Newton's 2nd Law conceptual problem

AI Thread Summary
The discussion centers on the application of Newton's 2nd Law to a rocket with a mass of 15,000 kg accelerating upward at 2.0 m/s². The initial calculation of 30,000 N as the thrust force is incorrect because it does not account for the rocket's weight. The correct formula includes both the gravitational force and the upward acceleration, leading to the equation F = 15,000 kg * (2 m/s² + 9.8 m/s²). This means the thrust force must actually be 179,700 N to overcome gravity and achieve the desired acceleration. Understanding the forces acting on the rocket is crucial for accurate calculations.
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A 15,000 kg rocket blasts off from Earth with a uniform upward acceleration of 2.0 m/s2 and feels no air resistance. The thrust force its engines must provide during this acceleration is 30,000 N upward.

Apparently this is FALSE!

I don't understand why
F=MA
<br /> 30000N=15000kg*2m/s^2<br />

indeed the math is true?

why is the answer false? What am I missing? Thanks!
 
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Because you have to also counter the rocket's weight, so the force must be equal to the rocket's weight plus an additional 30,000 N to cause a 2 m/s2 upward acceleration.

(It might be easier to see if you draw a free body diagram)
 
Expanding cjl's post:

<br /> 30000N=15000kg*2m/s^2<br />
is wrong; while
<br /> F=15000kg*(2m/s^2 + 9.8m/s^2)<br />
is the right expression.
 
Remember that F in Newton's 2nd law are all the forces that are acting on a body. Thus, besides the upward force, you also have a downward force, which is gravity.
 

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