<script>on mouseUpput the short name of this card into xTempput char 2 to 6 of xTemp & ".mov" into Movie--put Movieif Movie is not empty then-- put "Videos:NASA:" & Movie into pMovieplayMovie Movie,"Videos:NASA:"end ifend mouseUp</script>
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<text><span class="style10">otion and Force (4 of 5)Newton's laws of motion</span><span class="style7">Newton's laws of motion state relationships between the acceleration of a body and the forces acting on it. A </span><span class="style26">force</span><span class="style7"> is something that causes a change in the rate of change of velocity of an object. </span><span class="style26">Newton's first law</span><span class="style7"> states that a body will remain at rest or traveling in a straight line at constant speed unless it is acted upon by an external force. Notice that the force has to be an external one. In general, a body does not exert a force upon itself.The tendency of a body to remain at rest or moving with constant velocity is called the </span><span class="style26">inertia</span><span class="style7"> of the body. The inertia is related to the mass, which is the amount of substance in the body. The unit of mass is the </span><span class="style26">kilogram</span><span class="style7"> (kg).</span><span class="style26">Newton's second law</span><span class="style7"> states that the resultant force exerted on a body is directly proportional to the acceleration produced by the force. The unit of force is the </span><span class="style26">newton</span><span class="style7"> (N), which is defined as the force that, acting on a body of mass 1 kg, produces an acceleration of 1 m s-2.The mass of a body is often confused with its weight. The mass is the amount of matter in the body, whereas the </span><span class="style26">weight</span><span class="style7"> is the gravitational force acting on the body, and varies with location. The unit of weight is the newton. Thus a body will have the same mass on the Moon as on Earth, but its weight on the Moon will be less than on Earth since the gravitational force on the Moon is approximately one sixth of that on Earth. The same person, stepping on a set of compression scales at the bottom of a mountain and then at the top, would weigh less at the top because of the slight decrease in the gravitational force, which results from the slight increase in distance from the center of the Earth.Newton expressed his second law by stating that the force acting on a body is equal to the rate of change in its `quantity of motion', which is now called </span><span class="style26">momentum</span><span class="style7">. The momentum of a body is defined as the product of its mass and velocity.</span><span class="style26">Newton's third law</span><span class="style7"> states that a single isolated force cannot exist on its own: there is always a resulting `mirror-image' force. In Newton's words, `To every action there is always opposed an equal reaction.' This means that, because any two masses exert on each other a mutual gravitational attraction, the Earth is always attracted towards a ball as much as the ball is attracted towards the Earth. Because of the huge difference in their sizes, however, the observable result is the downward acceleration of the ball.The </span><span class="style26">principle of the conservation of momentum</span><span class="style7"> follows from this third law. This states that, when two bodies interact, the total momentum before impact is the same as the total momentum after impact. Thus the total of the components of momentum in any direction before and after the interaction are equal.In an </span><span class="style26">accelerating</span><span class="style7"> or </span><span class="style26">non-inertial</span><span class="style7"> frame of reference, Newton's second law will not work unless some fictitious force is introduced. For example, passengers on a circus merry-go-round feel as if they are being forced outward when the machine is operating. This is ascribed to a `center-fleeing' or `centrifugal force'. The passengers experience this because they are moving within the system; they are within an accelerating frame of reference. To an observer on the ground it appears that the passengers on the ride should fly off at a tangent to the circular motion unless there were a force keeping them aboard. This is the centripetal force and is experienced as the </span><span class="style26">friction</span><span class="style7"> between each passenger and the seat. If a passenger were to fall off, it would be because the centripetal force was not strong enough, not because the `centrifugal force' was too great.</span></text>
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<text><span class="style10"> gyroscope in low gravity</span><span class="style7"> demonstrates the laws of physics outside the force of gravity. It clearly shows Newton's first law which states that a body will remain at rest or at a constant speed unless it is acted upon by another force. The gyroscope tends to move in a single direction until the astronaut nudges it with the string.</span></text>
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<text>ΓÇó THE HISTORY OF ASTRONOMYΓÇó FORCES AFFECTING SOLIDS AND FLUIDSΓÇó THERMODYNAMICSΓÇó QUANTUM THEORY AND RELATIVITYΓÇó THE HISTORY OF SCIENCEΓÇó FUNCTIONS, GRAPHS AND CHANGE</text>
