Math Is Fun Forum
You are not logged in.
- Topics: Active | Unanswered
#1 Re: Help Me ! » I need help with a motion in two dimensions problem. » 2019-02-20 07:55:19
I have to study what you did there. I am familiar with the law of sines, but haven't done anything with it in soooo loong. I'm actually more interested in your alternative method and I don't know why. I'm grateful that you provided me with the alternative. I have to comb through that to figure out what's going on there. Thank you so much. Might have a question about it later. I have to step away from this a bit. I have been up all night burning myself out trying to figure this out.
#2 Help Me ! » I need help with a motion in two dimensions problem. » 2019-02-20 03:14:52
- CIV
- Replies: 4
http://www.nxtbook.com/nxtbooks/ngl/collegephysics11eAP/index.php?startid=72#/76
I was very fortunate to find the very pages on the internet. I'm doing Exercise #39 on page 73. I did Example 3.9 with out any issues, but Exercise 3.9 completely escapes me.
The illustration they provided has me a bit confused. I think the illustration is drawn to accommodate both parts of Exercise 3.9, but I'm not sure because I'm having trouble solving this.
I read the problem carefully over and over and I keep drawing a right triangle to represent the components of the vector and the vector itself, but the answer I'm getting isn't making any sense.
It says the river is flowing east at 5km/h. I made this the x-component which is named Vre(velocity of the river with respect to earth).
The boat is traveling 45deg south of east with respect to the the earth. I made this the vector with a magnitude of Vbe(velocity of boat with respect to earth). It wants me to solve for Vbe.
Im assuming Vbr(velocity of boat with respect to the river) is the y component, as it was for the other two similar problems I solved before this.
I keep ending up with a right triangle with two unknown sides, Vbe and Vbr. I know all the angles. I know that tan(45) = 1, therefore Opp = Adj, so 5km/h = 5km/h. Using Pythagorean Thereom I get Vbe = 7.07km/h, but that's not the answer the book has.
I could't help but assume that Vbr is 10km/h because that was the boats velocity for the last two problems, but if that true the right triangle with the 45deg angle doesn't make sense because both sides would have to be equal.
I'm completely lost and wasted so much time trying to solve this. Please help.
#3 Re: Help Me ! » Physics: Help with understanding Centripetal and Centrifugal forces. » 2018-11-23 18:57:41
Thanks for the replies. I didnt actually know there were replies because I never got an email saying there were replies. Anyways.
Thanks for the link. I read through it and it's clear now.
Thanks for you reply Bob. I understand everything you are saying, i just could not wrap my head around why centrifugal force is fictitious. I get it now and will drop the centrifugal thing and just stick with centripetal.
Thanks.
#4 Re: Help Me ! » Physics: Help with understanding Centripetal and Centrifugal forces. » 2018-11-22 14:53:07
Wait a second...... my book says that an example of a centrifugal force is the normal force preventing a mass from reaching the center of the earth. The ground exerts a normal force keeping you from sinking in to the earth. If I swing a yo-yo around in a circle over and over, the only thing between the yo-yo and the center is the string, but the string is whats exerting a centripetal force (tension) that's trying to pull the yo-yo to the center. There's a force moving the yo-yo away from the center, the centrifugal force, and because there's nothing between the yo-yo and the center to exert this force its therefore fictitious.
Am I right?
#5 Help Me ! » Physics: Help with understanding Centripetal and Centrifugal forces. » 2018-11-22 14:23:48
- CIV
- Replies: 5
Oh my goodness... I've and have been told that centrifugal force is fictitious yet it's mentioned and used to make calculations as if it's real. I'm so confused!
Things I understand:
- Centripetal force is a center-seeking force.
- Centripetal force is the tension of something holding a mass to the center of a circular path. As the angular velocity of this mass increases, so does the centripetal force.
- Centripetal force is observed from a reference point that is not relative to the rotating mass or center of rotation.
- Centrifugal force is a center-fleeing force.
- Centrifugal force is observed from a reference point that's relative to the rotating mass.
Is centrifugal force the lack of centripetal force? What is the force pulling/ pushing a mass away from the center? Centripetal force is pulling/ pushing the mass to the center, what force is moving the mass away from center? It's said that centrifugal is fictitious so..... I don't get it.
