Does a Boat Move When You Walk on It?

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Homework Help Overview

The discussion revolves around the physics of a boat floating on a river and the implications of a person moving on the boat. Participants explore the relationship between the movement of the person and the resulting motion of the boat, considering concepts such as center of mass and external forces.

Discussion Character

  • Conceptual clarification, Assumption checking, Exploratory

Approaches and Questions Raised

  • Participants question whether the boat moves when a person walks on it, examining the implications for the center of mass of the boat-person system. There are discussions about the conditions required for the center of mass to change and the role of external forces.

Discussion Status

The conversation is active, with various interpretations being explored regarding the movement of the boat and the person. Some participants suggest that the center of mass remains unchanged, while others consider the implications of movement in opposite directions to maintain this balance.

Contextual Notes

Participants are working under the assumption of negligible friction between the boat and the river, which influences their reasoning about motion and forces in this scenario.

alyafey22
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If someone is on a boat that rests in a placid river with negligible friction between the boat and the river ,, suppose that person moves a certain distance ,Does that boat move as well , if so why ?
 
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alyafey22 said:
If someone is on a boat that rests in a placid river with negligible friction between the boat and the river ,, suppose that person moves a certain distance ,Does that boat move as well , if so why ?
What do you think?
 
Doc Al said:
What do you think?
I think it will not move
 
alyafey22 said:
I think it will not move
So what happens to the center of mass of the boat+person system?
 
well ,, if the person moves for a certain distance , surely the center of mass will change in position ,but does that mean the boat has to move ?!
 
Can the center of mass of the system change? What is required for such a change?
 
yes it can ,, if we change the positions of the particles of that system with respect to a specified reference point .
 
alyafey22 said:
yes it can ,, if we change the positions of the particles of that system with respect to a specified reference point .
What is required to change the center of mass of a system that is at rest?
 
an external force .
 
  • #10
alyafey22 said:
an external force .
Right! Note that the problem states "negligible friction between the boat and the river". So what must be true about the center of mass of this system?
 
  • #11
Doc Al said:
So what must be true about the center of mass of this system?

it will stay the same before and after the movement of the man .
 
  • #12
alyafey22 said:
it will stay the same before and after the movement of the man .
Exactly. So, if the man moves a distance X closer to shore (say), how can you figure out how much the boat must move in order that the center of mass of the system stay put?
 
  • #13
Doc Al said:
Exactly. So, if the man moves a distance X closer to shore (say), how can you figure out how much the boat must move in order that the center of mass of the system stay put?

so the boat will move the same x-displacement as the man in order to maintain the same center of mass of the system .
 
  • #14
alyafey22 said:
so the boat will move the same x-displacement as the man in order to maintain the same center of mass of the system .
Why would you think that? Are they the same mass?
 
  • #15
Doc Al said:
Why would you think that? Are they the same mass?

there is a problem here because if the boat moves, the man as well will move .
 
  • #16
alyafey22 said:
there is a problem here because if the boat moves, the man as well will move .
Of course. So?

If the boat moves a certain distance to the left, how far must the man move? (Note that if you measure the man's movement with respect to the boat, then you have to add the movement of the boat to find his movement with respect to the shore.)
 
  • #17
Doc Al said:
Of course. So?

If the boat moves a certain distance to the left, how far must the man move? (Note that if you measure the man's movement with respect to the boat, then you have to add the movement of the boat to find his movement with respect to the shore.)

you mean if the man moves towards the right , so he is getting closer to the shore, the boat will move in the opposite direction (to the left ).
 
  • #18
alyafey22 said:
you mean if the man moves towards the right , so he is getting closer to the shore, the boat will move in the opposite direction (to the left ).
Sure. How else can the center of mass remain the same distance to shore?
 

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