Newtons Three Laws of Motion

Newton's first law of motion states that, unless acted upon by some external force, a body at rest remains at rest and a moving object continues to move forever in a straight line and at a constant speed. This property is known as inertia. The measure of an object's inertia is its mass (in effect, the amount of matter the object contains). The more massive an object, the greater its inertia.

Newton's first law explains why the planets move in nearly circular orbits—essentially because an external force (gravity) acts on each planet. Without gravity, the planets would all fly off in straight lines, like so many pocket watches.

Newton's second law states that the acceleration of an object is directly proportional to the force applied to the object and inversely proportional to the mass of the object. Pull two objects with the same force, and the more massive object will accelerate more slowly than the less massive one. We all know this intuitively. Your subcompact car's engine would have a much harder time accelerating than an 18-wheel truck!

Star Words

An Astronomical Unit (A.U.) is equal to the mean distance between the earth and the sun. It is a conveniently large unit for measuring the large distances in the solar system.

Star Words

An Astronomical Unit (A.U.) is equal to the mean distance between the earth and the sun. It is a conveniently large unit for measuring the large distances in the solar system.

Star Words

Star Words

Inertia is the tendency of a body in motion to remain in motion in a straight line and at a constant velocity unless acted on by an external force; it is also the tendency of a body at rest to remain at rest unless acted upon by an external force. Mass can be thought of as the amount of matter an object contains or as a measure of an object's inertia. You have experienced the inertia of your body when rounding the corner in a car. Your body wants to keep moving in a straight line, but the car door (fortunately) makes you move in a curved path.

Newton's third law of motion states that forces do not act in isolation. If object A exerts a force on object B, object B exerts an equal but opposite force on object A. A hammer, for example, exerts a force on the nail, driving it into the wall. The nail exerts an equal and opposite force on the hammer, stopping its motion.

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Telescopes Mastery

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