Question

A block weighing $30N$ is on a smooth horizontal surface and it is accelerated in the horizontal direction at a rate of ${4m/s}^2$ by a force. The net force in horizontal direction acting on the block is (g $={10m/s}^2$ ):

• A. 12 N
• B. 120 N
• C. 50 N
• D. 10 N

Answer 1

The correct option is A 12 N

Weight of the block $W=mg$

N is the normal reaction acting on the block due to the horizontal surface (mg is downward and N is upward)

So $N=mg$ as there is no acceleration in vertical direction.

The net force experienced by the block will be in the horizontal direction and will be equal to the product of mass and acceleration.
Mass: $m=\frac{W}{g}=\frac{30N}{10m/s^2}=3kg$

Hence, by Newton's second law of motion,

Force $F=ma=3kg\times 4m/s^2=12N$