• Document: Std 8 th PHYSICS. 1-Describing Motion
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Std 8th PHYSICS 1-Describing Motion Q I Choose the right answer 1. Uniform circular motion is called continuously accelerated motion mainly because -------- -direction of motion changes. 2. A cricketer hits a sixer. The cricket ball moves up with a velocity of 2 ms-1 And falls down . Its initial velocity while falling down will be I Fill in the Blanks 1. S.I unit of acceleration is metre per second square [ms-2 ] 2. Velocity has both speed and direction. 3. If an object starts from ‘A’ and comes back to ‘ A’its displacement will be zero II solve 1. An Object is moving in a circular path of radius 3.5 m. If it completes one full cycle. what will be the displacement and what is the distance travelled? Ans: There is no displacement because the body reaches the same point from where it started .Hence displacement is zero. Distance travelled is equal to the circumference of the circular path B ) Distance travelled by an object=Circumference of the path=2 ∏ r 2 x 22 x 3.5 7 =2 x 22 x 0.5 =22cm is the distance travel 2. An object changes its velocity from 30ms-1 to 40ms-1 in a time interval of 2 second. what is its acceleration Ans:U=30ms-1 V=40ms-1 t=2second Acceleration= Rate of change of velocity = Change of velocity time = 40 – 30 =10 =5 ms-2 2 2 3. A object at rest starts moving. It covers a distance of 3m in one second. It comes a further distance of 3m in two second in the same direction. what is its average velocity and acceleration. Ans: Average velocity=Total distance = 2+5 =7 = 2 1 ms-1 time 1+2 3 3 Acceleration is to be calculated in 2 stages by using the equation V = u + at Where V is the final velocity and u’ is the initial velocity. ‘a’ acceleration and t is time u= o v = 2/1= 2 , t= 1 V= u + at 2=0+ax1 a = 2 ms-2 II stage u=2 since the final velocity of stage I is the initial velocity at stage II a=5 u = 2 m/s t=2 s= ut + ½ at2 5= 2 x 2 + ½ a x 22 5= 4 + ½ a x 4 5 – 4 =2 a . . . a =1/2 A = 0.5 m / s2 II Answer the following 1) In a body is moving with uniform velocity in a given direction its acceleration will be zero why? Ans: There is no change in velocity in the case of uniform velocity therefore acceleration is zero. 2) Distingiush between speed the velocity. Speed Velocity Speed is total distance Velocity is travelled by an object displacement of the in unit time object in unit time. i.e speed= Distance covered /travelled Velocity = displacement time time 3. Distingiush between distance travelled and displacement. Ans: If an object covers a particular distance without the reference of direction is called distance travelled. If an object covers a distance in a particular direction it is called displacement. 3) what are uniform and non uniform motions? Ans: If an object covers equal distance in equal interval of time , it is uniform motion If an object covers unequal distance in equal intervals of time motion it is non uniform motion. 4) while mentioning acceleration the time is mentioned two times why? Ans: Velocity = displacement time Acceleration = velocity = displacement Time time Time = displacement A=V =d = d 2 time t t t2 t Hence acceleration has two times time or time2 FORCE AND NEWTON’S LAWS OF MOTION Textual Questions and Answers I. Four alternatives are given to each of the following incomplete statement / question. Chose the right answer. 1. Action and reaction a) act on the same body b) are equal c) are opposite d) act on different bodies (d) 2. If the forces acting on an object are balanced then

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