Question 4
A block of weight 40 N is sliding along a table. It is pulled with a horizontal force of 26 N and the dynamic friction is 10 N. Find its acceleration. (Take g = 10 m s−2.)
The right answer is the one marked above.
4 m s−2. Take it in three steps.
F = 26 − 10 = 16 N
m = 40 ÷ 10 = 4 kg
a = F ÷ m = 16 ÷ 4 = 4 m s−2
6.5 m s−2 comes from using the whole 26 N instead of the unbalanced 16 N. 16 m s−2 comes from forgetting to divide by the mass at all — and 40 N is a weight, not a mass, so it has to be divided by g first.