Newton's second law says that the acceleration of an object depends on the resultant force on it and on its mass. In symbols, resultant force = mass × acceleration, or F = m × a. Force is in newtons (N), mass is in kilograms (kg) and acceleration is in metres per second squared (m/s2).
The equation can be rearranged to a = F ÷ m and m = F ÷ a. For a fixed mass, doubling the resultant force doubles the acceleration. For a fixed force, doubling the mass halves the acceleration. In the lab this can be tested with a trolley, weights and light gates.
Always use the resultant force. A resultant force of 12 N on a trolley of mass 3 kg gives a = 12 ÷ 3 = 4 m/s2. A car of mass 800 kg has a driving force of 2000 N and a drag force of 400 N. The resultant force is 2000 − 400 = 1600 N, so the acceleration is 1600 ÷ 800 = 2 m/s2, in the direction of the resultant force.