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A block is at rest on an ramp

  1. Mar 9, 2015 #1

    tony873004

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    A block is at rest on a ramp with an incline of 30 degrees above the horizontal. Find the force of static friction between the block and the ramp.

    One might be tempted to say that it equals the resultant force down the ramp: mg sin30.

    Or...
    A book is at rest on a table. What is the net force acting on the book? One might be tempted to say 0.

    A few years ago on this forum we had a discussion about the word "rest" meaning v = 0, but not necessarily acceleration or any of the other derivatives of velocity.

    This block might have been slid up a frictionless ramp, and is momentarily at rest at its highest point, in which case there is no static friction.

    Likewise, the book might be sliding across a frictionless table into a breeze that momentarily brings it to rest before reversing its direction. Net force is not 0.

    Is there a better word than "rest" to describe an object that is not moving and not accelerating and not jerking, etc...

    "Stationary", "motionless"?
     
  2. jcsd
  3. Mar 9, 2015 #2

    Nugatory

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    "Rest" is fine. "Momentarily at rest" is customarily used for the other cases you mention, and the only complication is that people sometimes omit the "momentarily" qualification when it is obvious from the context that we're discussing a situation in which dv/dt is non-zero even though v is zero.
     
    Last edited: Mar 9, 2015
  4. Mar 9, 2015 #3

    Simon Bridge

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    "at rest" is the standard description, it is well known as are the problems with it - people usually figure out what is intended from context.
    If there is a possibility of misunderstanding, it is incumbent on the author to say more.

    Note: "stationary in the frame of..." is very common in special relativity.
    If you want to see how possible confusions arise and are dealt with, that is a good field to start with.
     
  5. Mar 9, 2015 #4

    tony873004

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    I found the old thread from 2012.
    https://www.physicsforums.com/threads/what-does-rest-mean.631292/
    Back then when I asked, I was getting my credential to teach high school physics. Now I teach and often make up my own questions. Even though the students might not notice, it's my intention to phrase questions as accurately as possible. After being told in that thread (and subsequently looking up the definition and verifying) that rest has nothing to do with the derivatives of velocity, I phrased some questions in a manner that confused the students: "A block is on an incline. It is at rest and is not accelerating..." Simply stating it was at rest would have been clearer to them, but it bothered me because according to the definition, it would have been ambiguous. It seems textbooks use "rest" in their questions to mean velocity and its derivatives = 0. So I guess I'll just follow convention and do the same. I was just wondering if there was a better word for staying put and continuing to stay put? Thanks for the replies!
     
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