A baby bouncer is a harness attached to a spring that hangs from a door frame - AQA - GCSE Physics - Question 7 - 2023 - Paper 1

In position A, the baby has gravitational potential energy as it is elevated above the floor. As the baby moves down to position B, this gravitational potential energy is converted into kinetic energy of the baby. When the baby reaches position B, the kinetic energy is then converted into elastic potential energy of the spring.

First, convert the extension from cm to meters: $e = 0.080 \, m$ The elastic potential energy (EPE) for a spring is given by the formula: $EPE = \frac{1}{2} k e^2$ Rearranging this formula to find the spring constant $k$ gives: $k = \frac{2 \times EPE}{e^2} = \frac{2 \times 4.0}{(0.080)^2} = \frac{8.0}{0.0064} = 1250 \, N/m$