A spring obeys Hooke's law. Its length is 0.180 m under a force of 3.0 N and 0.240 m under a force of 7.5 N. Which gives its spring constant and unstretched length?
75 N/m75\ \mathrm{N/m}75 N/m and 0.060 m0.060\ \mathrm{m}0.060 m.
31.25 N/m31.25\ \mathrm{N/m}31.25 N/m and 0.060 m0.060\ \mathrm{m}0.060 m.
31.25 N/m31.25\ \mathrm{N/m}31.25 N/m and 0.140 m0.140\ \mathrm{m}0.140 m.
75 N/m75\ \mathrm{N/m}75 N/m and 0.140 m0.140\ \mathrm{m}0.140 m.
102 exam-style questions on Edexcel GCSE Physics 15.1 Forces and springs, covering 15.1.1 Elastic and inelastic distortion, 15.1.2 The spring constant, 15.1.3 Work done in stretching a spring, and 15.1.4 Linear and non-linear force–extension. Each one has a worked solution and a mark scheme showing where the marks go.