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If a person on boat moves forward (from left to right of a boat)and if we draw the free body diagram we would observe that 1)mg is balanced by normal reaction 2)friction acts to the left side, on the man ,right? It's because friction always opposes relative motion 3)On the boat ,then,friction acts towards the right,but the boat moves backwards i.e to the left How should I understand this?

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    $\begingroup$ Friction does not cause the boat to move. Force causes the boat to move. Friction merely explains how a force can be transmitted between the person and the boat. $\endgroup$ – Solomon Slow Jun 16 '18 at 14:40
  • $\begingroup$ Could you explain ,more precisely,how it happens? $\endgroup$ – scisyhp Jun 16 '18 at 14:42
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    $\begingroup$ It's the same as when you walk on land. Friction is what enables you to move. Conservation of momentum is what causes the boat to move. $\endgroup$ – Lewis Miller Jun 16 '18 at 14:45
  • $\begingroup$ Don't think in terms of friction force, instead think in terms of forces acting upon the boat. There will be an F=mA force on the boat, caused by the mass of the moving person, and Newton's 3rd. The same would happen if the boat and the moving person were suspended in vacuum. $\endgroup$ – wbeaty Jun 16 '18 at 15:16
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When you walk you are essentially using the ground to propel yourself using the friction force as a medium. However, since the Earth is so big it does not move. Boats, however, are much smaller, so the force caused by you walking is much more evident.

While these types of questions you are talking about can be solved by analyzing the forces, it would be easier to use the center of mass to analyze it (no external force = the center of mass stay at the same spot).

Hope this helps!

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