CLASSICAL MECHANICS & THERMODYNAMICS - Chapter 2, Exercise 1 Solution
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Finding Force from Mass and Acceleration (F = ma)

PROBLEM
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A 1,200 kg car accelerates from rest to 20 m/s in 8 seconds. Use
F = ma to calculate the average net force the engine must supply.

SOLUTION
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Step 1: Find the acceleration using a Chapter 1 SUVAT equation.

    v = u + at

    u = 0 m/s   (starts from rest)
    v = 20 m/s
    t = 8 s

    20 = 0 + a(8)
    a = 20 / 8
    a = 2.5 m/s^2

Step 2: Apply Newton's Second Law, F = ma.

    F = ma
    F = 1200 x 2.5
    F = 3000 N

ANSWER: The engine must supply an average net force of 3,000 N
(3 kN).

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WHY THIS WORKS AS AN ANSWER

This problem requires chaining two chapters' own tools together: the
question gives a change in velocity over time but never states the
acceleration directly, so Chapter 1's own v = u + at equation is
needed first to extract it. Only once acceleration is known can
Newton's Second Law, F = ma, convert that acceleration into the real
force responsible for producing it. This two-step structure - find
the motion's own acceleration first, then find the force behind it
second - is the standard real-world pattern used throughout mechanics,
since force is rarely given directly; it is almost always inferred
from how an object's motion actually changed.