Question 8
An object has a mass of 6 kg. Its velocity increases uniformly from 5 m s−1 to 13 m s−1 over 4 s. What force must have been applied?
The right answer is the one marked above.
12 N. The force cannot be found directly — the acceleration has to come first.
a = (13 − 5) m s−1 ÷ 4 s = 8 ÷ 4 = 2 m s−2
F = m a = 6 kg × 2 m s−2 = 12 N
Two steps: get a from chapter 2's acceleration formula, then feed it into this chapter's F = ma.
19.5 N comes from using 13 without subtracting the initial velocity (6 × 13 ÷ 4). Do not forget it is the change in velocity that counts.