Question 8
An object weighs 20 N. An unbalanced force applied to move it horizontally produces an acceleration of 2 m s−2. What is that unbalanced force? (Take g = 10 m s−2.)
The right answer is the one marked above.
4 N. F = ma needs the mass, not the weight — so the mass has to be found first.
m = weight ÷ g = 20 N ÷ 10 m s−2 = 2 kg
F = m a = 2 kg × 2 m s−2 = 4 N
40 N comes from treating the weight as the mass and working out 20 × 2. Weight is in newtons; what goes into F = ma is the mass, in kilograms.