P18: A nonlinear spring model could include a cubic term. That is, the force is approximated by F(x) = –kx + k’x3 . a) If all quantities are in our usual (SI) units, what are the units of k and k’? b) For a particular spring, the constants are k = 300 and k’ = 5x104 . How much does the potential energy in the spring change when it is stretched from x = 0.02 m to 0.05 m?

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The boat's velocity relative to the riverbank can be resolved into two components. the component going north is 2.7 m/s and the component going west is 1.2 m/s.we add these two vectors and then find the magnitude of the resultant to get the speed.the magnitude of the resultant vector from adding can be obtained by the pythagorean theorem.    to two significant figures, the boat's speed relative to the riverbank is 3.0 m/s.