Kinematics

25.A body falls freely from the top of tower. It covers 36 % of total height in the last second, before striking the ground level, the height of the tower is

solution: a body falls freely from the tower ,it means it has initial velocity is zero. we have formula ,The total time taken is ‘t’ S= ut +(1/2)at2 Total height (H)=(1/2)g(t)2 …………. (1) For remaining 64% covered in (t-1) seconds (64/100)H=1/2g(t-1)2 ……………….(2) Equation (1)/Equation(2) 1/(64/100)=(t)2 /(t-1)2 (t/t-1)=10/8 t=5sec then, Total height (H)=(1/2)g(t)2 =(1/2)*10*5*5 H =125m

20.A bullet loses 1/20 of its velocity on passing through a plank. The least number of planks required to stop the bullet is

solution: velocity of bullet after passing from plank ,its velocity decreases . initial velocity(u)=u final velocity(v)=initial velocity-1/20 of initial velocity v=u-u/20 v=19u/20 we have formula for one plank, (v)2-(u)2=2as, (19u/20)2-(u)2=2as 39u2/400=2as ……….(1) for n planks: then width of one plank will be ‘s’ width of n planks will be ns after passing from n planks …

20.A bullet loses 1/20 of its velocity on passing through a plank. The least number of planks required to stop the bullet is Read More »

19.A car initially at rest starts with a uniform acceleration and gains a velocity of 20m/s in 4s.The displacement of the car is

solution: a car initially starts at rest (u)=0 it is moving with uniform acceleration and the car gains velocity(v)=20m/s. the time taken by the car is (t)=4s we have the formula v=u+ at 20=0+4a 20=4a a=5m/s2 we got the uniform acceleration a =5m/s2 then ,for displacement S=ut+1/2at2 =0*4+1/2*5*4*4 =1/2*5*2*4 =40m or displacement=(u+v)/2*t =(0+20)/2*4 =40m The …

19.A car initially at rest starts with a uniform acceleration and gains a velocity of 20m/s in 4s.The displacement of the car is Read More »

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