#6 Help Me ! » Can someone please help me with this: 2x + 4cosx = 0 » 2017-10-31 21:41:53
- CIV
- Replies: 1
I'm doing some Calculus 1 homework and I've come to a problem that I just cant figure out. I need to find where the slope of the equation below is zero.
I got the derivative without any trouble: 2x + 4cosx.
I can't figure out how to find the solution(s) though. I know that cosx = 0 at PI/2, but that isn't helping me here. I know I can factor out 2 as well, so: 2(x+2cosx) = 0.
What do I do?
#7 Re: Help Me ! » Help with solving the volume of a right triangular pyramid. » 2017-09-27 14:31:15
Im going to have to sit down and do it again. You, myself, and my friend all got different answers. Thanks Bob.
#8 Help Me ! » Help with solving the volume of a right triangular pyramid. » 2017-09-26 12:56:09
- CIV
- Replies: 3
There's a right triangular pyramid with a lateral surface area of 195 root 3 and a slant height of 13. What's the volume?
That's the problem. I got 150 root 3 u^3 as an answer. A friend of mine got 459.86 u^3.
I saw my friends work. I understand some of it, but other parts not so much. Eventually I'll sit down, when I have more time, and figure out what she did.
I just want to get a third person, or more, to try it for an answer. Thanks.
#9 Re: Help Me ! » Mean Value Theorem » 2017-03-18 06:09:34
I made a mistake.... darn it. Sorry.
It's:
It's continuous and I just figured out that it's not differentiable at 0. The slope at zero is undefined.
I always make mistakes like that when posting online. Usually when I post online my brain is hurting from trying to figure things out.
#10 Help Me ! » Mean Value Theorem » 2017-03-18 05:45:46
- CIV
- Replies: 3
The book I'm using says that the Mean Vaule Theorem does not apply with this function:
Why is this? It's continuous and differentiable right?
Using the MVT formula I get:
Then I solve for it's derivative and get:
Then I determine where the slope is 1/2:
I don't understand. The book says as the answer is "Does not apply".
#11 Help Me ! » Whats the official way to calculate percent difference? » 2017-03-14 16:13:38
- CIV
- Replies: 1
I know it to be the difference divided by the average and multiplied by 100. My one professor said to calculate the % difference you basically: 100((a - b) / b). I'm doing linear approximation in my math book and it wants the % difference and it mentioned the same thing: 100((approx - exact) / exact). What is this? I'm confused.
#12 Re: Help Me ! » Help with an optimization problem.... please. » 2017-03-14 16:06:03
I meant 9 / root 13. I still don't follow what your doing there.
#13 Re: Help Me ! » Help with an optimization problem.... please. » 2017-03-13 18:05:37
Yes. I am getting
continuing with my previous post#8
How did you get 9 / root 5? I don't understand what you mentioned.
#14 Re: Help Me ! » Help with an optimization problem.... please. » 2017-03-13 05:26:32
I just solved another optimization problem, part a of this problem, on my own without any help, except checking my calculations online. I didn't google the actual problem to see how its set up. Totally got it right and I'm so happy. Totally awesome:)
The problem: If you had a piece of wire 60 units in length, where would you cut this wire to form a square and a circle with A: the minimum combined area and B: the maximum combined area.
It involved making calculations with pi, it was confusing me a bit. Im not used to working with something like 2(pi)x as a term. Just need to solve for the max now.
#15 Re: Help Me ! » Help with an optimization problem.... please. » 2017-03-13 04:06:06
The book has this as the answer:
This book doesn't usually have wrong answers so Im really trying to exhaust myself here to make sure it's not me.
Again, the book has 8 / root 5 as the answer to part one too. LOL. So I'm really scratching my head here.
#16 Re: Help Me ! » Help with an optimization problem.... please. » 2017-03-13 00:28:45
hi CIV,
Looking again at your working, it seems we are using a different interpretation of part 2. 2 root13 is the distance across the sea from start to finish. So you aren't making any use of the walking speed. I thought the question meant row straight to the shore, as quickly as possible, and then walk 6 miles along the shore. I assumed this as part 2 repeats the walking speed. Why say this if no walking is involved?
