A car at rest remains as such unless the engine forces the wheels to rotate and initiate its motion. The same car requires brakes for it to achieve rest state. Let’s think of a hypothetical situation where you give a thrust, just enough to move it from rest state, to the car and leave it. The car, obviously, moves certain distance and comes back to rest. Is it possible that the same experiment can be done elsewhere in the universe and the results are different? The answers were dug out by scientists and are discussed below.
A moving object tends to remain in motion and an object at rest tends to be at rest. This tendency needs to be countered by the exertion of force. As we have learnt during our Physics tuition classes, this behaviour of physical entities is termed as ‘inertia’. Inertia is the reason we need fuel to move our cars from rest and brakes to bring them back to rest. Inertia can be observed in every form of motion. In case of circular motion, it is ‘moment of inertia’. Inertia, in case of linear motion, is directly proportional to the mass of the object. An easy example for you to relate with inertia is; moving a bicycle is easy compared with moving a car from their respective rest states.
Similarly it is relatively easy to bring a moving bicycle to rest and don’t even think of stopping a car with bare hands. But the thing common to both stopping a moving object or moving the one at rest is the application of ‘force’. A car moving at a very high speed requires more force to attain rest state (remember the screeching noise tyres make when someone suddenly stops their car). On the other hand taking a car from rest to very high speeds also requires lot of force (now recall the shrill engine makes when a car is accelerated).
A large change in speed requires large amounts of force. The same change in the speed, when brought up on an object in a relatively small interval of time, demand more force. This requires the introduction of a new quantity called as acceleration. Acceleration is rate of change of velocity. So more acceleration requires more force. Two things learnt here are the intervention of force in changing the motion state (rest or moving) of an object, secondly this force required is proportional to acceleration or deceleration (when the object is brought to rest) of the object. This is exactly what Newton’s first law of motion says and is represented as ‘F=ma’.
Aristotle earlier believed that an object set into motion by application of force comes to rest, eventually, when the force is removed. But this is later found to false. The object comes to rest because of the frictional forces. When the same scenario is repeated in a relatively friction-less environment like space, the object moves with constant speed even after the removal of the force that effected its motion. Hence force is essential to bring a change in the speed of the objects.