Newton's 2nd Law conceptual problem

[[V]]

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!

 

[cjl]

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)

 

[Sakha]

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.

 

[MasterX]

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.