Newton's Second Law Rocket Problem

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Homework Help Overview

The discussion revolves around a physics problem involving Newton's Second Law, specifically related to a person attempting to accelerate vertically from the Earth's surface using a rocket. The original poster is trying to determine the necessary thrust to achieve an upward acceleration of 5.0 m/s², given their mass and the mass of the rocket.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the forces acting on the person, including weight and normal force, and question the conditions necessary for lift-off. There is discussion about the implications of being weightless and the need to overcome gravitational force to achieve upward acceleration.

Discussion Status

Participants are actively engaging with the problem, questioning assumptions about forces and discussing the relationship between thrust, weight, and acceleration. Some guidance has been offered regarding the force equation and the conditions for lift-off, but no consensus has been reached on the final solution.

Contextual Notes

There is an emphasis on the absence of friction and air resistance in the problem setup, and participants are clarifying the definitions of weight and normal force in the context of the scenario.

mailmas
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Homework Statement


A person whose mass is 20 kg needs to accelerate vertically from the surface of the Earth at 5.0m/s^2
and is trying to pick which rocket he should strap to his back. How much thrust
does he need if each rocket has a mass of 30 kg?

Homework Equations


F=ma

The Attempt at a Solution


F = m*a
F = 50*5 = 250N
I don't understand why the answer is 740N
no friction, no air resistance, only gravity and the normal force
 
Last edited:
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Is there a force he has to overcome before he can lift off?
 
mjc123 said:
Is there a force he has to overcome before he can lift off?

I assumed he was on the surface of the Earth.
 
He is. What forces are acting on him?
 
mjc123 said:
He is. What forces are acting on him?
I'm pretty sure there is no friction and air resistance. Just his weight and the Normal Force from the surface that are acting on him. Also his mass is 20kg not his weight my bad.
 
What would he have to do to be just touching the surface of the Earth in such a way that the normal force is zero?
 
mjc123 said:
What would he have to do to be just touching the surface of the Earth in such a way that the normal force is zero?
Would he have to be weightless?
 
He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?
 
mjc123 said:
He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?
He would have to have a force in the opposite direction with the same magnitude as his weight so that Fnety = 0?
 
  • #10
mjc123 said:
He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?
In order for him to accelerate upwards 5m/s^2 he would have to first overcome the acceleration from Earth.
 
  • #11
Yes
 
  • #12
mjc123 said:
Yes
Thanks :)
 
  • #13
The normal force equals the apparent weight, so it doesn't help you get off the ground. If you applied an upward force mg/2, for example, his apparent weight would be mg/2, so the normal force would be mg/2. The total upward force would still be mg, balancing his weight, so he wouldn't lift off. You have to exceed mg to get an upward acceleration. Your force equation should be F - mg = ma, hence F = m(a + g)
 
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  • #14
mjc123 said:
The normal force equals the apparent weight, so it doesn't help you get off the ground. If you applied an upward force mg/2, for example, his apparent weight would be mg/2, so the normal force would be mg/2. The total upward force would still be mg, balancing his weight, so he wouldn't lift off. You have to exceed mg to get an upward acceleration. Your force equation should be F - mg = ma, hence F = m(a + g)
Thank you for your help!
 

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