Newton's 2nd Law conceptual problem

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Discussion Overview

The discussion revolves around a conceptual problem related to Newton's 2nd Law, specifically regarding the forces acting on a rocket during its upward acceleration from Earth. Participants explore the implications of gravitational force in calculating the net force required for the rocket's acceleration.

Discussion Character

  • Conceptual clarification, Technical explanation, Debate/contested

Main Points Raised

  • One participant asserts that the thrust force of 30,000 N is sufficient for the rocket's acceleration based on the formula F=MA.
  • Another participant counters that the thrust must also account for the weight of the rocket, suggesting that the total force required is the sum of the gravitational force and the force needed for acceleration.
  • A third participant expands on this by providing the correct expression, indicating that the force should equal the mass times the sum of the acceleration due to gravity and the upward acceleration.
  • Another participant emphasizes that Newton's 2nd Law considers all forces acting on the body, including both upward thrust and downward gravitational force.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are competing views on the correct application of Newton's 2nd Law in this scenario. Some participants believe the initial calculation is valid, while others argue for the inclusion of gravitational force in the total thrust calculation.

Contextual Notes

The discussion highlights the importance of considering all forces acting on an object, particularly in scenarios involving acceleration and gravity. There are unresolved aspects regarding the interpretation of the forces involved and the assumptions made in the 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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