Work Done on Jet by Engines & Catapult during Launch

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The discussion focuses on calculating the work done on a jet during its launch from an aircraft carrier. The jet has a mass of 17,000 kg and experiences a thrust force of 160 kN from its engines. The initial attempt to calculate work using the formula W=Fdcos(θ) incorrectly applied a negative cosine value, leading to an erroneous result. It was clarified that since the force is in the direction of the displacement, the work should be considered positive, and the cosine factor is unnecessary in this context. The key takeaway is that the work done by the engines contributes positively to the jet's kinetic energy during launch.
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Homework Statement


A 17000kg jet takes off from an aircraft carrier via a catapult. The gases thrust out from the jet's engines exert a constant force of 160 kN on the jet;the force exerted on the jet by the catapult is plotted in the figure b.

a)Determine the work done on the jet by the gases expelled by its engines during launch of the jet.
b)Determine the work done on the jet by the catapult during launch of the jet.

GIANCOLI.ch07.p13.jpg


Homework Equations


W=Fdcos\theta

The Attempt at a Solution



a)Since no displacement is stated outright, I assumed that it is 85m from the plot.
160kN=160,000N
W=160000*85*cos(180)=-1.36*10^7

This was an incorrect answer. What did I do wrong?
 
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Symstar said:

Homework Statement


A 17000kg jet takes off from an aircraft carrier via a catapult. The gases thrust out from the jet's engines exert a constant force of 160 kN on the jet;the force exerted on the jet by the catapult is plotted in the figure b.

a)Determine the work done on the jet by the gases expelled by its engines during launch of the jet.
b)Determine the work done on the jet by the catapult during launch of the jet.

Homework Equations


W=Fdcos\theta

The Attempt at a Solution



a)Since no displacement is stated outright, I assumed that it is 85m from the plot.
160kN=160,000N
W=160000*85*cos(180)=-1.36*10^7

This was an incorrect answer. What did I do wrong?

Since it is the Force over positive distance and the Force is in the direction of Positive distance, then the work is positive. You don't need Cosθ. The positive force goes to build positive KE which is increasing V2 not decreasing.
 
Thanks, I was a little thrown off by the thrust out of the back of the plane I guess.
 

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