what is upthrust?(英文答)

Upthrust = Buoyancy

In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i.e., a liquid or a gas) in which it is fully or partially immersed, due to the pressure difference of the fluid between the top and bottom of the object. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least to seem lighter. Buoyancy is important for many vehicles such as boats, ships, balloons, and airships.

Upthrust is the force that pushes an object up

and makes it seem to lose weight in a fluid.

(Remember, a fluid means a liquid or a gas).