When the forces on an object are not balanced, there is a resultant force and the object accelerates. Newton's second law links the three quantities: force = mass × acceleration, or F = m × a. Force is measured in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s2). The force in the equation is the resultant force, and the acceleration is in the same direction as that force.
The equation shows two things. For the same mass, a larger resultant force gives a larger acceleration. For the same resultant force, an object with a larger mass has a smaller acceleration, so heavy objects are harder to speed up.
Rearranging gives acceleration = force ÷ mass. A 5 kg trolley with a resultant force of 20 N has an acceleration of 20 ÷ 5 = 4 m/s2. Doubling the resultant force to 40 N doubles the acceleration to 8 m/s2, so acceleration is directly proportional to force. A car of mass 1000 kg with a resultant force of 3000 N accelerates at 3 m/s2. A force of 12 N on a mass of 3 kg gives 12 ÷ 3 = 4 m/s2.