Hello friends this video on laws of motion part one is brought to you by exam fear.com no more fear from exams the topics which we'll cover in this lesson are introduction Aristotle's fallacy the law of inertia Newton's first law of motion Newton's second law of motion Newton third law of motion conservation of momentum equilibrium of a particle common forces in mechanics circular motion and solving problems in mechanics let's think of what causes motion we have already discussed in our previous lessons that motion is nothing but change in position of an object with respect to time correct so what causes an object to move an external force is needed to make a stationary body move you think of any common example around you for example you have a ball the ball will stay at rest until and unless an external force is applied on it just now we saw that this ball was pushed by the hand because of which it started rolling that means this external Force which is given by the hand made the ball to move that means the motion of the ball was caused due to an external Force applied by the hand similarly we need an external Force to stop a moving body let us suppose a ball is rolling it will come to stop or it will come to rest only when the force produced by the hand will try to stop it that means an external Force external force is the cause of motion an external force can make a body which is at rest to move it can also cause a moving body to come to rest so here we found that force causes our stationary body to move and a moving body to come to rest but what about uniform motion uniform motion I hope all of you are aware of what is uniform motion uniform motion means a body is moving with constant velocity that means the velocity of the body is same throughout the journey so in case of a body moving with uniform velocity that time we do not exactly see some force acting on it or whether there is a force which is acting on it so what is the physics behind or what is the cause of motion of unifor in case of uniform motion so in the next slide we will discuss the cause of uniform motion the first law which was given to describe the cause behind uniform motion was given by Aristotle which is popularly known as Aristotle's law of uniform motion this law stated that an external force is required to keep a body in uniform motion that means when a body is in uniform motion an external Force has to be present continuously that means as long as an external force is present on a body the body will be in uniform motion so how did Aristotle come to this conclusion Aristotle basically thought of such a scenario let us suppose this is Ronnie and this is a box which Ronnie ties to a string correct so this is the Box Ronnie tied a string to the box and what he is doing he will he is just walking and pushing the string that means he is applying some Force what was happening was he applied some Force because of which the box was also moving after some time he left the thread he is no more holding the thread the Box comes to rest even though Ronnie is moving observe it once again in this case Ronnie is holding the thread tied to the box and he is dragging it now he leaves the thread and walks off the body stops moving that means initially the body was in motion as soon as the force was left or the force was taken back the body came to rest so Aristotle concluded from such con from such observation that an external force is continuously needed to keep a body in uniform motion if a body is in uniform motion that means there is a force which is acting on it so in brief that is what Aristotle's law conveyed but after some time it was observed that the Aristotle's law proved out to be false so what was Aristotle's fallacy I mean where did Aristotle fail Aristotle's law said that an external force is required to keep a body in uniform motion right so that is what we had observed he dragged the the box when he left it the Box stopped moving but it was later observed that this scenario which happened here or which Aristotle observed it proves something else the box was moving initially whenever a box is lying on the ground there is a force which is acting at the surface of contact that is at this point like suppose let us consider the scenario from here right now the box is at rest whenever the box is at rest there is a force of friction which is acting between the surface of the box and the floor so there is a force of friction which always opposes motion it doesn't want that the body should move right now this guy in order to make the Box move he is applying a force in this direction now when this Force when this force is greater than the frictional force then the body is moving till this point now when he leaves giving this force that means this Force becomes equal to zero at this point but this force that is the frictional force is still acting so in this case the frictional force becomes greater than the force applied because the force applied is zero therefore it opposes motion and frictional force opposes motion so the body body comes to rest so what did we see in this scenario the external it was not that the external Force kept the body in uniform motion external force was needed to make the body move and when the external force was taken aback due to friction the body came to rest so the entire the major role in making the body move and stopping the body was played by the frictional force so the conclusion was that an external force is required to keep a body in uniform motion only if the resistive forces like frictional force or viscous forces are present thank you visit exam fear.com to watch free educational videos try free online tests get the best quality study materials study from the best tutors and mentors and much more thanks once again