 m/s:

3 m/s^2 * 5 s = 15 m/s = `dv

15 m/s + 3 m/s = 18 m/s = fin. vel.

(18 m/s + 3 m/s) / 2 = 10.5 m/s = vAve

10.5 m/s * 5 s = 52.5 m = displacement

In more abbreviated form:

a * `dt = `dv

v0 + `dv = vf

(vf + v0) /2 = vAve

vAve * `dt = `ds so

`ds = (vf + v0) / 2 * `dt.

**

STUDENT QUESTION

If we have the formula vf= v0 + a * dt, then we would substract the v0 from both sides

to isolate the a * dt algebraically, so our formula would be vf-v0= a* `dt,

how is this in comparison to the initial velocity v0 + the change in velocity(dv) = to

the final velocity(vf).

If we multiply the acceleration(a) times time(dt) we find the change in velocity

(dv).......we then add the initial to the change to find the final.......

Why do we add the initial to the change in velocity to find the final?

INSTRUCTOR RESPONSE

The initial velocity is v0, the final velocity is vf, so the change in velocity is `dv =

vf - v0.

Thus your early result vf-v0= a* `dt shows that a * `dt is equal to `dv.

In general the change in any quantity is equal to its final value minus its initial

value.

It follows immediately from this that if you add the change in the quantity to its

original value, you get its final value.

The following two statements say the same thing:

statement 1: If the temperature starts at 20 degrees and ends up at 35 degrees then it

changed by +15 degrees.

statement 2: If the temperature starts at 20 degrees and changes by +15 degrees then it

ends up at 35 degrees.

We generalize this to the two symbolic statements

If a quantity Q changes from Q0 to Qf then the change is `dQ = Qf - Q0.

If a quantity Q starts out at Q0 and changes by `dQ, then it ends up at Qf.

These statements can be expressed as two equations

`dQ = Qf - Q0 and

Qf = Q0 + `dQ

These two equations are algebraically equivalent: you can get the second by adding Q0 to

both sides of the first, or you can get the first by subtracting Q0 from both sides and

reversing sides.

A third equation also follows:

Q0 = Qf - `dQ,

which can be interpreted in terms of the preceding examples into obvious statements.

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Self-critique (if necessary):

I'm confused because I thought that what I needed to do was see what I knew about how to

find initial velocity, acceleration, and elapsed time to re-work equations to use only

v0, a, and `dt in my final answers. And, that the final answers were for final velocity

and displacement. The only good thing that I can say, is that when I plug in the

numbers you gave for the example, I get the same answers you did for final velocity and

displacement.

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Self-critique Rating: 2

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Question: `qWhat is the displacement `ds associated with uniform acceleration from

velocity v0 to velocity vf in time interval `dt?

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Your solution:

First you have to find the average velocity by adding the initial velocity to the final

velocity and dividing by two. Then you multiply the average velocity by the elapsed

time to get the displacement.

The formula is `ds=[(v0+vf)/2] * `dt

confidence rating #$&*: 2

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Given Solution:

Since accel is uniform vAve = (v0 + vf) / 2.

Thus displacement is

`ds = vAve * `dt = (v0 + vf) / 2 * `dt,

which is the first equation of uniformly accelerated motion. **

STUDENT QUESTION

I failed to make reference to uniformly accelerated motion.

What exactly is the difference between uniformly accelerated motion and average

acceleration??? Will we be asked to

differentiate between the two for problems, or is this something we should be able to

determine on our own easily???

INSTRUCTOR RESPONSE

Uniformly accelerated motion is motion in which the acceleration is uniform, unchanging.

If motion is uniformly accelerated, then the acceleration is constant, so the

acceleration at any instant is equal to the average acceleration.

If motion is uniformly accelerated, then since the slope of the velocity vs. clock time

graph represents acceleration, the slope is constant; i.e., the graph is a straight

line.

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Self-critique (if necessary): OK

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Self-critique Rating: 2

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Question: Describe the flow diagram we obtain for the situation in which we know v0, vf

and `dt.

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Your solution:

The first line is

`dt v0 vf

The next lines are

`vf-v0=`dv

`dv/`dt=a

(v0+vf)/2=vAve

vAve*`dt=`ds

If I know the beginning and final velocities, I can immediately find two pieces:

the change in velocity and the average velocity.

If I know the change in velocity and the change in time, I can find acceleration

If I know the average velocity and the elapsed time, I can find the displacement.

confidence rating #$&*: 2

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Given Solution:

`a** The first level in the diagram would contain `dt, v0 and vf.

From v0 and vf we can easily reason out `dv, so v0 and vf would connect to `dv in the

second level.

The second level would also contain vAve, also obtained from v0 and vf and therefore

connected from vf in the first level to v0 in the first level.

