Answer the physics question 3 and 4

Mass $m = 20 \text{g} = 20 \times 10^{-3} \text{kg} = 0.02 \text{kg}$
Extension $x = 4 \text{cm} = 4 \times 10^{-2} \text{m} = 0.04 \text{m}$
Force constant $k = 200 \text{Nm}^{-1}$

The elastic potential energy stored in the rubber is converted into the kinetic energy of the stone.

The velocity with which the stone leaves the catapult is $\boxed{\text{4 m/s}}$.

Compression $x = 0.02 \text{m}$
Force constant $k = 400 \text{Nm}^{-1}$

The energy stored in a spring is given by the elastic potential energy formula:

The energy stored in the spring is $\boxed{\text{0.08 J}}$.