NCERT Solutions Class 9th Science Chapter -7 Motion Notes

NCERT Solutions Class 9th Science Chapter – 7 Motion

NCERT Solutions Class 9th Science Chapter – 7 Motion

Chapter – 7

Motion

Notes

Rest: Abody is said to be in a state of rest when its position does not change with respect to a reference point.

(i) Motion: A body is said to be in a state of motion when its position change continuously with reference to a point. Motion can be of different types depending upon the type of path by which the object is going through. Circulatory motion/Circular motion – In a circular path. (ii) Linear motion-In a straight line path. (iii) Oscillatory/Vibratory motion-To and fro path with respect to origin. Physical quantity: There are seven basic physical quantity. Every quantity is written intwo parts: first write the magnitude of the physical quantity and then write the unit of the quantity, i.e., magnitude per Unit Physical Quantities can be grouped into two: i) Scalar quantities ii) Vector quantities i)Scalar quantities: Those Physical quantities that has only magnitude but no direction. i.e. speed, distance, mass, volume, time, temperature, work, electric current. ii) Vector quantities: Those physical quantities that has both magnitude as well as direction. i.e. velocity, force, momentum, displacement, acceleration etc. and The actual path or length travelled by an object during its journey from its initial position to its final position is called the distance. It is denoted by Distance is a scalar quantity which requires only magnitude but no direction to explain it. Example, Ramesh travelled 65 km. (Distance is measured by odometer in vehicles.) The actual deitance (covered by an object) in between its Initial and final position is called displacement. It is denoted by S. Displacement is a vector quantity requiring both magnitude and direction for its explanation. Example, Ramesh travelled 65 km south-west from Clock Tower. Displacement can be zero (when initial point and final point of motion are same) Example, circular motion. Distance and displacement are denoted by ‘S’.

Uniform and Non-uniform Motions: Uniform Motion – When a body travels equal distance in equal interval of time, then the motion is said to be uniform motion. eg. movements of hands of a clock rotation and revolution of the earth.

Non-uniform Motion In this type of motion, the body will travel unequal distances in equal intervals of time. eg. motion of a car on a busy road. Continuous increase in slope of curve indicates accelerated non-uniform motion. Non-uniform motion is of two types: Continuous decrease in slope of curve indicates decelerate non-uniform motion. (i) Accelerated Motion: When motion of a body increases with unequal time.

(ii) De-accelerated Motion or Non uniform Retardation When motion of a body decreases with unequal interval of time. Continuous decrease in slope of curve indicates deaccelerated non-uniform motion.Speed: The measurement of distance travelled by a body per unit time is called speed. It is denoted bySpeed = Distance travelled\Time taken V= S \ t SI unit = m/s (meter/second) If a body is executing uniform motion, then there will be a constant speed or uniform motion. If a body is travelling with non-uniform motion, then the speed will not remain uniform but have different values throughout the motion of such body For non-uniform motion, average speed will describe one single value of speed throughout the motion of the body. Average speed = Total distance travelled \ Total time take It is necessary to write the unit of every quantity in the answer of numerical questions:

Graphical Representation of Equation (i) Distance-Time Graph: (s-t graph) (a) s/t graph for uniform motion : The slope of a distance – time graph represent speed of an object speed of an object moving with uniform speed can be determined by:V= S2-S1 \ T2 -T1 (b) s/t graph for non-uniform motion : Continuous increase in slope of curve indicates accelerated non-uniform motion. Continuous decrease in slope of curve indicates decelerated non-uniform motion. (c) s/t graph for a body at rest: V= S2-S1 \ T2 -T1 But =  S2-S1  V= 0 \ T2 -T1 ro V=0

(ii) Velocity-Time Graph : (v/t graph) (a) v/t graph for uniform motion :a= V2 -V1\T2 -T2 a= 0\T1-T2 ro a=o Distance covered by the object in time t, or t, is :- distance (s) = AC x CD = area of rectangle (b) v/t graph for non-uniform motion : (A) v/t graph for accelerated (uniform) motion : a= change in velocity \time taken ⇒ a =V – U\t2-t1 Final velocity (v) – Initial Velocity (u) \t2-t1 In uniformly accelerated motion, there will be equal increase in velocity in equal interval of time throughout the motion of body.

(B) v/t graph for accelerated (non-uniform) motion: Here if, t2 – t1 = t2– t3 Then, v2 – v1 ≠ v3 -v2 Or v2 -v1 \ t2-t1 ≠v3 – v3 \t3– t2 Or a2 ≠ a3(C) v/t graph for deaccelerated (uniform) motion : Here, v2 – v1 =v3 -v2 If t2 – t1 = t3 – t2 Then, v2– v1 \t2 -t1 = v3-v2 \t3 – t2 Or a ;=a2 (D) v/t graph for deaccelerated (non-uniform) motion: Hera v2 -v1 ≠v3 – v2 if t2– t1 =t3 -t2 Then v2– v1 \ t2 -t1\ = v3– v2 \ t3 -t2Or a;≠ a2

Uniform Circular Motion If a body is moving in a circular path with uniform speed, It is motion is called uniform circular motion.In such a motion the speed may be same throughout the motion but its velocity (which is tangential) is different at each and every point of its motion due to continuous change in direction. Thus, uniform circular motion is an accelerated motion. so, velocity of an object in a circular motion is : V = 2лг\ t

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