What would happen to the people on Earth if it suddenly stopped :
- a) to rotate on itself
- b) to revolve around the sun
a) If the Earth were to suddenly stop rotating on its axis:

The immediate effect would be a catastrophic change in the weather. The side of the Earth facing the sun would experience extreme heat, while the side facing away from the sun would be plunged into freezing temperatures. This could lead to massive storms, hurricanes, and tornadoes as the atmosphere tries to adjust to the sudden change.

Additionally, the sudden stop would cause huge tsunamis in the oceans, and everything not anchored to the ground would be thrown forward due to the momentum of the Earth's rotation, resulting in massive destruction of infrastructure and property.

b) If the Earth were to suddenly stop revolving around the sun:

The immediate effect would be a sharp drop in temperature on the surface of the Earth. The Earth would be plunged into perpetual darkness and the temperature would drop to below freezing very quickly.

Over time, the lack of sunlight would lead to the death of most plants and animals, leading to a catastrophic collapse of the food chain and eventually, the extinction of most life on Earth.

In both cases, the effects would be catastrophic and likely lead to the end of human civilization. It's important to note that neither of these scenarios is physically possible, as the laws of physics dictate that the Earth must rotate and revolve around the sun to maintain its current state.