Basic Physics

Page 2 - Seeking answers? Join the AnandTech community: where nearly half-a-million members share solutions and discuss the latest tech.

Wnh5001

Senior member
Dec 1, 2005
408
0
76
96 is correct, yay thanks ppl.. guess imma try to learn integrals and deriatives soon, i have a feeling 80-90 % of the people in that class already had calculus -.-'. in recitation and labs ive been lucky to have some smart people help me along, >_>
 

jchu14

Senior member
Jul 5, 2001
613
0
0
Okay here?s a quick crash course on Physics mechanics (and applicable calculus)

ALWAYS understand the concept first!!!

The physics concept is this:

---> is derivative
Displacement-->velocity-->acceleration
derivative of displace is velocity, derivative of velocity is acceleration

to tak the derivative of something is to find the slope or rate of change. I also like to think of it as 'how fast something is changing'.

In a physical sense, what is velocity? It's how fast you're moving from point a to point b, or how fast you're chaing your displacement.

By the same token, what is accleration? when you accerlate your car, you're changing how fast you're going. So acceleration=how fast velocity is changing.

Now does it make sense that:
derivative of displace is velocity, derivative of velocity is acceleration or
velocity is how fast you're changing displcement (location), and acceleration is how fast you're changing velocity.


Conversely, antiderivative (or integral) of something is the complete opposite of derivative. hense:

==>> is integrate
acceleration==>>velocity==>>displacement

Integration is the "summing" of a function, or finding area under a curve.
So velocity is the 'sum' of all of the acceleration and
displacement is the 'sum' of all of the velocity.

This may seem a little abstract, but just remember integration is the opposite of derivative.

in short:
derive to go from left to right, integrate to go from right to left
displacement--velocity--acceleration

Now that you have an understanding of that, here's how you would FIND the derivative (and solve problems!)

calculus crash course:

The simplest and most useful derivative technique is the power rule. Here's what you do:

POWER RULE:

Given: f(x)=x^a where a is a constant and you want to find deriative of the function, f'(x), all you do is take 'a' and multiply the x by it. then subtract 1 from a.
so you have f(x)=x^a so f'(x)=a*x^(a-1)

example:
a) f(x)=x^2 differentiate and you get f'(x)=2*x^(2-1)=2x
b) f(x)=3x^2 differnetiate and you get f'(x)=2*3*x(2-1)=6x
c) here's interesting one.... f(x)=4x+12,

one of the properties of derivatives is that you can chop the function right where the add is and derive each part, then sum them together.

so you have another way to rewrite 4x+12 is 4x^1+12x^0. remember anthing to the 0th power is 1.

Then you just do the power rule for each part separated by +.

f(x)=4x^1+12x^0
differentiate
f'(x)=4*1*x^(1-1)+12*0*x^(0-1)=4
this also shows that the derivative of a constant is always 0...justify this in your mind.

Just knowing that gives you the tool to go Left to right: displacement-->velocity-->acceleration



Now you want to go right to left:
The power rule for integration is essentailly the 'backwards' of differentiation.

Given f'(x)=x^a, f(x)=(1/(a+1))*x^(a+1)+c ................... c is constant of integration

It's easier to see in an example:

example:
a) f'(x)=x^2 so integrate: f(x)=(1/(2+1))*x^(2+1)=(1/3)x^3+c
b) f'(x)=6x so integrate: f(x)=(6/2)x^(1+1)=3x^2+c remember x=x^1
c) f'(x)=4+x integrate: f(x)=(4/(0+1))*x^(0+1)+1/(1+1)*x^(1+1)=4x+(1/2)x^2+c


the plus c term is there because you 'lose' a constant whenever you differentiate. the derivative of 4x and 4x+9999999999 is the the same. However, unless you're given an initial condition(IC), you'll never know what the c is. In the OP the IC is v(2)=16, using that bit of info, you can find c.

VERY important that you put the c down and sovle for it whenever you are given initial condition.


Now you have 2 very simple tools (you'll learn LOTS more tool in your calc class). You can go from acceleration and up (increasing power of x) and displacement and downard (decreasing power of x).

now a simple exercise if to take the power rule and start diff/int. your 'formulas' in teh OP. You'll figure out that there formulas are nothing more a statement of when then formulas apply. The golden rule of doing well in intro physics is never plug and chug into equations without understanding WHY the equations are the way it is.


