Robert has just bought a new model rocket, and is trying to measure its flight c

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 11.4 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 0.763 kg, how much work is done by the drag force on the rocket during its ascent?

Explanation / Answer

Let Wd be the work done by drag

As the engine is putting Ft force to lift the rocket for cut off height He.

so the work done by the engine will be

We = Ft * He = 11.4* 19 = 216.6 J

This energy gets converted to KE, PE and a fraction of this is lost due to air drag.

now when the rocket reaches the peak height of its flight, at this time the kinetic energy become zero because its velocity becomes zero

At this time all the energy that the rocket has the gravitational potential energy PE.

PE = M g Hp = 0.763* 9.8* 23.5 = 175.72 J

Drag Force will then be:

Wd = We - PE =216.6- 175.7 = 40.9 Joules

or 41 joules is the Work done by drag force

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