# A vehicle of mass 840 kg pecorre a straight and level road at 72 Km / h. Suddenly, the brakes are applied reducing the speed to 36 km / h. Calculate: a) A vehicle of mass 840 kg pecorre a straight and level road at 72 Km / h. Suddenly, the brakes are applied reducing the speed to 36 km / h. Calculate: a) The work of the force exerted by the brakes on the variation of speed b) The force applied by the brakes

2
by Castieldoidao

2014-07-22T19:18:19+05:30

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M = 840 kg         u = 72 * 5 / 18 = 20 m/s              v = 36 * 5 /18 = 10 m/s
average deceleration = (v-u) / t = 10 / t
force by breaks = m a = 8400 / t
Impulse exerted by breaks = 8400 N-sec
work by force = change in K.E. = 0 - 1/2 * 840 * (20*20 - 10*10) = - 126000 J
= same as force exerted * distance traveled

2014-07-29T16:50:39+05:30

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Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
Mass = 840 kg,
Initial velocity = 72 km/hr = 72 × 5/18 = 20 m/s
Final Velocity = 36 km/hr = 36× 5/18 = 10 m/s
Let time be t.
Acceleration = (v-u)/t = (20-10)/t = 10 / t
Force by the brakes = Mass × Acceleration = (840×10/t) = 8400/t
Force exerted by breaks = 8400 N/sec (answer a)

Work by force = Change in Kinetic Energy
= 0-1/2 ×840×(20×20-10×10) =  -126000 Joules (answer b)