Question

A 10 kg mass rests on a table. What acceleration will be generated when a force of 20 N is applied and encounters a frictional force of 15 N?

Answer 1

The frictional force is oposing the force that we apply to this mass. Therefore the overall force is $F_{total}=F_{applied}-F_{friction}=20-15 =5 (Newton)$
The second principle of physics tells us that the acceleration of a mass is direct proportional to the applied force, the mass of that object being the proportionality constant.
$F=m*a$
Hence $a= \frac{F}{m}=5/10 =0.5 (m/s^2)$