Newton's second law of motion states that "the rate of change of momentum of a body is proportional to the applied force and takes place in the direction of the force". It implies that the force is proportional to the product of mass and acceleration. If we express force (F) in newtons (SI unit of force), mass (m) in kilograms and acceleration (a) in metres per second squared, we can write the second law as

F = ma

It is obvious that if the same force acts on 1kg and 2kg masses, the acceleration produced in the 1kg mass will be twice of that produced in the 2kg mass.

In travelling the same distance, a car consumes more fuel on a crowded road that on a free road. This happens because the car has to stop and start quite often in a crowded road. The repeated acceleration requires a force, in accordance to the second law, which ultimately comes from the fuel. On a free road the car runs at almost uniform speed requiring fewer accelerations and hence less fuel consumption.

F = ma

It is obvious that if the same force acts on 1kg and 2kg masses, the acceleration produced in the 1kg mass will be twice of that produced in the 2kg mass.

In travelling the same distance, a car consumes more fuel on a crowded road that on a free road. This happens because the car has to stop and start quite often in a crowded road. The repeated acceleration requires a force, in accordance to the second law, which ultimately comes from the fuel. On a free road the car runs at almost uniform speed requiring fewer accelerations and hence less fuel consumption.