The force that propels a bottle rocket is the difference in pressure between the inside and outside of the bottle. When you light the fuse, it ignites the fuel, which creates hot gas. This gas expands rapidly, increasing the pressure inside the bottle. The pressure inside the bottle is now greater than the pressure outside, so the gas pushes the water and the bottle upwards.

The shape of the bottle also helps to create lift. The wider part of the bottle is at the bottom, so the air flowing over the top of the bottle is moving faster than the air flowing over the bottom. This creates a difference in pressure, which lifts the bottle up.

The amount of thrust that a bottle rocket produces depends on several factors, including the type of fuel used, the size of the bottle, and the amount of water in the bottle.

The heavier the rocket is, the less thrust it will produce. This is because the force that is pushing the rocket up is the difference between the pressure inside the bottle and the pressure outside, so if the rocket is heavier, the pressure inside the bottle needs to be greater to lift it off the ground.

The more water in the bottle, the less thrust it will produce. This is because water is denser than air, so it takes more pressure to compress water than it does air. So, if there is more water in the bottle, the pressure inside the bottle needs to be greater to lift the rocket off the ground.

The type of fuel used also affects the amount of thrust that a bottle rocket produces. Some fuels, such as sugar or baking soda, produce more gas than others, so they create more thrust.