Question

A cubical block of wood of side $5cm$ is placed on a table. Find the thrust by the block of wood on the table, if density of wood is $5gc{m^{ - 3}}$ and $g = 10m{s^{ - 2}}$
A. $425N$
B. $5N$
C. $6.25N$
D. $7N$

Answer 1

Hint: In this question, we need to determine the thrust by the block of wood on the table such that cubical block of wood of side $5cm$ is placed on a table and the density of the wood is $5gc{m^{ - 3}}$. For this, we will first calculate the mass of the wood and then, use the relation between the thrust and the mass to evaluate the result.

Complete step by step answer:
Thrust is defined as the perpendicular force applied on a surface. The only force experienced by the table is the weight of the block. Hence we need to calculate the weight of the wooden block.
For weight, we need the mass of the body. But in the question mass is not directly given. We need to find the mass of the block from the density and volume of the block.
The block is in the shape of a cube. Hence the volume of the block in SI units is given as
$
 V = {(0.05)^3}{m^3} \\
 = 1.25 \times {10^{ - 4}}{m^3} \\
 $
The density of the block is given in the question as $5gc{m^{ - 3}} = 5000kg{m^{ - 3}}$
Now the mass of the block is given as the product of volume and density of the wooden block.
$
 m = \left( {1.25 \times {{10}^{ - 4}} \times 5000} \right)kg{m^{ - 3}} \cdot {m^3} \\
 = 0.625kg \\
 $
Finally after getting the mass of the block, we need to multiply the mass and acceleration due to gravity to get thrust
Thus,
$
 Thrust = 0.625 \times 10N \\
 = 6.25N \\
 $
So, the correct answer is “Option C”.

Note:
- The thrust is defined as the perpendicular force on a surface. Mathematically, it is given as the product of the mass and the acceleration.
- Be careful with the units while calculating. The units given in options are in Newton, which is a standard unit.
- Thus, all the other quantities should also be calculated in standard units.