Roller coaster loop conservation of energy?

[Seiya]

A roller coaster car of mass 1500 kg starts a distance H = 25 m above the bottom of a loop 15 m in diameter (Figure 7-24). If friction is negligible, find the downward force of the rails on the car when it is upside down at the top of the loop.

Uhm iwell i figured out the KE and the PE at the 15m height of the loop... and then i tried calculating the centripetal force using F=(mv^2)R ... but i can't figure out how to get the downard force of the rails on the car? :(

Any hints? Thanks

 

[Chi Meson]

How much of the centripetal force will be provided by gravity? How much more centripetal force than that does it need?

 

[Seiya]

provided by gravity will be mg... ill think about it a bit more...

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dammit when i ask such stupid questions i feel really stupid :P

so easy...FN+mg=mV^2/R

thats it.. :(

 

[Chi Meson]

Not easy, hard. Physics is hard. It only seems to be easy after the fact.

 

[dekoi]

Physics is not hard.
If anything, they try to make things simpler than in any other science... so much assuming happens, such as: "there is no air resistance", and "there is no friction", and "it is a perfect circle".

We're not living in a utopia! Imagine if all that was taken into account in the homework questions, then physics could be called hard.

I know I also post questions in this forum, I'm not some "genius", but overall as long as you take the time to learn the concept, nothing is hard.

Of course some questions will be tricky, but that happens in any subject.

Just had to comment

 

[Chi Meson]

Physics is not hard.
If anything, they try to make things simpler than in any other science... so much assuming happens, such as: "there is no air resistance", and "there is no friction", and "it is a perfect circle".

We're not living in a utopia! Imagine if all that was taken into account in the homework questions, then physics could be called hard.

I stick with my statement. Physics is hard. That's why we (teachers at the intro level) try our hardest at making things as simple as possible. I'm glad you think it is easy. Go with it. For some people it always makes the most perfect sense. FOr some people it is the easiest class they ever took in high school (it was for me). But I am here to tell with with several years of experience as a HS teacher, that some very very smart people simple do not understand what is going on. Most people do not study physics, and of those who do, most people will tell you how hard it is.

 

[dekoi]

You are a high school teacher?

I'm actually in university right now, my program is Physics and its Applications (Physics/Astronomy/Math), and I am going to be a high school teacher also.

Where I live in Ontario there is a high demand for Physics/Math teachers right now-- I'm not really sure why. Maybe not many people want to do it?
Do you mind taking a look at my electric charges question? I don't understand why nobody's replying to it. Surely someone in this forum must know how to solve it.