CLIFFS:
PHYSICS IF FUN!!!
displacement<-->velocity<-->acceleration
where --> is derivation, and <-- is integration.
don't forget +c when you do integration, solve!


edit: flipped the direction "now you have the tool to go from left to right "
 

Wnh5001

Senior member
Dec 1, 2005
408
0
76
ok thanks for posting all this info, i searched for some physics sites online, and am planning to get extra tutoring. ive been re-reading this crappy txtbook for a while now.. i will read over what u wrote right now!!.. i cant fall behind =(...
 

jchu14

Senior member
Jul 5, 2001
613
0
0
One last bit about tutoring before I go to bed. Assuming that you go to UT Austin (this looks awfully like UTHW), there are many resources for you, especially if you're an engineer.

1) UTLC (UT learning center) offers free drop-in tutoring almost all day all week. There's also free 20 hour /semester of 1 on 1 tutoring if you receive ANY kinds of financial aid (grants, loans, etc) These 1 on 1 tutors are paid tutors have been tested and trained.

2) JCL and Kinsolving engineering study tables. Another place where you can get free drop-in tutoring on weeknights. Natural science people also have their own drop=in tutoring at JCL.

3) your major's (honor) society (AIAA, Sigma gamma tau, IEEE, AICHE, ASME etc). Almost all honor societies I know have free tutoring, either drop-in (often there's so few people tat it's almost like 1 on 1). These people have been in your shoe before and can often offer you advice on all of your classes.

4) Your profs and TAs!!! I know my TA was BORED to hell during his office hours because noone ever came, they can be a great help. =).

Best of luck on your (future) homeworks and don't get discouraged =).
 

Wnh5001

Senior member
Dec 1, 2005
408
0
76
No i go to Penn State University, they have also the university learning center thing + extra assistance. the hw system is from webassign.
Im a 2nd semester freshman, currently exploring computer engineer. and thanks again for the help and confidence. i feel i can actually master this...
 

Cybore

Member
Apr 7, 2005
30
0
61
Can someone post how to solve the problems using the equations posted:
V=Vo + at
X-Xo = Vot + 1/2at^2
v^2=Vo^2 + 2a(X-Xo)
X-Xo=1/2(Vo+v)t
X-Xo=vt-1/2at^2
took physics last year but already forgot how to solve this. jchu14's solution seems kinda hard to comprehend since apparently uses calculus.
 

yhelothar

Lifer
Dec 11, 2002
18,409
39
91
Originally posted by: hypn0tik
Originally posted by: Compnewbie01
Would that mean that the initial velocity (Vo) is 6 m/s and that it would be going 36 m/s at t = 6 seconds? Since it was 16 m/s at t = 2 seconds, it accelerated 10 m/s during that time.

That's exactly what I got.

V = Vo + at
Sub in the point you know:
16 = Vo + 5 (2)

Vo = 6.

Sub in second point:

V = 6 + 5 (6) = 36m/s.

Alternate solution:
a(t) = 5
Integrate to find v(t)

v(t) = 5t + c
v(2) = 16 --> c = 6

v(6) = 5(6) + 6 = 36m/s.

Yup, I'm doing that now in my entry level physics. It's not calc, it's just algebra.
 

Wnh5001

Senior member
Dec 1, 2005
408
0
76
Ok, ive read it and taken down the examples, seems like i can understand the derivatives part, with the power rule and that a derivative of constant=0. i understand the integrals part a little. im goin to try and learn some more from my calculus txtbook and practice more examples.
 

DaShen

Lifer
Dec 1, 2000
10,710
1
0
Originally posted by: jchu14
Originally posted by: PottedMeat
Heh is this the UT Physics Homework Service?

haha i was thinking that the whole time. I know UTHW all too well.

LONGHORNS REPRESENT! Aerospace Engineering c/o '08 (hopefully!) =)

Hahaha, I remember that stuff. Hilarious.
 

