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MasteringPhysics: Assignment Print View
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Chapter 04 - Newton’s Laws Of Motion
Due at 11:59pm on Tuesday, September 30, 2008
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Newton's 1st Law
To understand Newton's 1st law.
states this first law of motion:
An object subject to no net force maintains its state of motion, either at rest or at constant speed in a right line.
This law may be stated as follows: If the sum of all forces acting on an object is zero, then the acceleration of that object is zero.
Mathematically this is just a special case of the 2nd law of motion,
, prompting scholars to advance the
following reasons (among others) for Newton's spelling it out separately:
1. This expression only holds in an inertial coordinate system--one that is not accelerating--and this law really says you
have to use this type of coordinate system (i.e., Newton's laws won't work inside an accelerating rocket ship.)
2. This was a direct challenge to the Impetus theory of motion, described as follows:
A mover, while moving a body, impresses on it a certain impetus, a certain power capable of moving this
body in the direction in which the mover set it going, whether upwards, downwards, sideways or in a
circle. By the same amount that the mover moves the same body swiftly, by that amount is the impetus that
is impressed on it powerful. It is by this impetus that the stone is moved after the thrower ceases to move
it; but because of the resistance of the air and the gravity of the stone, which inclines it to move in a
direction opposite to that towards which the impetus tends to move it, this impetus is continually
weakened. Therefore the movement of the stone will become continually slower, and at length, the
impetus is so diminished or destroyed that the gravity of the stone prevails over it and moves the stone
down towards its natural place.
A. C. Crombie,
Medieval and Early Modern Science
This theory is sometimes called the Animistic theory of motion since it envisions a "life force" being associated with
Newton's 1st law is often very difficult to grasp because it contradicts various common-sense ideas of motion that they have
acquired from experinece in everyday life. For example, unnaccounted for forces like friction might cause a ball rolling on the
playground to eventually stop, even though no obvious forces seem to be acting.
When studying Newtonian mechanics, it is best to remember this as two laws:
1. If the
force (i.e., sum of all forces) acting on an object is zero, the object will keep moving with constant velocity
(which may be zero).