How forces affect motion

For the book to remain stationary, the resultant force must equal zero. This means all forces acting on the book must be balanced.

The only two forces acting on the book are its weight (12 N downward) and the normal reaction force from the table (upward). For these to balance: $\text{Normal reaction}=\text{Weight}=12\text{ N}$

The normal reaction force always acts perpendicular to the surface, pushing the object away from it. Since the table is horizontal, the normal reaction acts upward.

The normal reaction force must be 12 N upward to balance the 12 N weight acting downward, giving a resultant force of zero.

We need to find the normal reaction force that keeps the book stationary. A stationary object has balanced forces — the resultant force must be zero.

'12 N downward' is wrong — if both the weight and the normal reaction acted downward, the total downward force would be 24 N with nothing pushing up. The book would accelerate downward through the table, which is impossible.

'0 N' is wrong — if there were no normal reaction force, the only force on the book would be the 12 N weight downward. The book would accelerate downward, which contradicts it being stationary.

'6 N upward' is wrong — a 6 N upward force cannot balance a 12 N downward weight. The resultant would be 6 N downward, so the book would still accelerate downward.

'12 N upward' is correct. A 12 N upward normal reaction exactly cancels the 12 N downward weight, giving a resultant force of zero and keeping the book stationary.

We are given: mass $m=2$ kg, resultant force $F=10$ N. We need to find acceleration $a$. The formula from Newton's Second Law is $F=m\times a$.

Rearranging $F=m\times a$ to solve for $a$: $a=\frac{F}{m}$

$a=\frac{10\text{ N}}{2\text{ kg}}=5{\text{ m/s}}^2$

The acceleration of the skateboard is 5 m/s².

Using Newton's Second Law: $F=m\times a$, rearranged to $a=\frac{F}{m}$. We divide force by mass: $\frac{10}{2}$.

'20 m/s²' would come from multiplying $10\times 2=20$, which incorrectly uses multiplication instead of division. This confuses the rearrangement of $F=ma$.

'0.2 m/s²' would come from $\frac{2}{10}=0.2$, which has the fraction upside down — dividing mass by force instead of force by mass.

'8 m/s²' has no mathematical basis from the given values and is simply a distractor — $10-2=8$ uses subtraction, which is not how $F=ma$ works.

'5 m/s²' is correct: $a=\frac{F}{m}=\frac{10}{2}=5{\text{ m/s}}^2$.