# M4 - Relative motion

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#1
A river is running at 1.5ms^-1. A man who can row at 2.5ms^-1 in still water wishes to cross to the nearest point on the opposite bank of a river which is 200m wide.

Velocity of the water is 1.5ms^-1 relative to earth. That is clear.

Why isn't the velocity of the man 2.5ms^-1 relative to earth? This book says its relative to the water. In this situation the water is running, shouldn't the velocity of the man relative be greater than 2.5ms^-1 for him to end up at the nearest point on the opposite side?
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5 years ago
#2
(Original post by AlmostNotable)
A river is running at 1.5ms^-1. A man who can row at 2.5ms^-1 in still water wishes to cross to the nearest point on the opposite bank of a river which is 200m wide.

Velocity of the water is 1.5ms^-1 relative to earth. That is clear.

Why isn't the velocity of the man 2.5ms^-1 relative to earth? This book says its relative to the water. In this situation the water is running, shouldn't the velocity of the man relative be greater than 2.5ms^-1 for him to end up at the nearest point on the opposite side?
You row relative to the water
you move as far an observer on the ground different to how you row
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#3
(Original post by TeeEm)
You row relative to the water
you move as far an observer on the ground different to how you row
So if the river is running at 100ms^-1, the man's velocity would still be 2.5ms^-1 relative to the water?
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5 years ago
#4
(Original post by AlmostNotable)
So if the river is running at 100ms^-1, the man's velocity would still be 2.5ms^-1 relative to the water?
yes
this describes his physical speed...
however what an outside observer sees is different
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