Tiêu đề 06. WORK- ENERGY AND POWER(MOBILE).pdf ✅

SPARK PCMB | DOWNLOAD APP NOW| +91-920-630-6398 06. WORK- ENERGY AND POWER #QID# 38252 (1.) Ten litre of water per second is lifted from well through 20 m and delivered with a velocity of 10 ms −1 , then the power of the motor is (a.) 1.5 kW (b.) 2.5 kW (c.) 3.5 kW (d.) 4.5 kW ANSWER: b EXPLANATION: (b) P = total energy t = mgh + 1 2 mv 2 t = 10 × 10 × 20 + 1 2 × 10 × 10 × 10 1 = 2000 + 500 = 2500 W = 2.5 kW #QID# 38253 (2.) A glass ball is dropped from height 10 m. If there is 20% loss of energy due to impact, then after one impact, the ball will be upto (a.) 2 m (b.) 4 m (c.) 6 m (d.) 8 m ANSWER: d EXPLANATION: (d)
SPARK PCMB | DOWNLOAD APP NOW| +91-920-630-6398 Clearly, 80% energy is retained after impact ∴ h ′ = 80 100 × 10 = 8 m #QID# 38254 (3.) Under the action of a force, a 2 kg body moves such that its position x as a function of time t is given by x = t 3/3, where x is in metre and t in second. The work done by the force in the first two seconds is (a.) 1.6 J (b.) 16 J (c.) 160 J (d.) 1600 J ANSWER: b EXPLANATION: (b) v = dx dt = d dt ( t 3 3 ) = t 2 When t = 0, then v = 0, when t = 2, then v = 4 m/s Work done in first two second = change in KE W = 1 2 m[(4) 2 − (0) 2 ] = 1 2 × 2 × 16 = 16 J #QID# 38255 (4.) A truck of mass 30,000 kg moves up an inclined plane of slope 1 in 100 at a speed of 30 kmph. The power of the truck is (Given g = 10 ms −2 ) (a.) 25 kW (b.) 10 kW (c.) 5 kW (d.) 2.5 kW ANSWER: a EXPLANATION: (a)
SPARK PCMB | DOWNLOAD APP NOW| +91-920-630-6398 As truck is moving on an incline plane therefore only component of weight (mg sin θ) will oppose the upward motion Power = force × velocity = mg sin θ × v = 30000 × 10 × ( 1 100) × 30 × 5 18 = 25 kW #QID# 38256 (5.) A body of mass 2kg makes an elastic collision with another body at rest and continues to move in the original direction with one fourth of its original speed. The mass of the second body which collides with the first body is (a.) 2 kg (b.) 1.2 kg (c.) 3 kg (d.) 1.5 kg ANSWER: b EXPLANATION: (b) m1 = 2 kg and v1 = ( m1−m2 m1+m2 ) u1 = u1 4 [Given] By solving we get m2 = 1.2 kg #QID# 38257 (6.) Two masses of 1 g and 4g are moving with equal kinetic energies. The ratio of the magnitudes of their linear momenta is (a.) 4 : 1 (b.) √2 ∶ 1 (c.) 1 : 2 (d.) 1 : 16 ANSWER: c EXPLANATION: (c)
SPARK PCMB | DOWNLOAD APP NOW| +91-920-630-6398 p = √2mEk or p ∝ √m [∵ Ek is given to be constant] ∴ P1 P2 = √ m1 m2 = √ 1 4 = 1 2 #QID# 38258 (7.) An elastic ball is dropped from a height h and it rebounds many times from the floor .If the coefficient of restitution is e, the times interval between the second and the third impact, is (a.) ev/g (b.) e 2v/g (c.) e 2 √( 8h g ) (d.) e 2 √( h g ) ANSWER: c EXPLANATION: (c) t = √( 2h g ) The second impact occurs after an additional times = 2 √2h1g = 2e √ 2h g The third impact occurs after an additional time = 2 √2h2g = 2e 2 √ 2h g