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# Hooke's Law questions Watch

1. Got a couple of simple Hooke's Law questions in a text book I'm using, but I cannot get the answers given in the book.

Question 1: What force is required to stretch a spring with spring constant 0.2 N/cm a distance of 5 cm?

Question 2: A vertical spring stretches 5 cm under a load of 100 g. Determine the spring constant.

The formula for Hooke's Law is F= kx (K = constant and x = extension)

So for Q1 I simply multiply 0.2 N/cm with 5 cm and get a force of 1 Newton.

But the book says it's 0.01 N. ??

For Q2 I work out the force of the 100g mass: (0.1 Kg x 10 N/Kg for gravity = 1 Newton)
Then I divide Force by Extension which is 1 N / 5 cm = 0.2 N/cm

But book says it's 0.05 N/cm ??

What am I doing wrong please?
2. Hmmm.... the book clearly states that it is fine to give a constant as N/cm or even N/mm so long as the other unit corresponds. I still don't see how they get those particular answers.

If question 1 was supposed to be 0.2 N/m then yes... I convert the 5 cm to 0.05 m and then the calculation does result in 0.01 N for the Force. But it is clearly 0.2 N/cm so I leave the 5 cm alone.

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Updated: September 20, 2016
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