Gravitation doubt

  • #1
Gravitation doubt....

This is supposedly a simple conceptual problem but I still have a doubt about this:

A satellite is orbitting the earth with a constant speed when suddenly the force of gravitation disappears(no explanation as to how that happened!). Then the satellite will:

a) continue to move in its orbit with the same velocity.
b) move tangentially to the orginal orbit with the same velocity.
c) become stationary in its orbit.
d) move towards the earth.

Now I know I can safely eliminate (a) and (d).

My answer was (b), since by conservation of momentum in the absence of any external force, the body will continue moving in a straight line with constant speed.

But the MCQ book I was referring to says that the answer is (c) giving the explanation that when the gravitational force becomes zero, then centripetal force on satellite becomes zero and therefore, the satllite will become stationary in orbit.

I am puzzled..... should i throw the book out of my window or is its answer correct? :confused:
 

Answers and Replies

  • #2
463
7
According to Newton's law of inertia, an object already moving will continue to move in a straight line at a constant speed unless acted on by an outside force. Thus, to make an object move in a circular path, an outside force must act on the object. Centripetal force is the force that pushes or pulls an object inward so that it will move in a circular path. The word centripetal means to seek the center.

When you whirl a stone tied to a string in a circle, you must constantly pull on the string to keep the stone from flying off in a straight line. The force the string applies to the object is the centripetal force. Centripetal force operates in other scenarios as well, acting, for example, on a car to make it travel around a curve. In this case, the force is produced by the friction between the tires and the pavement. Likewise, Earth's gravity exerts a centripetal force on a communications satellite that prevents it from flying off into space by keeping it in orbit.
http://www.teachersdomain.org/3-5/sci/phys/mfe/zcentrip/
This says that the centripetal force exerted on a satellite is created by the Earth's gravitational force. Without a gravitational force, there would be no centripetal force and the satellite would fly off on a tangent from the point where gravity disappeared. I'm hesitant to argue with a textbook, but this source indicates that the answer the text gives is wrong. It happens sometimes.
 
  • #3
Gza
444
0
nu_paradigm said:
This is supposedly a simple conceptual problem but I still have a doubt about this:

A satellite is orbitting the earth with a constant speed when suddenly the force of gravitation disappears(no explanation as to how that happened!). Then the satellite will:

a) continue to move in its orbit with the same velocity.
b) move tangentially to the orginal orbit with the same velocity.
c) become stationary in its orbit.
d) move towards the earth.

Now I know I can safely eliminate (a) and (d).

My answer was (b), since by conservation of momentum in the absence of any external force, the body will continue moving in a straight line with constant speed.

But the MCQ book I was referring to says that the answer is (c) giving the explanation that when the gravitational force becomes zero, then centripetal force on satellite becomes zero and therefore, the satllite will become stationary in orbit.

I am puzzled..... should i throw the book out of my window or is its answer correct? :confused:



I would like to ask who wrote that MCQ book? Think of the problem in simpler terms with everyday objects. Take a rock tied to a string for instance. The string in this case, being analogous to the force of gravity in keeping the rock (or the planet) in circular motion, by providing the centripital acceleration to perform such a manuever. I'm sure you've swung an object around that's tied to a string when you were a kid, and i'm pretty confident that upon release, you've hardly noticed the object continue to execute circular motion! It flies tangentially away from the circular flight-path traversed by the rock/planet.
 
  • #4
Gza
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0
huckleberry, quick on the draw as always, lol :rofl:
 
  • #5
Meir Achuz
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Throw out the book.
 
  • #6
Thanks people.... the book's outta here... it wasn't a textbook anyway. Just had questions for practice.
I've come across other mistakes too!!
As for who wrote it... u don't need to panic. It's an Indian publication and here often cheap substitutes for good books have errors. We're used to it!
I just needed some practice for questions so went and got it.... now I need to get another one... damn.
 
  • #7
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I hate to break this to any naive kids out there, but school textbooks are full of rubbish. The late Mario Iona spent decades trying to have corrections made, and he was largely ignored by the authors and the publishers. Some people just want to run a printing press.
 
  • #8
SGT
More wrong ideas from teachers and textbooks here.
 
  • #9
Meir Achuz
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"Throw out the book." Not all books.
 
  • #10
But hey, it doesn't mean the author is bad, it just means he's special! Some greek philosopher (can't remember who) was certain that a stone being throwed would move in it's initial direction until it loses it speed. Then it would fall strait down along a vertical line to earth. Completely against common sense, of course. But this might be just rubbish aswell, just as the crap about Einstein being bad in maths in school. And I think I read it in the textbook I had six-seven years ago =)
 
  • #11
Chi Meson
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I have yet to look through a SAT II prep book in physics that does NOT contain either a totally bad question, or totally wrong explanation.
 
  • #12
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Havent had a good ol'e book burning in about 50 years!
 
  • #13
208
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skeptic said:
I hate to break this to any naive kids out there, but school textbooks are full of rubbish. The late Mario Iona spent decades trying to have corrections made, and he was largely ignored by the authors and the publishers. Some people just want to run a printing press.


would the halliday/resnick/walker physics text be a good text with minimum errors?
 
  • #14
ZapperZ
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quasi426 said:
would the halliday/resnick/walker physics text be a good text with minimum errors?

I think what is meant by "school texbooks" here are high school texts. Unless Halliday/Resnick have started writing high school physics text, college level intro physics are usually quite good in weeding out the obvious, even not-obvious errors. They have to, or else the school, or even the professor teaching the class, will not adopt it. There is no single school board or school administrator that can force the use of a particular college text (at least, not in US universities). So one full of errors would not last long.

Zz.
 
  • #15
13
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SGT said:
More wrong ideas from teachers and textbooks here.
Ironically, there is a glaring mistake on that website - one rather erroneous chemical reaction!
 

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