7. A 1500 kg car traveling at 100 km/h is towing a 250 kg trailer. The coefficie

Question

7. A 1500 kg car traveling at 100 km/h is towing a 250 kg trailer. The coefficient of friction between the tires and the road is 0.8 for both the car and trailer Approximately what energy is dissipated by the brakes if the car and trailer are braked to a complete stop? (A) 96 kJ (B) 390 k.J (C) 580 kJ (D) 680 kJ 8. A 12 kg aluminum box is dropped from rest onto a large wooden beam. The box travels 0.2 m before con- tacting the beam. After impact, the box bounces 0.05 m above the beams surface. Approximately what impulse does the beam impart on the box? (A) 8.6 N.s (B) 12 N s (C) 36 N.s (D) 42 Ns 5. The 170 kg mass, m, shown is guided by a frictionless rail. The spring is compressed sufficiently and released such that the mass barely reaches point B. 1 m compressed L m# 170 kg position What is most nearly the kinetic energy of the mass at point A? (A) 20J (B) 220 J (C) 390 J (D) 1700 J

Explanation / Answer

7)

Energy Dissipated by the brakes = the change in kinetic energy of the car + trailer.

Therefore Kinetic energy = 1/2*M*V^2

KE =1/2*(1500 + 250)*(100*5/18)^2 (1 km/hr=5/18 m/sec)

KE = 675 KJ = 680 KJ (approx)

8)

Given :-

m = 12 kg, h1 = 0.2, h2 = 0.05

v1 = sqrt(2gh1)

v1 = sqrt(2 x 9.81 x 0.2) = 1.980 m/s

v2 = sqrt(2gh2)

v2 = sqrt(2 x 9.81 x 0.05) = 0.99 m/s

Impulse = m(v1 - v2) = 12*(1.980 - 0.99)

Impulse = 12 N-s (approx)

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