The third level would contain an a, which is reasoned out from `dv and `dt and so is

connected to `dv in the second level and `dt in the first level.

The third level would also contain `ds, which follows from vAve and `dt and is therefore

connected to vAve in the second level and `dt in the first level. **

STUDENT QUESTION:

I'm not sure what is meant by a flow diagram. I know that we can determine 'ds from the

equation 'ds=(v0+vf)/2* 'dt. Then I can use 'ds to find other possible information by

plugging this and other information into other equations.

INSTRUCTOR RESPONSE

The instructor's response developed into an entire document, a bit too long to include

in this query without interrupting the flow. The document has been posted at

http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/flow_diagrams.htm

and should be very useful to anyone who is having trouble with the idea of flow

diagrams.

STUDENT COMMENT

Flow diagrams are useful in that they give us something to logically grind out. It's

not enough to know that there are formulas to find variables. True learning is when a

person can take whats given, twist it and manipulate it, and find other answers.

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Self-critique (if necessary):

I'm still working on these. Mine get a little confusing because sometimes the

connecting lines overlap. I think the more of these I do, the better I will remember

the connections between this information. I tried to open the web document referenced

in the question to compare mine to yours and read what you had to say, but it wouldn't

open.

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Self-critique Rating: 2

@&

It would be good to submit a question form about that link.

*@

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Question: `qDescribe the flow diagram we obtain for the situation in which we know v0,

vf and `dt.

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Your solution:

v0 vf `dt

vf-v0=`dv

`dv/`dt=a

(v0+vf)/2=vAve

vAve*`dt=`ds

confidence rating #$&*: 2

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Given Solution:

`a** The flow diagram shows us the flow of information, what we get from what, usually

by combining two quantities at a time. How we get each quantity may also be included.

From vf and v0 we get `dv, shown by lines from vf and v0 at the top level to `dv. From

vf and v0 we also get and vAve, shown by similar lines running from v0 and vf to vAve.

Then from vAve and `dt we get `ds, with the accompanying lines indicating from vAve and

`dt to `ds, while from `dv and `dt we get acceleration, indicated similarly. **

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Self-critique (if necessary): OK, but still shaky

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Self-critique Rating: 2

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Question:

Suppose we have two points on a straight-line graph of velocity vs. clock time.

How do we construct a trapezoid to represent the motion on the intervening interval?

What aspect of the graph represents the change in velocity for the interval, and why?

What aspect of the graph represents the change in clock time for the interval, and why?

What aspect of the graph represents the acceleration for the interval, and why?

What aspect of the graph represents the displacement for the given interval, and why?

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Your Solution:

To construct the trapezoid, you would make a line from the first and final points

straight down to the x axis. We'll call the first point (x1,y1) and the final point

(x2, y2). The line segments for the trapezoid would be (x1,0), (x1, y1), (x2, y2), and

(x2,0).

The part of the graph that represents the change in velocity for the interval would be

y2-y1. This is because the y axis represents the velocity and this will tell the

difference between the ending velocity and the initial velocity.

The aspect of the graph that represents the change in clock time for the interval would

by x2-x1. This is because the x axis represents time. This would be the final time

minus the intial time, which would yield the change in time.

The part of the graph that represents the acceleration for the interval would be the

slope. Slope is rise/run, or (y2-y1)/(x2-x1). This would tell how much change there is

in velocity for each unit of time.

The part of the graph that represents displacement is the area in the trapezoid that was

created by connecting these points vertically to the x axis. In order to find the area,

you have to find the length of the 2 altitude lines, add them together and divide by

two. This is the same as finding the mid-point on the line or the average velocity.

@&

Just to be sure: To find the area you then multiply this average, which gives you the average 'graph altitude', by the width.

*@

Multipying the average velocity by the elapsed time (x2-x1) will give you the

displacement. So the displacement of the object equals the area of the trapezoid.

confidence rating #$&*:

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Question: `qPrinciples of Physics and General College Physics Students: Prob. 1.26:

Estimate how long it would take a runner at 10 km / hr to run from New York to

California. Explain your solution thoroughly.

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Your solution:

I looked on-line and found that it is approximately 3900 km from New York to California. In order to find out how many hours it takes to run 3900 km at 10 km/hr, you have to divide 3900 km by 10 km/hr. In order to do that, you actually invert the second fraction and multipy 3900 km/1 * 1hr/10km. The km cancel each other out, and you end up with 3900/10 hr, or 390 hours.

confidence rating #$&*: 2

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Given Solution:

It is about 3000 miles from coast to coast.

A km is about .62 mile, so 3000 miles * 1 km / (.62 miles) = 5000 km, approximately.

At 10 km / hr, the time required would be

5000 km / (10 km / hr) =

500 km / (km/hr) =

500 km * (hr / km) =

500 (km / km) * hr =

500 hr.

Be sure you understand the units of this calculation. Units should be used at every

step of every calculation.

