A rocket is launched vertically upwards. It burns its fuel in 10 seconds, reaching a maximum height at 30 seconds. After that, it continues to move upwards for another 20 seconds before starting to fa

At point B, the rocket reaches its maximum height, meaning its instantaneous vertical velocity is zero. However, it is still under the influence of gravity. Since upward motion is considered positive, the acceleration due to gravity acts downwards.
The gradient at B is $\boxed{-9.8 \text{ m/s}^2}$ (or approximately $-10 \text{ m/s}^2$).

Since gravity is the only significant force, the rocket's acceleration is the acceleration due to gravity.

The shaded area above the time axis (from 0 to B) represents the total upward displacement of the rocket. The shaded area below the time axis (from B to E) represents the total downward displacement of the rocket.

The rocket starts from the ground and returns to the ground at point E. This means its net displacement from its starting point to its end point is zero. Therefore, the magnitude of the upward displacement must be equal to the magnitude of the downward displacement.
The magnitude of the shaded area above the time axis is equal to the magnitude of the shaded area below the time axis. This is because the total displacement of the rocket from launch to landing is zero.

The graph should start at B (speed = 0) and move into the "rocket moving downwards" region (negative velocity). The curve should initially be less steep than the original B-C curve, indicating a smaller acceleration due to the parachute's drag. It should then flatten out to a constant, small negative velocity (terminal velocity), which is much smaller in magnitude than the speed reached at E in the original graph. The time taken to reach the ground will be longer than the original time to E.