The passengers in a roller coaster car feel 40% heavier than their true weight as the car goes through a dip with a 34 m radius of curvature. What is the car's speed at the bottom of the dip?

I don't really even know how to get started on this problem, this is like what i have so far.. I don't think im even pluggin the numbers in correctly... i got so confused on this chapter D:>

PS: Double click to enlarge.

BuGzlToOnl

United States5918 Posts

December 09 2008 00:33 GMT

Is w suppose to be your angular momentum?

YPang

United States4024 Posts

December 09 2008 00:34 GMT

On December 09 2008 09:33 BuGzlToOnl wrote:
Is w suppose to be your angular momentum?

wapp = apparent weight

w=weight

micronesia

United States24745 Posts

December 09 2008 00:40 GMT

The passengers in a roller coaster car feel 40% heavier than their true weight

This means the normal force exceeds the weight obviously. Specifically, Fn = 1.4 * W

as the car goes through a dip with a 34 m radius of curvature.

r = 34m and the direction of the centripetal acceleration (and net force) is up

What is the car's speed at the bottom of the dip?

We need the free body diagram.

^ Fn
|
|
[ ]
|
|
\/ W

Newton's Second Law: Fnet = ma
+ Show Spoiler +

Fn-W = m v^2 / r

Substitute in Fn = 1.4 W

1.4W - W = mv^2/r
W(.4) = mv^2/r
.4*mg=mv^2/r
0.4*(9.8) = v^2 / 34
v = 11.54 m/s

In my head I hope that's correct XD

Agro_Z

United States138 Posts

December 09 2008 00:41 GMT

Hmm, which physics are you in? I'm in physics B, but in my circular motion chapter, we never had to deal with apparent weight.

vAltyR

United States581 Posts

December 09 2008 00:42 GMT

Don't think about it in terms of force. Think about it in terms of acceleration. Try it again with just acceleration instead of force, if you still have problems check the spoiler.

+ Show Spoiler +

40% heavier than normal weight is 1.4*g, where g is the acceleration due to gravity at the earth's surface. centripetal acceleration is determined by the formula a=v^2/r. Therefore, 1.4*g=v^2/r. you know g and r, so it's just a matter of plugging it in and solving.

micronesia

United States24745 Posts

December 09 2008 00:49 GMT

On December 09 2008 09:41 Agro_Z wrote:
Hmm, which physics are you in? I'm in physics B, but in my circular motion chapter, we never had to deal with apparent weight.

This sounds bad. Double check an ap physics b review book I suggest.

Cobalt

United States441 Posts

December 09 2008 01:21 GMT

On December 09 2008 09:49 micronesia wrote:

Show nested quote +

This sounds bad. Double check an ap physics b review book I suggest.

My physics class hasn't even gotten past ballistic motion, and mine is the highest physics class my school offers.

Kentor

United States5784 Posts

December 09 2008 01:35 GMT

The weight you feel is due to the normal force acting on you. There are only two forces on you at the dip, that is N going up and mg going down. The net force has to equal mv^2/r since it's assumed that the car is traveling on a circle. So
N - mg = mv^2/r
1.4mg - mg = mv^2r
.4g = v^2/r
v=sqrt(.4rg)

Empyrean

17020 Posts

December 09 2008 01:47 GMT

#10

On December 09 2008 10:21 Cobalt wrote:

Show nested quote +

My physics class hasn't even gotten past ballistic motion, and mine is the highest physics class my school offers.

This late in the year? Even for high school level AP Physics courses, kinematics should've been finished by three weeks at the latest. The AP Physics tests aren't exactly cakewalks, either. They're much more difficult than, say, the AP Calculus tests or the Chem/Bio tests (the non-hard science APs such as history, gov, psych, enviro sci etc., are much easier, don't even worry about them). Honestly, AP Physics B is probably the most difficult out of the AP tests, followed by AP Physics C (paradoxically enough).

If you're planning on taking the test for any sort of college credit, I would definitely recommend seeing what sort of outside tutoring you could get (at a reasonable price, of course. Try bored college students in the major?) and forming a study group yourself to go over concepts in review books.

If not, then just enjoy your physics class. Physics is one of the most interesting things you'll ever learn in school, in my opinion.

Also AP Language tests are a bitch if you aren't a native speaker.

Oh, and for the next two days I absolutely hate physics because my final is on Wednesday and I haven't started studying ahhhhhhhhh fuck my life.

micronesia

United States24745 Posts

December 09 2008 02:32 GMT

#11

On December 09 2008 10:47 Empyrean wrote:
Honestly, AP Physics B is probably the most difficult out of the AP tests, followed by AP Physics C (paradoxically enough).

I've heard a lot of people say this and I emphatically disagree :-/

I found the AP Physics B to be relatively easy relative to the C... I actually didn't take the C but I've since graded them :p

Empyrean

17020 Posts

December 09 2008 02:34 GMT

#12

Well, what I've found to be the trend (and definitely correct me if I'm wrong, because you obviously have much more experience in this), is that out of the schools that offer both AP Physics B and C, they do a much better job preparing you for the AP C test because of the sheer quantity of material for the AP B test that many students just can't handle. At least that's what it was like at my school and a few others.

Malongo

Chile3472 Posts

December 09 2008 02:36 GMT

#13

micronesia just solved it perfectly, people of the world learn to do free body diagram before posting.

YPang

United States4024 Posts

December 09 2008 03:02 GMT

#14

oo thanks micronesia! :D

micronesia

United States24745 Posts

December 09 2008 03:13 GMT

#15

On December 09 2008 11:34 Empyrean wrote:
Well, what I've found to be the trend (and definitely correct me if I'm wrong, because you obviously have much more experience in this), is that out of the schools that offer both AP Physics B and C, they do a much better job preparing you for the AP C test because of the sheer quantity of material for the AP B test that many students just can't handle. At least that's what it was like at my school and a few others.

Hm I guess if you have a strong math background then the ap physics c isn't that bad. I didn't take calculus in high school so I couldn't take ap physics C. I took regular physics a year ahead, and then took AP Physics B so since half of it was repeat... the 'sheer quantity of material' didn't feel so bad to me.

Pendragon

Canada23 Posts

December 09 2008 03:25 GMT

#16

On December 09 2008 11:36 malongo wrote:
micronesia just solved it perfectly, people of the world learn to do free body diagram before posting.

ACII free body diagrams aren't cool.

SpiritoftheTunA

United States20903 Posts

December 09 2008 03:26 GMT

#17

the normal force is always equal to the apparent weight of anything, and in this case, its 1.4 times Fg

BuGzlToOnl

United States5918 Posts

December 09 2008 04:04 GMT

#18

Lol, took my Physics I final this morning at 8AM and although some of what people are saying most of it is already lost. Hoping for at least half of them right.