Anyway, let's go back to part 1 first. Is the answer we both have considered to be correct by the book?
Bob
Yes. The answer for part 1 is correct. I corrected my typo.
Ok so yea I didn't type out part b word for word because I thought what I wrote was pretty clear, but that OK. Here's exactly what part b says:
If she walks at 3 mi / hr, what is the minimum speed at which she must row so that the quickest way to the restaurant is to row directly (with no walking)?
That's word for word. I probably should of said row directly to restaurant:/ Now you see what I mean about the book mentioning walking speed for part b? I know the distance to the restaurant directly by sea is 2 root 13 and I know what the quickest time is, so all I need is the rate at which to row to at least achieve the quickest time. That would be the minimum rate at which to row, which really isnt all that much faster than the original 2 mi / hr rowing speed.
#17 Re: Help Me ! » Horrible Equations!!!!! » 2017-03-12 16:49:42
Why are we even bothering with homework that is 10 years out of date?
Besides which, I'm not sure I like the PRONTO. THANX .... (1) Pronto seems a bit demanding; (2) Thanks isn't even spelt correctly
.
Bob
Exactly. HA HA HA.
#18 Re: Help Me ! » Help with an optimization problem.... please. » 2017-03-12 13:45:38
You're right, it was a typo. I don't understand what you did and what you got isn't what the book has as the answer:(
#19 Re: Help Me ! » Differntiation » 2017-03-12 03:50:05
This is a question I had asked: http://www.mathisfunforum.com/viewtopic.php?id=23783
I made a serious effort to do the problem and made sure that everyone could see this. It is homework for me, but clearly you can see that I'm trying.
#20 Re: Help Me ! » Differntiation » 2017-03-12 03:46:53
Someone else will have to show you, because I don't see an effort being made here. Just a word of advice... if you keep posting like this, eventually people will stop wanting to help you. You make us feel like we're doing your homework for you. That of course is just my opinion.
#21 Re: Help Me ! » Horrible Equations!!!!! » 2017-03-12 02:44:57
I think he missed his deadline.
Probably.
#22 Re: Help Me ! » Differntiation » 2017-03-12 02:43:46
You first. What have you done to try and solve this? Show us.
#23 Help Me ! » Help with an optimization problem.... please. » 2017-03-12 02:11:06
- CIV
- Replies: 16
It's the rowing and walking problem.
Basically, you're 4 miles out at sea from the closest point on shore and from that point on shore is a restaurant 6 miles away. You row at a speed of 2mi/hr and walk at a speed of 3mi/hr.
A: At what point on the shore should you land to minimize the travel time.
B: If you walk at a speed of 3mil/hr, what's the minimum speed at which you should row so that the quickest way is to row directly.
Now I already searched online for help and I got the answer to part A, which is:
I calculated the quickest travel time as well:
I compared the quickest time with the limits of the interval, that being 0 and 6. When x = 0, T = 4 hours. When x = 6, T is about 3.61 hours.
Now... the second part has me pulling my hair out because I feel like it's such an easy thing to figure out, but I'm not getting what the book has listed as the answer:
So, what I know is that you will be rowing a distance of:
I also know that the quickest travel time is:
So what I need now is the rate at which you must row.
If I use my answer to calculate total direct rowing time I get about 3.49 hours which is what it was before with some rowing and walking. This is the MINIMUM rowing rate. If I use the answer in the book I get 2.88 hours????
Am I missing some here? Is the answer in the book wrong? Did I not understand the question? I wrote part B here just about word for work just in case I misunderstood. I thought it was weird when it read "If you walk at a speed of 3mi/hr", because what does that have to do with the minimum rowing speed for rowing directly to be the fastest route.
Thanks everyone.
#24 Re: Help Me ! » Extreme values » 2017-03-12 01:00:49
The only roots are 0 and 0. So (0,1) is a possible local extreme.
Why you not wote (0,1)
How y axis is 1
The location of the extrema is x = 0. The location of the tangent line, when it's slope is zero, is (0,1)
#25 Re: Help Me ! » Horrible Equations!!!!! » 2017-03-10 03:28:49
If you want the answers, you have to show us your best effort at trying to solve them first.