Question 5
A box of mass 5 kg is pushed along horizontal ground with a force of 25 N, against a frictional force of 20 N. What is the box's acceleration?
The right answer is the one marked above.
1 m s−2. Find the unbalanced force first — it is that, not the applied force, which sets the acceleration.
unbalanced force = 25 N − 20 N = 5 N
a = F ÷ m = 5 N ÷ 5 kg = 1 m s−2
5 m s−2 comes from ignoring the friction and using the full 25 N — the commonest slip here. The F in F = ma is the resultant force.