The corresponding symbolic solution:

vAve = `ds / `dt; we want to find `dt so we solve to get

`dt = `ds / vAve. Substituting `ds = 5000 km and vAve = 10 km/hr we have

`dt = 5000 km / (10 km/hr) = 500 hr.

STUDENT SOLUTION (with some inconsistencies in units)

The student's estimate of the distance was 4000 km, which is perfectly OK:

To find out how much time it takes to travel this far, I took 4000 km and divided it by

10 km/h.

This was set up as follows:

4000 km / 10 km

This becomes 400 km * 1 hr

Our kilometers cancel out and we are left with 400 hours to run from New York to

California.

INSTRUCTOR RESPONSE

I would certainly accept your solution, with little or no penalty at the level of Phy

121.

However your use of units does have some contradictions, and you will understand units

better if you understand them:

In the first place, 4000 km / (10 km) = 400, not 400 km. The km divide out.

400 represented the number of 10 km intervals in a 4000 km trip.

Since average speed is 10 km/hr, meaning that a 10 km interval is covered each hour, it

therefore takes about 400 hours to complete the trip.

Note also that the calculation given in your solution as 400 km * 1 hr would be 400 km *

hr, not the 400 hr you intend.

Finally, to use the fact that v_Ave = `ds / `dt:

The time to cover distance `ds at average speed v_Ave is `dt = `ds / v_Ave, and that the

units of v_Ave are km / hr. So to be entirely correct, the correct calculation could

read

`dt = `ds / v_Ave = 4000 km / (10 km/hr) = 400 hr.

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Self-critique (if necessary):

I was unsure whether to start with miles or kilometers. Since it asked in kilometers, I used that to start with. Also, I verified at 2 sources that 3900 km was the distance. I am unsure when we get these kinds of problems where the information is supposed to come from. Also, I did not explain it using the formulas, but I do understand that I start with vAve = `ds/`dt. In order to get to `dt, I would multipy both sides by `dt, so I would have `dt*vAve=`ds and then divide both sides by vAve, so that I would get `dt=`ds/vAve. This would give me 3900 km / (10km/hr), and I would proceed as I explained above.

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Self-critique Rating: 2

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Question: All Students: Estimate the number heartbeats in a lifetime. What

assumptions did you make to estimate the number of heartbeats in a human lifetime, and

how did you obtain your final result?

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Your solution:

I looked up heartbeats per minute and US average life expectancy. The range of heartbeats per minute is 60-80, so I went with 70. The average life expectancy in the United States is 80. I set up my problem as follows:

70 heartbeats/min * 60 min/hr * 24 hr/day * 365 days/year * 80 years = 2,943,360,000 heartbeats in a lifetime. I would imagine that, since most of the numbers I used have 2 significant figures, my answer would best be expressed as 2.9 X 10^9.

confidence rating #$&*: 2

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Given Solution:

Typical assumptions:

At 70 heartbeats per minute, with a lifetime of 80 years, we have

70 beats / minute * 60 minutes/hour * 24 hours / day * 365 days / year * 80 years =

3 billion beats, approximately.

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Self-critique (if necessary): OK

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Self-critique Rating: 3

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Question: University Physics Students Only: Problem 1.55 (11th edition 1.52) (i.e.,

Chapter 1, Problem 52): Angle between -2i+6j and 2i - 3j. What angle did you obtain

between the two vectors?

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Your solution:

confidence rating #$&*:

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Given Solution:

For the given vectors we have

dot product =-2 * 2 + 6 * (-3) = -22

magnitude of first vector = sqrt( (-2)^2 + 6^2) = sqrt(40)

magnitude of second vector = sqrt( 2^2 + (-3)^2 ) = sqrt(13)

Since dot product = magnitude of 1 st vector * magnitude of 2d vector * cos(theta) we

have

cos(theta) = dot product / (magnitude of 1 st vector * magnitude of 2d vector)

so that

theta = arccos [ dot product / (magnitude of 1 st vector * magnitude of 2d vector) ]

= arccos[ -22 / ( sqrt(40) * sqrt(13) ) ] = arccos ( -.965) = 164 degrees, approx.. **

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Self-critique (if necessary):

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Self-critique Rating:

Add comments on any surprises or insights you experienced as a result of this

assignment.

** I had to get a little help from a friend on vectors, but now I think I understand

them. They are not as difficult to deal with as I thought. **"

Self-critique (if necessary):

------------------------------------------------

Self-critique rating:

Add comments on any surprises or insights you experienced as a result of this

assignment.

** I had to get a little help from a friend on vectors, but now I think I understand

them. They are not as difficult to deal with as I thought. **"

Self-critique (if necessary):

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Self-critique rating:

#*&!

&#Your work looks good. See my notes. Let me know if you have any questions. &#