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November 17th, 2007, 09:25 AM   #1
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Relative Motion Question

Hi guys, been givin this question for a tutorial and tis really annoying me, not sure what it states. would be brill to get some help...

V=velocity

Three cars travelling with constand V along a straight road. car A is travelling in same direction as B but 10 mph faster whilst C is travelling in opposite direction with speed of 100mph relative to A.

What is the V of C relative to B? (90?)
What is the V of B relative to C? (-90?)
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November 17th, 2007, 06:49 PM   #2
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You are correct:


A is going 10 mph faster than B, so B is going 10 mph in the other direction (with respect to A). Since C is going 100 mph in the other direction (with respect to A), C is going 100 - 10 = 90 mph.

I would use the positive number +90 with a qualifier of some sort in both cases; If you follow the reference frame, C with respect to B should be -90, but if you follow the statement that "C ... speed of 100 miles relative to A", than C with respect to B should be +90.

Bloody physicists are always so imprecise with their use of language.
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November 18th, 2007, 12:27 AM   #3
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Ok but the question states A is travelling in the same direction as B.
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