Newtons law and canoe question

[cherylrondo]

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.

 

[phinds]

does the person moving forward exert any force on the canoe?

 

[cherylrondo]

I think he exerts applied force on the canoe

 

[Drakkith]

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?

 

[Andrew Mason]

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

 

[Drakkith]

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.

 

[sophiecentaur]

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.

 

[Drakkith]

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.

 

[sophiecentaur]

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.