1) Describing motion :- i) Motion :- is the change in position of a body with time. Motion can be described in terms of the distance moved or the displacement. ii) Distance moved :- is the actual length of the path travelled by a body. iii) Displacement :- is the length of the shortest path travelled by a + ody from its initial position to its final position. &qg :- If a body starts moving in a straight line from origin O and moves through C and B and reaches A and then moves back and reaches C through B, then Distance travelled = 60 + 35 =95 km Displacement =25km °, c \ BF A IRE BI 0 5 10 15 20 25 30 35 40 45 50 55 60 km2) Uniform motion and Non uniform motion :- i) Uniform motion :- If a body travels equal distances in equal intervals of time, it is said to be in uniform motion. ii) Non uniform motion :- If a body travels unequal distances in equal intervals of time, it is said to be in non uniform motion. iii) Speed :- of a body is the distance travelled by the body in unit time. Distance- Speed = Time If a body travels a distance s in time t then its speed v is s The SI unit of speed is metre per second m/s orms Since speed has only magnitude it is a scalar quantity. iv) Average speed :-is the ratio of the total distance travelled to the total time taken. Sethi betesciteretion Average speed = Total time taken3) Speed with direction :- The rate of motion of a body is more meaningful if we specify its direction of motion along with speed. The quantity which specifies both the direction of motion and speed is velocity. i) Velocity :- of a body is the displacement of the body per unit time. Displacement. Velocity = Time taken “ince velocity has both magnitude and direction, it is a vector quantity. Average velocity :- is the ratio of the total displacement to the total ume taken. Total displacement Average velocity = Total time taken Average velocity is also the mean of the initial velocity u and final velocity v. a aig ki Average velocity = a = 2 Speed and velocity have the same units m/s or ms4) Rate of change of velocity :- During uniform motion of a body in a straight line the velocity remains constant with time. In this case the change in velocity at any time interval is zero (no change in velocity). During non uniform motion the velocity changes with time. In this case the change in velocity at any time interval is not zero. It may be positive (+ ve) or negative (- ve). The quantity which specifies changes in velocity is acceleration. Acceleration :- is the change in velocity of a body per unit time.( or the rate of change of velocity.) Change in velocity Acceleration = ———— Time If the velocity of a body changes from initial value u to final value v in time t, then acceleration ais veu t The SI unit of acceleration is ms * Uniform acceleration :- If the change in velocity is equal in equal intervals of time it is uniform acceleration. Non uniform eration :- If the change in velocity is unequal in equal intervals of time it is non uniform acceleration. 25) Graphical representation of motion :- a) Distance — Time graphs :- The change in the position of a body with time can be represented on the distance time graph. In this graph distance is taken on the y - axis and time is taken on the x — axis. i) The distance time graph for uniform speed is a straight line ( linear ). This is because in uniform speed a body travels equal distances in equal intervals of time. We can determine the speed of the body from the distance — time graph. For the speed of the body between the points A and B, distance is (s, - s,) and time is (t, -t,). You Cot s jes) _ a & t (t,t) 8 20-10 10 5 20 = 2 0-5 5 7 2 mines 10 0 15 20 x 10 Time (s) Distance — time graph for a body moving with uniform speedii) The distance — time graph for non uniform motion is non linear. This is because in non uniform speed a body travels unequal distances in equal intervals of time. Distance (m) s é COC 101520 Time (s) Distance — time graph for a body moving with non uniform speedb) Velocity — time graphs :- The change in the velocity of a body with time can be represented on the velocity time graph. In this graph velocity is taken on the y — axis and time is taken on the x — axis. i) If a body moves with uniform velocity, the graph will be a straight line parallel to the x — axis . This is because the velocity does not change with time. To determine the distance travelled by the body between the points A and B with velocity 20 km h* Y 40 30 s=vxt v= 20 km h* = AC or BD t= t,-t, =DC = AC (t,-t,) = AC XCD area ofthe rectangle ABDC —g Velocity (km h") x 5 10 415 20 Time (s) Velocity - time graph for a body moving with uniform velocityii) If a body whose velocity is increasing with time, the graph is a straight line having an increasing slope. This is because the velocity increases by equal amounts with equal intervals of time. The area under the velocity — time graph is the distance (magnitude of displacement) of the body. The distance travelled by a body between the points A and E is the area ABCDE under the velocity - time graph. area ABCDE area of rectangle ABCD + area of triangle ADE 1 3=ABX BC+ --- (AD XDE) 2 Velocity (m s") 30 Time (s) Velocity - time graph for a body moving with uniform accelerationiii) If a body whose velocity is decreasing with time, the graph is a straight line having an decreasing slope. This is because the velocity decreases by equal amounts with equal intervals of time. iv) If a body whose velocity is non uniform, the graph shows different variations. This is because the velocity changes by unequal amounts in equal intervals of time. Y 40 sok 3 Velocity (ms) 8 1 10 15 20 9 10 «15 20 Time (s) Time (s) Velocity - time graph for a uniformly Velocity - time graph for decelerated motion non uniform acceleration6) Equations of motions by graphical method :- The motion of a body moving with uniform acceleration can be described with the help of three equations called equations of motion. The equations of motion are :- i) veutat ii) s=ut+ “at? 4) 2as =v? -u? where u - is the initial velocity v - is the final velocity a-is acceleration t- is the time s-is the distance traveleda) Equation for velocity — time relation ( v = u + at) :- Consider a velocity — time graph for a body moving with uniform acceleration ‘a’, The initial velocity is u at A and final velocity is v at B in time t. Perpendicular lines BC and BE are drawn from point B to the time and velocity axes so that the initial velocity is OA and final velocity is BC and time interval is OC. Draw AD parallel to OC. We observe that BC = BD +DC=BD+0A Substituting BC = v and OA=u “Je get v=BD+u or BD=v-u Change in veloci Acceleration = BD a a AD v-ue | Time (s) Velocity - time graph for a uniformly accelerated motionb) Equation for position — ti it + % at?) :- Consider a velocity — time graph for a body moving with uniform acceleration ‘a’ travelled a distance s in time t. The distance traveled by the body between the points A and B is the area OABC. s = area OABC ( which is a trapezium ) = area of rectangle OABC + area of triangle ABD ie relation (s = 1 | =OAXOC + -- (ADXBD) 8 2 im ch Substituting OA = u, OC = AD=t, é fs BD=v-u=at 2 br} 3 HH We get gre eee t 1 > 1b ro s=uxt+--(t x at) t 2 HEH EH | or|s = ut + at? ° Time (8) Velocity - time graph for a uniformly accelerated motionon — velocity relation (2as Consider a velocity — time graph for a body moving with uniform acceleration a’ travelled a distance s in time t. The distance travelled by the body between the points A and Bis the area OABC. s = area of trapezium OABC (OA + BC) X OC se 5 i E Substituting OA=u, BC=v and OC=t &% (ut+v)Xt £ We get s = g 8 S From velocity - time relation 7 (v-u) & a a f (v tu)X(v-u) Time (s) or) 2as = v?—Uu?| Velocity— time graph for a uniformly accelerated motion7) Circular motion :- The motion of a body in a circular path is called circular motion. Uniform circular motion :- Ifa body moves ina circular path with uniform speed, its motion is called uniform circular motion. Uniform circular motion is accelerated motion because in a circular motion a body continuously changes its direction. The circumference of a circle of radius ris given by 2nr. If a body takes time t to go once around the circular path, then the velocity v is given by 2nr