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I'd like to know why a young modulus for a material is constant whereas the spring constant isn't.Is it because the spring constant depends on the extension which depends on the length and area of the wire?Just wanted to make sure :)

Also,I have these two questions I'm not getting the right answers for so please help !

Q . A student standing on a platform at a railway station notices that the first two carraiges of an arriving train pass her in 2 s and the next two in 2.4s.The train is decelerating uniformly.Each carraige is 20m long.When the train stops,the student is opposite the last carraige.How many carraiges are there in the train?

The correct answer is eight but I keep getting a wrong answer.I tried finding out the acceleration by finding out the two velocities at 2 and 2.4s and then using v^2=u^2+2as but don't get the right answer.

Q .Two equations for power dissipated in a resistor are P=I^2R and P=V^2/R.The first suggests that the greater the resistance R of the resistor,the more power is disspated.The second siggests the opposite,the greater the resistance,the less the power.Explain this inconsistency

Thank you!

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# Homework Help: A level questions help

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