Question

Robert has just bought a new model rocket, and is trying to measure its flight characteristics. The rocket engine package claims that it will maintain a constant thrust of 13.8 N until the engine is used up. Robert launches the rocket on a windless day, so that it travels straight up, and uses his laser range-finder to meaure that the height of the rocket when the engine cuts off is 21.2 m. He also measures the rocket\'s peak height, which is 25.7 m. If the rocket has a mass of 0.763 kg, how much work is done by the drag force on the rocket during its ascent?

Answer 1

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Robert has just bought a new model rocket, and is trying to measure its flight characteristics. The rocket engine package claims that it will maintain a constant thrust of 12.9 N until the engine is used up. Robert launches the rocket on a windless day, so that it travels straight up, and uses his laser range-finder to meaure that the height of the rocket when the engine cuts off is 19.0 m. He also measures the rocket\'s peak height, which is 23.5 m. If the rocket has a mass of 0.713 kg, how much work is done by the drag force on the rocket during its ascent?

Before the engine runs out,

Increase in Kinetic Energy + Increase in gravitational potential energy = Work done by Engine - Work done by drag force

0.5*0.713*V² + 0.713*g*19 = 12.9*19 - F_d*19 ..eq(1)

after the engine runs out,
Work done by drag force + increase in gravitational potential energy = loss in kinetic energy
now

h = 23.5-19 = 4.5 m
F_d*h + 0.713*g*h = 0.5*0.713*V² ..eq(2)

solve both the eq to get answer.