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Unformatted text preview: 4.1.2 The little known codicil to Newton's 3rd law
Many books, e.g. Marion, state Newton's laws in something like this form:
1. A body remains at rest or in uniform motion unless acted upon by a
force.
2. A body acted upon by a force moves in such a manner that the time
rate of change of momentum equals the force.
3. If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction.
This statement of 3 is WRONG! The correct statement is
30. If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction AND ALONG THE SAME LINE.
Newton knew this, but modern text book writers have garbled it. B B OK NOT OK!
A A (a) (b) If (b) were possible, then we could take the two particles A and B , nail
one to the hub of a wheel, the other to the center, and watch as the wheel
accelerated up to innite angular velocity.
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 Fall '09
 dion

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