# Newtons law and canoe question

1. Mar 18, 2014

### 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.

2. Mar 18, 2014

### phinds

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

3. Mar 18, 2014

### cherylrondo

I think he exerts applied force on the canoe

4. Mar 18, 2014

### Staff: Mentor

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?

5. Mar 19, 2014

### Andrew Mason

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

6. Mar 19, 2014

### Staff: Mentor

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

7. Mar 19, 2014

### sophiecentaur

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

8. Mar 19, 2014

### Staff: Mentor

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.

9. Mar 19, 2014

### 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.