Statics asked what holds still. Dynamics asks what happens when the arrows do NOT balance, and the answer is one line: $F = ma$. Read it aloud — F equals m a. Letter by letter: $F$ is the net force, in newtons, meaning every push and pull on the body already summed; $m$ is the mass in kilograms; and $a$ is the acceleration in $\mathrm{m/s^2}$ that the leftover force produces. Whichever of the three the problem leaves blank is the one you solve for.

The word net is doing enormous work there, and it is where most marks are lost. Feed this relation a gross force — the motor's pull without the friction it fights — and it will hand you a wrong answer with flawless arithmetic. Free-body diagram first, sum the arrows, and only then let Newton speak.

Run the units check for free: $\mathrm{kg \cdot m/s^2}$ is precisely a newton — no accident, since that is where the newton came from. Push the units through every rearrangement you write. If they refuse to cancel into the answer's units the rearrangement is wrong, no appeal; units that DO work out never prove you right. The check is a one-way street, and it is still the cheapest insurance in engineering.