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College Physics HELP! I'm Lost! Throw me a life Ring!

S

Shippy

Golden Member
Hi. I'm taking college Physics (2030) and am lost. I was wondering if anyone knew a web site or good book (like a cliffnotes or something) that could help me out. I just had my Midterm yesterday and I think I blew it. I have only 5 weeks until my final. I don't want to end up paying for this class. Any and all help is appreciated.


Shippy

PS: I never took Physics in High School (graduated in 1990), and like an idiot I skipped the introduction to Physics class (1030).
 
try the website of the publisher of your book. they usually have a good series of tools to help. one of my biology books offered a free cd rom so went ahead and got it, hey it was free. it had all these really cool animations and narrations on the book, was pretty slick. if you are lucky you may be able to find something like that.
 
The book is called College Physics. I believe it's algebra based (not calculus). Pretty much everything we do is word problems and knowing which formula to use for the given variables. That's where I'm stuck. Here's a couple of examples:

A cannon of Mass 500 kg is fired while on a level road 50 m away from a hill of 11 degree slope. The bullet, of mass 50 kg, leaves the cannon traveling at 250 m/s in the direction away from the hill. The cannon rolls back towards the hill, then starts rolling up the hill, which is frictionless. The road, on the other hand, has a coefficient of friction of 0.33 between the cannon and the pavement. How far up the hill does the cannon reach before coming to a stop?

This question here kicked my butt. Another one that has me in a daze is:

In the first 15 m of a 500 m sprint in the Winter Olympics and Ice Skater pushes off from the ice with a force 1.5 times her weight applied at an angle of 41 degrees with th ehorizontal. She then maintains a constant velocity for the rest of the race. What is her time for the race?

Now, I can do simple conservation of energy problems, but these complex ones have me stumped. That's why I was looking for some tutorials or something that could break this down to me easier. Any direction would be appreciated.

Shippy
 
Hehe...

Me and some of my buddies at PSU used to joke about opening a Physics store...

In the store we would incluse such things as...

-Frictionless surfaces
-Strings of negligible mass
-Perfect Vacuums

and the like.

Look on the bright side... IMO you are on the easier portion of the physics universe. I loved that stuff, but hated the portion on fields and waves.

You should be able to find some really good material to help you. Check for one of those couple of page laminated study guides on the subject matter. I have a degree in Chemistry, and have one that I still use today for everyday calculations.

check Barcharts Study Guides

Hope that helps.
 
Here is the website for the book Berkeley used for its honors quantum course:

http://www.whfreeman.com/physics/

For E&M, there is a book that YOU MUST HAVE... it's "Electricity and Magnetism, Berkeley Physics Course Volume 2" by Edward Purcell. It is absolutely the best physics text that I've encountered.

God I LOVE physics! Can't believe I'm done with it.
 
U can always ask my dad, he is PhD in Physics, but if u want him to tutor u, it's gonna cost ya. Probably say around $50/hr! If ur interested, let me know!
 
Ack, mecahnics is useful but so annoying, lol. I am learning heat and optics right now and it's a little annoying but interesting.

BTW, shippy, i think both of those problems sound like momentum problems, possibly dealing with conservation of energy, such as kinetic or potential energy.
 
The dirty little secret to doing ok in physics classes is that you don't have to understand whats going on, you just have to know when to use what equation to get the right answer.
 
That's what it looks like I'm going to have to do. Since I only go to classes 2 nights a week it's kind of hard. Work 40+ hours then go home and taking care of my wife and kid. Then going to class. I think I may have bitten off more than I can chew.
 
those problems are easy man, you just need some conservation of momentum, conservation of energy, and speed distance equations. It's cake.
 
you know mass of cannonball and velocity, thus you know the Kinetic Energy (KE).
this is equal to the KE of the cannon moving in the opposite direction. you know the mass, thus you know the velocity (initial) of the cannon.

