Explain, with the help of an example the difference between the terms thrust and pressure. Which one of the two has same SI unit as that of Force? (b) Consider a wooden block of mass 5 kg and dimensions 40 cm x 20 cm x 10 cm with its faces 20 cm × 10 cm and 40 cm × 20 cm kept on the table, in turn, in which case will the pressure exerted by the box on the table be more? Justify your answer by doing mathematical calculations.

(a) Pressing the flat end of a ball point pen against the hand with some force and pressing the pointed tip of the same pen against the hand with the same force. Pressing on pointed tip produces greater pain because the area on which same force acts decreases and the effect of this force increases.

The total force acting on an area is called thrust whereas the force acting per unit area is called pressure. Thrust has the same SI Unit, Newton as that of force.

(b) Pressure = {Force \over Area}

Force = mg 5 x 9.8 N

When 20 cm x 10 cm is kept on the table:

Pressure = {5×9.8 N \over 0.2×0.1 m^2}

= 2450 Nm^-2

When the block is kept on the table with its face 40 cm x 20 cm,

Pressure = {5×9.8 N \over 0.4×0.2 m^2} = 612.5 Nm^-2

Thus, pressure exerted will be more when block with face 20 cm x 10 cm is kept on table.