A 45 kg child jumps off a wall 1.8 m high…

Step 1. (a) $v = \sqrt{2 \times 10\ \text{m/s}^2 \times 1.8\ \text{m}} = 6\ \text{m/s}$.

Step 2. (b) $45\ \text{kg} \times 6\ \text{m/s} = 270\ \text{kg m/s}$.

Step 3. (c) $\dfrac{270\ \text{kg m/s}}{0.06\ \text{s}} = 4500\ \text{N}$.

Step 4. (d) $\dfrac{270\ \text{kg m/s}}{0.5\ \text{s}} = 540\ \text{N}$.

Step 5. (e) Her weight is $450\ \text{N}$: $10$ times less than the straight-legged force, but close to the bent-knee one.