Newton's 2nd Law conceptual problem

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[V]

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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
[tex]
30000N=15000kg*2m/s^2
[/tex]

indeed the math is true?

why is the answer false? What am I missing? Thanks!
 

cjl

Science Advisor
1,754
343
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)
 
295
0
Expanding cjl's post:

[tex]
30000N=15000kg*2m/s^2
[/tex]
is wrong; while
[tex]
F=15000kg*(2m/s^2 + 9.8m/s^2)
[/tex]
is the right expression.
 
51
0
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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