What are the three laws of motion?
For centuries the problem of motion and its causes was a central theme of natural philosophy,an early name for what we call physics.It was not until the time of Galileo and Newton,however,that dramatic progress was made.Isaac Newton,born in England in the year of Galileo’s death,is the principal architect of classical mechanics.He carried to full fruition the ideas of Galileo and others who preceded him.His three laws first presented (in 1686) in his philosophiae Naturalis principia mathematica,usually called the principia.In his principia Newton stated the three fundamental laws of motion,which are the basis of Newtonian mechanics.
Before Galileo’s time most philosopher’s thought that some influence or “force” was needed to keep a body moving.They thought that a body was in its “natural state” when it was at rest.For a body to move in a straight line at constant speed,for example they believed that some external agent had to continually propel it ;otherwise it would”naturally” stop moving.
If we wanted to test these ideas experimentally we would first have to find a way to free a body from all influences of its environment or from all forces.This is hard to do,but in certain cases we can make the forces very small.If we study the motion as we make the forces smaller and smaller,we shall have some idea of what motion would be like if the external forces were truly zero.
Let us place our test body,say a block,on a rigid horizontal plane.If we let the block slide along this plane,we note that it gradually slows down and stops.This observation was used,in fact,to support the idea that motion stopped when the external force ,in this case the hand initially pushing the block ,was removed.We can argue against this idea,however,by reasoning as follows.Let us repeat our experimental,now using a smoother block and a smoother plane and providing a lubricant.We note that the velocity decreases the more slowly than before.Let us used still smoother blocks and surfaces and better lubricants.We find that the block decreases in velocity at a slower and slower rate and travels farther each time before coming to rest.You may have experimented with an air track,on which objects can be made to float on a film of air; such a device comes close to the limit of no friction,as even a slight tap on one of the gliders can send it moving along the track at a slow and almost constant speed.We can now extrapolate and say that if all friction could be eliminated the body would continue indefinitely in a straight line with constant speed.An external force is needed to set the body in motion,but no external force is needed to keep a body moving with constant velocity.
It is difficult to find situation in which no external force acts on a body.The force of gravity will act on an object on or near the earth,and resistive forces such as friction on air resistance oppose motion on the ground or in the air.Fortunately ,we need not go to the vacuum of distant space to study motion free of external force,because,as for as the overall translational motion of the body is concerned,there is no distinction between a body on which no external force acts and a body on which the sum or resultant of all the external force is zero.We usually refer to the resultant of all the forces acting on the body as the “net” force.For example ,the push of our hand on the sliding block can exert a force that contracts the force of friction on the block,and an upward force of the horizontal plane contracts the force of gravity.The net force on the block can then be zero,and the block can move with constant velocity.
This principle was adopted by newton as the first of his three laws of motion:
What is newton’s first law of motion?
“Consider a body on which no net force acts.If the body is at rest,it will remain at rest.If the body is moving with constant velocity it will continue to do so.”
Newton’s first law is really a statement about reference frames.In general,the acceleration of a body depends on the reference frame relative to which it is measured.However,the laws of classical mechanics are valid only in a certain set of reference frames,namely,those from which all observers would measure the same acceleration for a moving body.Newton’s first law helps us to identify this family of reference frames if we express it as follows:
“If the net force acting on the body is zero,then it is possible to find a set of reference frames in which that body has no acceleration.”
The tendency of a body to remain at rest or in uniform linear motion is called inertia,and newton’s first law is often called the law of inertia.The reference frames to which it applies are called inertial frames.You will recall from that discussion that observers in different inertial reference frames (moving with constant velocity relative to one another) all measure the same value of acceleration.Thus there is no just one frame in which the acceleration happens to be zero;There is a set of all inertial frames in which the acceleration is zero.
If there is a net interaction between the body and objects present in the environment,the effect may be to change the “natural’ state of the body’s motion.To investigate this,we must now examine carefully the concept of force.