you know the weight of the cannon. you know coeffiecient of friction (assume to be dynamic CoEf.) Thus you know the force of friction operating in the opposite direction of the cannon motion. If you know force and mass, you know acceleration. If you know acceleration, you know what speed it'll be going when it hits the hill 50 meters away (one of the basic kinematics equations)

You know velocity of cannon at base of the hill. Along with the mass, you know it's KE at the base of the hill. That's gonna be equal to the potential energy the cannon has when it's at the top of the roll. And since you know it has an eleven degree slope, finding the distance it rolls up is pretty trivial.


problem 2:

For the first 15meters, she pushes with 1.5 times her weight. You know her mass, you know g, thus you know her weight. Multiply by 1.5, you know the force F.
F=ma Thus you know a, acceleration.
you know acceleration for a given distance, thus you know her final velociy at the end of 15 meters, and you know the time it took her to do those 15 meters.
using final velocity as the speed for the rest of the race (485 meters) you know the rest of the time.
add the two times together and you're done.


I think i did em right.. correct me if i didn't

-patchy
 
ApacheXMD,
After going to the movies to see The Sixth Day (I give it 2 thumbs up!), I came back home and looked at the problems some more. I believe I figured out how to do problem # 2. The only problem with your explanation is that I don't have her Mass. All i know is that the force she exerts is 1.5 times her wight (weight = mass * gravity). Anyways, here's what I did:

F*D=1/2MV^2 (The Force * the Distance = 1/2 the Mass * Velocity Squared)
1.5*M*9.8m/s^2*cos41*15m=1/2MV^2 (The Mass will cancel out!)
166.4m^2/s^2=1/2V^2
332.8m^2/s^s=V^2
18.2m/s=v (her final velocity at 15m)

I know V=D/T (Velocity = Distance / Time)
18.2m/s=485m/t
26.64s=t (the time it takes her to travel 485m)

To find the time it took her to travel the initial 15m I needed to find the accelleration:

a=(v.f^2-v.i^2)/2D (The final Velocity squared - Initial velocity squared, divided by 2*D)
a=((18m/s)^2-(0m/s)^2)/2*15m
a=(324m^2/s^2)/30m
a=10.8m/s^2 (For the 15m she accellerated at 10.8 meters/seconds squares)

Then I put the accelleration into this:

v.f=v.i-at (Velocity final = Velocity initial - acelleration*time)
18.2m/s=0m/s+10.8m/s^2)t
1.69s=t

Then I added the two times to get: 28.33s so she would win the silver medal.

Does this sound right?
I'm going to try your advice about the 1st problem. Thanks for giving me some insight!
 
ApacheXMD,

Ok, for the 1st problem. 1st I found the velocity of the cannon:

m.1v.1+m.2v.2 = 0 (The mass of the cabullet * velocity + the mass of the cannon * velocity = 0)
50kg*250m/s + 500kg*v.2 = 0
12500kgm/s = -500kg*v.2
25m/s = v.2 (the cannon moves 25 m/s)

Next, to find the velocity after the cannon travels 50 meters:

v.f^2 = v.i^2 + 2ax (the final velocity squared = the initial velocity squared + 2*acelleration*distance)
the acelleration can be broken down as coeffecient of friction * gravity.
v.f^2 = v.i^2 + 2*uk*g*x
v.f^2 = (25m/s)^2 + 2(.33)*(-9.8m/s^2)*(50m)
v.f^2 = 625m^2/s^2 - 323.4m^2/s^2
v.f^2 = 301.6m^2/s^2
v.f = 17.4m/s (the velocity after traveling the 50m)

Finally, how far up the hill does the cannon travel. I know the initial velocity and the final velocity has to = 0, so:

v.f^2 = v.i^2 + 2ax (this time the acelleration comes from gravity * sin11, the angle of the hill)
v.f^2 = f.i^2 + 2(sin11)*(-9.8m/s^2)x
0 = (17.4m/s)^2 - (3.74m/s^2)x
(3.74m/s^2)x = 302.76m^2/s^2
x = 80.95m

How does this look to you? Thanks again for all your help!

 
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