Tiêu đề 01. UNITS AND DIMENSIONS2.pdf ✅

02 UNITS AND DIMENSIONS #QID# 36639 (1.) An aeroplane flies 400m north and 300m south and then flies 1200m upwards, then net displacement is (a.) 1500m (b.) 1400m (c.) 1300m (d.) 1200m Ans: d Exp: (d) An aeroplane flies 400m north and 300m south so the net displacement is 100m towards north. Then it flies 1200m upward so ( ) ( ) 2 2 r 100 1200 = + =1204 m 1200 m #QID# 36640 (2.) A very large number of balls are thrown vertically upwards in quick succession in such a way that the next ball is thrown when the previous one is at the maximum height. If the maximum height is 5m , the number of ball thrown per minute is (take 2 g 10 ms− = ) (a.) 120 (b.) 80 (c.) 60 (d.) 40 Ans: c Exp: (c) Maximum height of ball = 5 m So velocity of projection  = = u 2gh 10 m / s Time interval between two balls (time of ascent)
u 1 1 sec min g 60 = = = So number of ball thrown per min = 60 #QID# 36641 (3.) A constant force acts on a body of mass 0.9 kg at rest for 10s. If the body moves a distance of 250 m , the magnitude of the force is (a.) 3N (b.) 3.5N (c.) 4.0N (d.) 4.5N Ans: d Exp: (d) u 0,S 250m,t 10sec = = =   2 1 1 2 2 S ut at 250 a 10 a 5m / s 2 2 = +  =  = So, F ma 0.9 5 4.5N = =  = #QID# 36642 (4.) The speed of body moving with uniform acceleration is u. This speed is doubled while covering distance S , its speed would be become (a.) 3u (b.) 5u (c.) 11u (d.) 7u Ans: d Exp: (d) As ( ) 2 2 2 2 2 v u 2as 2y u 2as 3u = +  = + = Now, after covering an additional distance s , if velocity becomes v ,then,
( ) 2 2 2 2 2 2 v u 2a 2s u 4as u 6u 7u = + = + = + =  =v 7u #QID# 36643 (5.) The variation of velocity of a particle moving along a straight line is shown in the figure. The distance travelled by the particle in 4s is (a.) 25 m (b.) 30 m (c.) 55 m (d.) 60 m Ans: c Exp: (c) ( ) 1 1 S 1 20 1 20 20 10 1 1 10 2 2 =   +  +  +  +  = 55 m #QID# 36644 (6.) A body falling for 2 seconds covers a distance S is equal to that covered in next second. Taking 2 g 10m / s = , S = (a.) 30 m (b.) 10 m (c.) 60 m (d.) 20 m Ans: a Exp: (a) If u is the initial velocity then distance covered by it in 2 sec
( ) 1 1 2 S ut at u 2 10 4 2u 20 i 2 2 = + =  +   = +  Now distance covered by it in 3rd sec rd ( ) ( ) 3 1 S u 2 3 1 10 u 25 ii 2 = +  − = +  From (i) and (ii), 2u 20 u 25 u 5 + = +  =  =  + = S 2 5 20 30 m #QID# 36645 (7.) A ball A is thrown up vertically with speed u and at the same instant another ball B is released from a height h . At time t , the speed of A relative to B is (a.) u (b.) 2u (c.) u gt − (d.) ( ) 2 u gt − Ans: a Exp: (a) At time t Velocity of A, v u gt A = − upward Velocity of B, v gt B = downward It we assume that height h is smaller than or equal to the maximum height reached by A , then at every instant A v and B v are in opposite directions  = + V v V AB A B = − + u gt gt [Speeds in opposite directions get added]