Newtons law and canoe question

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Discussion Overview

The discussion revolves around the application of Newton's laws of motion to a scenario involving a person moving in a canoe. Participants explore the reasons behind the canoe moving in the opposite direction when the person steps forward, examining which of Newton's laws best explains this phenomenon.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that Newton's first law applies, noting that the canoe remains at rest until acted upon by an external force.
  • Another participant questions whether the person moving forward exerts any force on the canoe.
  • A different participant asserts that the person exerts an applied force on the canoe.
  • Some participants argue that while the first law explains the canoe's state of rest after both the person and canoe stop moving, it does not clarify why the canoe moves when the person steps forward.
  • There is a discussion about the center of mass, with some participants suggesting that the first law indicates the center of mass cannot change its state of motion without an external force.
  • Others contend that the original question does not require a focus on the center of mass and can be answered more simply.
  • One participant mentions that Newton's third law might also be relevant, indicating that all laws are interconnected in this context.

Areas of Agreement / Disagreement

Participants express differing views on which of Newton's laws best explains the canoe's movement, with no consensus reached on a definitive answer. Some emphasize the importance of the center of mass, while others argue for a more straightforward explanation.

Contextual Notes

Participants reference external sources, such as Wikipedia, to support their claims, indicating a reliance on definitions and interpretations of Newton's laws. The discussion reflects varying levels of understanding and assumptions about the laws of motion.

cherylrondo
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If you can get to your feet in a canoe and move towards the front of the canoe, it will move in the opposite direction. Explain why.

I thought Newton's first law applies to this question (after the person steps on the canoe, the canoe still maintain at rest), but i am not sure how to explain it.
 
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does the person moving forward exert any force on the canoe?
 
I think he exerts applied force on the canoe
 
Per wiki: http://en.wikipedia.org/wiki/Newton's_laws_of_motion

1. First law: When viewed in an inertial reference frame, an object either remains at rest or moves at a constant velocity, unless acted upon by an external force.

If the canoe moves backwards when you step forwards, then a force was applied to the canoe, so the first law isn't we're looking for here. It DOES explain why the canoe stays are rest once both you and the canoe stop moving, but it doesn't explain why the canoe moved.

Look at the other two laws in the link above. Which one do you think explains why the canoe moved backwards when you stepped forwards?
 
Drakkith said:
Per wiki: http://en.wikipedia.org/wiki/Newton's_laws_of_motion



If the canoe moves backwards when you step forwards, then a force was applied to the canoe, so the first law isn't we're looking for here. It DOES explain why the canoe stays are rest once both you and the canoe stop moving, but it doesn't explain why the canoe moved.

Look at the other two laws in the link above. Which one do you think explains why the canoe moved backwards when you stepped forwards?

Would the first law not say that the motion of the centre of mass of the canoe/person cannot change unless an external force acts? So, for example, the canoe/person are initially at rest in an inertial reference frame (the reference frame of the centre of mass of the canoe/person). If the person changes position in that frame, the canoe must also change position so that the centre of mass remains unchanged.

While the first law does not explain the mechanism by which the canoe changes position to offset the change in position of the person, it does seem to explain the result: the centre of mass cannot change its state of motion.

AM
 
Andrew Mason said:
Would the first law not say that the motion of the centre of mass of the canoe/person cannot change unless an external force acts?

Sure, but that is not what the OP was asking.
 
Drakkith said:
Sure, but that is not what the OP was asking.

I think it is. He just hasn't mentioned the centre of mass - needed to be pointed in that direction, perhaps.
 
sophiecentaur said:
I think it is. He just hasn't mentioned the centre of mass - needed to be pointed in that direction, perhaps.

What? The op's question is simply "why does the canoe move when I step forward". The simplest answer has nothing to do with center of mass and whatnot. Of course, a complete understanding of the larger scenario would.
 
You could say it's more to do with N3, I suppose. But they're all tie together really. I don't feel strongly either way.
 

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