Hope this helps u.
Newton's First Law
Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It may be seen as a statement about inertia, that objects will remain in their state of motion unless a force acts to change the motion. Any change in motion involves an acceleration, and then
Newton's Second Law applies; in fact, the First Law is just a special case of the Second Law for which the net external force is zero.
Newton's First Law contains implications about the fundamental symmetry of the universe in that a state of motion in a straight line must be just as "natural" as being at rest. If an object is at rest in one frame of reference, it will appear to be moving in a straight line to an observer in a reference frame which is moving by the object. There is no way to say which reference frame is "special", so all constant velocity reference frames must be equivalent.
Centripetal Force Example
The string must provide the necessary
centripetal force to move the ball in a circle. If the string breaks, the ball will move off in a straight line. The straight line motion in the absence of the constraining force is an example of
Newton's first law. The example here presumes that no other net forces are acting, such as horizontal motion on a frictionless surface. The
vertical circle is more involved.
Newton's Second Law
Newton's Second Law as stated below applies to a wide range of physical phenomena, but it is not a fundamental principle like the
Conservation Laws. It is applicable only if the
force is the net external force. It does not apply directly to situations where the mass is changing, either from loss or gain of material, or because the object is traveling close to the speed of light where relativistic effects must be included. It does not apply directly on the very small scale of the atom where quantum mechanics must be used.
Data can be entered into any of the boxes below. Specifying any two of the quantities determines the third. After you have entered values for two, click on the text representing to third to calculate its value.
Limitations on Newton's 2nd Law
but, though extremely useful, it is not a fundamental principle like the conservation laws. F must be the net external force, and even then a more fundamental relationship is
The net force should be defined as the rate of change of
momentum; this becomes
only if the mass is constant. Since the
mass changes as the speed approaches the
speed of light, F=ma is seen to be strictly a non-relativistic relationship which applies to the acceleration of constant mass objects. Despite these limitations, it is extremely useful for the prediction of motion under these constraints.
though extremely useful, it is not a fundamental principle like the conservation laws. F must be the net external force, and even then a more fundamental relationship is
The net force should be defined as the rate of change of
momentum; this becomes
Variable Mass Applications
The generalization of
Newton's 2nd Law to apply to variable mass systems takes the form
The term involving the derivative of the mass is responsible for the thrust in
rocket propulsion and must be included in any problem where the mass changes.
Newton's Second Law Illustration Newton's 2nd Law enables us to compare the results of the same force exerted on objects of different mass.  | Index |
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Newton's Third Law
Newton's third law: All forces in the universe occur in equal but oppositely directed pairs. There are no isolated forces; for every external force that acts on an object there is a force of equal magnitude but opposite direction which acts back on the object which exerted that external force. In the case of internal forces, a force on one part of a system will be countered by a reaction force on another part of the system so that an isolated system cannot by any means exert a net force on the system as a whole. A system cannot "bootstrap" itself into motion with purely internal forces - to achieve a net force and an acceleration, it must interact with an object external to itself.
 | Without specifying the nature or origin of the forces on the two masses, Newton's 3rd law states that if they arise from the two masses themselves, they must be equal in magnitude but opposite in direction so that no net force arises from purely internal forces. |
Newton's third law is one of the fundamental symmetry principles of the universe. Since we have no examples of it being violated in nature, it is a useful tool for analyzing situations with are somewhat counter-intuitive. For example, when a small truck collides head-on with a large truck, your intuition might tell you that the force on the small truck is larger. Not so!
Newton's Third Law Example
Newton's third law can be illustrated by identifying the pairs of forces which are involved in supporting the blocks on the spring scale.
Presuming that the blocks are supported and at equilibrium, then the net force on the system is zero. All the forces occur in Newton's third law pairs.
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Moderation Team
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