Motion – CBSE Notes for Class 9 Science
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Facts that Matter
• An object is said to be in motion when its position changes with time.
• We describe the location of an object by specifying a reference point. Motion is relative. The total path covered by an object is said to be the distance travelled by it.
• The shortest path/distance measured from the initial to the final position of an object is known as the displacement.
• Uniform motion: When an object covers equal distances in equal intervals of time, it is said to be in uniform motion.
• Non-uniform motion: Motions where objects cover unequal distances in equal intervals of time.
• Speed: The distance travelled by an object in unit time is referred to as speed. Its unit is m/s.
• Average speed: For non-uniform motion, the average speed of an object is obtained by dividing the total distance travelled by an object by the total time taken.
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• Velocity: Velocity is the speed of an object moving in definite direction. S.I. unit is m/s.
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• Acceleration: Change in the velocity of an object per unit time.
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Graphical representation of motions
(i) Distance-time graph
For a distance-time graph time is taken on x-axis and distance is taken on y- axis.
[Note: All independent quantities are taken along the x-axis and dependent quantities are taken along y-axis.]
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(ii) Velocity-time graph
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Equation of motion by graphical methods
(i)velocity-time relation:
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(ii) Equation for position-time relation:
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(iii) Equation for position-velocity relation:
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• Uniform circular motion: When a body moves in a circular path with uniform speed, its . motion is called uniform circular motion.
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