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Newton's second and third law questions Where am I wrong?

  1. Jun 25, 2006 #1
    I have some homework that is due on Wednesday (so I have some time to finish this), but I'm trying to get most of it done soon. I'm having trouble answering some Newton's laws questions and I don't understand why my answers are not correct. Here is what I have thus far:

    You push a box horizontally along level ground at constant velocity. The magnitude of the friction force from the ground on the box is EQUAL TO the magnitude of your force BECAUSE OF NEWTON'S SECOND LAW.

    You are standing on a hill where the ground is sloped. The magnitude of the normal force from the ground on you is EQUAL TO the magnitude of the normal force from you on the ground BECAUSE OF NEWTON'S THIRD LAW.

    You are standing at a place where the ground is level. The magnitude of your weight is EQUAL TO the magnitude of the normal force from the ground on you BECAUSE OF NEWTON'S THIRD LAW.

    You push a box horizontally along level ground but friction is causing the box to slow down. The magnitude of the friction force from the ground on the box is GREATER THAN the magnitude of your force BECAUSE OF NEWTON'S SECOND LAW.

    You are facing your (very large and very strong) friend, Commander Worf. Worf gets very angry and pushes you to the ground. Worf remains stationary (a = 0). The magnitude of the force from Worf on you is EQUAL TO the magnitude of the force from you on Worf BECAUSE OF NEWTON'S THIRD LAW.

    You are standing on a hill where the ground is sloped. The magnitude of the normal force from the ground on you is EQUAL TO the magnitude of your weight BECAUSE OF NEWTON'S THIRD LAW.


    The parts in caps are where I have to fill in answers. I have thought every single one of these over and over and I keep coming up with these answers, yet my online homework system tells me that it's wrong. Can anyone tell me where I'm going wrong and why? Any input is appreciated. Thanks.
     
  2. jcsd
  3. Jun 25, 2006 #2

    Hootenanny

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    Here is your problem. Perhaps if you try drawing a freebody diagram, note the following two points;

    (1)The normal reaction force always acts perpendicular to the slope.
    (2)Gravity will always act straight down.

    Perhaps, in future this type of question would be more appropriate in the homework forums.:smile:
     
    Last edited: Jun 25, 2006
  4. Jun 25, 2006 #3
    Ok well I figured it would be MORE THAN, but I entered that and it said I had already tried it before and it was incorrect. Then I tried LESS THAN and it said it was still incorrect.

    I thought this WAS the homework forums? Do you/anyone see any other possibilities for error?
     
    Last edited: Jun 25, 2006
  5. Jun 25, 2006 #4
    Anyone have any idea?
     
  6. Jun 26, 2006 #5
    3) is wrong, normal force and weight ARE NOT action reaction pair. the action reaction pair should be weight and the gravitational attraction you excert on the earth.
    it should be newton first law (or second law) it is quite ambiguous what things they want... well you can try both...

    last one: if the "normal force" does not include friction, then yeah it should be less than and i guess according to newton's first law.
     
  7. Jun 26, 2006 #6

    nrqed

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    I would agree
    I would agree
    I would disagree (tim_lou already explained)
    Iwould agree
    I would agree
    again, a problem here. Those two forces are not action-reaction pairs.
     
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