SaturnX

Diamond Member
Jul 16, 2000
3,415
0
76
Originally posted by: Wnh5001
here is another problem not worth anything though, do it if u are bored and need to refresh your physics lol...>_>

"A drowsy cat spots a flowerpot that sails first up and then down past an open window. The pot was in view for a total of 0.45 s, and the top-to-bottom height of the window is 2.50 m. How high above the window top did the flowerpot go?"

i already tried to answer it and used up my five total tries. my setup looked like 2.5=.225v-1/2(9.8m/s^2)(.225)^2 and solved for v, inputted back into some equation to find that displacment above the window = 4.09 m but it seems it was wrong, meh, maybe wrong sig figs.. >_<


Here's this solution:

You set up the initial equation:

2.5 = 0.225Vo - (0.5)*(-9.8)*(0.225)^2
Solve for v

you should get:

Vo = 10.00

Now using the equation:

v^2=Vo^2 + 2a(Y-Yo)

you know at that at the highest point, V = 0, but a = -g, and Y-Yo = Ymax

Ymax = (Vo)^2 / 2g

So Ymax = 5.10

Distance travelled ABOVE the windows = 5.10 - 2.50 = 2.60m

Now this is taking the base of the window as being 0 m

The trick in the question, as you seen to understand, that the time given is for both up and down travel, so you only use half for one direction.

The next trick is using the correct sign for the accelerations due to gravity. When an object is travelling upwards it experiences -9.8m/s^s, NOT postive accel.

--Mark





 

KillerCharlie

Diamond Member
Aug 21, 2005
3,691
68
91
Has anyone here (namely engineers) have an introductory "calculus-based physics" where you actually USED calculus other than integrating/differentiating simple functions to find displacement/velocity/acceleration? It's really a joke... they should call it algebra-based physics.

At least in the rest of the engineering classes we got our fair share of calc, ODEs, and PDEs. CFD FTW!
 

KillerCharlie

Diamond Member
Aug 21, 2005
3,691
68
91
Originally posted by: jchu14
Originally posted by: PottedMeat
Heh is this the UT Physics Homework Service?

haha i was thinking that the whole time. I know UTHW all too well.

LONGHORNS REPRESENT! Aerospace Engineering c/o '08 (hopefully!) =)

I'm aerospace too... here's one equation (inequality) I'm sure you've learned by now:

Aerospace Engineering > *

:)
 

jchu14

Senior member
Jul 5, 2001
613
0
0
Originally posted by: Cybore
Can someone post how to solve the problems using the equations posted:
V=Vo + at
X-Xo = Vot + 1/2at^2
v^2=Vo^2 + 2a(X-Xo)
X-Xo=1/2(Vo+v)t
X-Xo=vt-1/2at^2
took physics last year but already forgot how to solve this. jchu14's solution seems kinda hard to comprehend since apparently uses calculus.


Unfortunately, these equations don't apply in problem the OP posted. These equations ONLY works when the acceleration is constant. It's very useful in problems like projectile motion where the only force exerted on the object is a constant -9.8 m/s^2 due to gravity.

When the acceleration is non-constant (function of time, velocity, displacement etc), you need calculus to solve it.

Originally posted by: KillerCharlie

I'm aerospace too... here's one equation (inequality) I'm sure you've learned by now:

Aerospace Engineering > *

:)

kick-ass =). Aerospace engineers are so damn cool and good at phyics to boot! :p
 

Wnh5001

Senior member
Dec 1, 2005
408
0
76
Originally posted by: SaturnX
Originally posted by: Wnh5001
here is another problem not worth anything though, do it if u are bored and need to refresh your physics lol...>_>

"A drowsy cat spots a flowerpot that sails first up and then down past an open window. The pot was in view for a total of 0.45 s, and the top-to-bottom height of the window is 2.50 m. How high above the window top did the flowerpot go?"

i already tried to answer it and used up my five total tries. my setup looked like 2.5=.225v-1/2(9.8m/s^2)(.225)^2 and solved for v, inputted back into some equation to find that displacment above the window = 4.09 m but it seems it was wrong, meh, maybe wrong sig figs.. >_<


Here's this solution:

You set up the initial equation:

2.5 = 0.225Vo - (0.5)*(-9.8)*(0.225)^2
Solve for v

you should get:

Vo = 10.00

Now using the equation:

v^2=Vo^2 + 2a(Y-Yo)

you know at that at the highest point, V = 0, but a = -g, and Y-Yo = Ymax

Ymax = (Vo)^2 / 2g

So Ymax = 5.10

Distance travelled ABOVE the windows = 5.10 - 2.50 = 2.60m

Now this is taking the base of the window as being 0 m

The trick in the question, as you seen to understand, that the time given is for both up and down travel, so you only use half for one direction.

The next trick is using the correct sign for the accelerations due to gravity. When an object is travelling upwards it experiences -9.8m/s^s, NOT postive accel.

--Mark

i actually did my calculation wrong at the last part, the answer is 5.11 m.