Is force a necessary condition to persist a motion?
This is part of a series on common misconceptions.
Is this true or false?
Everything that moves must eventually come to a stop. In order for there to be continuous motion, there must always be a force acting on it.
Why some people say it's true: Any object which moves eventually comes to stop. Thus, to keep it in motion, a force must be applied.
Why some people say it's false: The object stops because of friction. Without friction, it will continue moving.
The statement is \( \color{red}{\textbf{false}}\).
Explanation:
Day-to-day observations without sufficient knowledge of forces and Newton's laws may lead to this conclusion. The point of confusion here is that objects which are left moving do not keep moving on their own and come to a halt after some time.
Let's illustrate this. A person is driving a car on a straight road. The car runs out of fuel and the engine stops running. Even if the driver does not apply the brakes, the car will still slow down and eventually stop. To move the car further, he will need to get out and push it. This gives the illusion that a force is necessary to keep the car moving.
To catch the fallacy in the above illustration, focus on forces acting on the car. When the car is left to move on its own, there are still some forces acting on it. The main force which was responsible for stopping the car is friction. The ground applies frictional force on the tires to stop the car. Similarly, air drag also stops the car, though to a lesser extent. Thus, when the car is left to move, there are forces which are opposing its motion and the car does not come to rest on its own.
As a result, it can be concluded that the objects maintain their state of rest or motion until they are compelled by a net force. This property of matter is known as inertia.
Query: Is there a daily life situation in which an object keeps on moving in the absence of forces?
Reply: It is very difficult to have a situation in which there are no forces at all. Many forces act at the same time and not all the forces can be perfectly eliminated. Forces like air drag, friction, the buoyancy of air, gravitational force of the earth, etc. affect the motion of objects.
Query: Can any experiment be done to show that without force, the motion will persist until eternity?
Reply: An experiment was done by Galileo Galilei to prove the same thing. Take an inclined plane as shown below and release a block from the top.The inclined surface is rough, and thus the block comes to rest after traveling a distance \(x\) on horizontal ground. If roughness of the inclined plane and the ground is reduced, then the distance moved by the block on the inclined plane is increased. Therefore, if the roughness tends to zero, then the distance moved by the block will tend to infinity.
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