What is an example of something that is negatively buoyant?

A block of wood having a mass $15 \mathrm{kg}$ and dimensions $40\mathrm{cm} \times 30\mathrm{cm} \times 20 \mathrm{cm}$ is kept on a table-top, It is made to lie on the table-top in three ways with its sides of dimensions

(a) $40 \mathrm{cm} \times 30 \mathrm{cm}$

(b) $30 \mathrm{cm} \times 20 \mathrm{cm}$

(c) $20 \mathrm{cm} \times 40 \mathrm{cm}$
 

The pressure exerted by the wooden block on the table-top will be

Give reasons: A body experiences more buoyant force on the earth than on the moon.

State whether upthrust will increase or decrease if the volume of the solid increases.

Give reasons: It is easier to swim in the sea than in the river.

Naveen and Paul are friends. Paul wanted to celebrate his birthday at home. He bought balloons filled with hydrogen gas from a balloon man. Naveen helped in decorating Paul's room with balloons and cards. Why balloons filled with hydrogen rise up in the air?