Question

Why a force is needed to keep a block of cork inside water

Answer 1

Hi my dear friend,

As the density of the cork less than that of water. Magnitude of the buoyance force acting on the body inside the water is greater than the weight of the body. Hence, a force needed to keep a block of cork inside water to over come the net upward force acting on the cork.

~Thank you

Answer 2

Because the density of cork is less than the density of water.The buoyant force acting on the cork inside water is more than the weight of the cork.Hence to keep it submerged under water a force is needed so that it can overcome the net buoyant force on  cork.