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Direct proportionality question watch

1. Hello, I feel a bit embarrased for asking this as I ought to know really, but the physics papers I've been sitting have had a few answers which state that 'so and so is not proportional to so and so because the straight line graph does not pass through the origin'. I thought that as long as the graph was linear, one variable was directly proportional to the other; the offset from the origin was due to a constant, i.e. y=mx+c. Am I wrong?

Example: (taken from the Jan 2010 OCR AS G452 Paper) 'Is the frequency directly proportional to the temperature?' Answer: (Test: constant f:T /straight line graph through origin) (1)

2. (Original post by Benjamin.F)
Hello, I feel a bit embarrased for asking this as I ought to know really, but the physics papers I've been sitting have had a few answers which state that 'so and so is not proportional to so and so because the straight line graph does not pass through the origin'. I thought that as long as the graph was linear, one variable was directly proportional to the other; the offset from the origin was due to a constant, i.e. y=mx+c. Am I wrong?

Example: (taken from the Jan 2010 OCR AS G452 Paper) 'Is the frequency directly proportional to the temperature?' Answer: (Test: constant f:T /straight line graph through origin) (1)

no

if y is proportional to x, it is in the form of i.e. no constant added

to be directly proportional, the line has to pass through the origin as well as be linear
3. Oh okay. That's my fail, so much for A-level maths. Thanks!

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Updated: January 4, 2013
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