Newton's Second Law Rocket Problem

In summary, the person needs a thrust of 740N to overcome the force of gravity and lift off from the surface of the Earth, taking into account his mass of 20kg and the mass of the rockets. The normal force and weight of the person are not useful in getting off the ground, as they must be exceeded in order to achieve an upward acceleration.
  • #1
mailmas
46
0

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
 
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  • #2
Is there a force he has to overcome before he can lift off?
 
  • #3
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.
 
  • #4
He is. What forces are acting on him?
 
  • #5
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.
 
  • #6
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?
 
  • #7
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?
 
  • #8
He isn't weightless. His weight is still acting on him. What would he have to do to appear effectively weightless?
 
  • #9
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!
 

FAQ: Newton's Second Law Rocket Problem

What is Newton's Second Law Rocket Problem?

Newton's Second Law Rocket Problem is a mathematical problem that involves calculating the acceleration and velocity of a rocket based on the forces acting upon it. It is based on Newton's Second Law of Motion, which states that the net force on an object is equal to its mass multiplied by its acceleration.

What are the forces that act on a rocket in this problem?

The main forces that act on a rocket in this problem are thrust, gravity, and air resistance. Thrust is the force that propels the rocket forward, gravity is the force that pulls the rocket towards the center of the Earth, and air resistance is the force that opposes the rocket's motion through the air.

How do you calculate the acceleration of a rocket in this problem?

To calculate the acceleration of a rocket, you can use the formula a = F/m, where F is the net force acting on the rocket and m is the mass of the rocket. First, you must calculate the net force by adding up all the forces acting on the rocket. Then, you can divide the net force by the mass of the rocket to find the acceleration.

Can you use this problem to determine the maximum height and distance a rocket can reach?

Yes, you can use this problem to determine the maximum height and distance a rocket can reach. By calculating the acceleration and velocity of the rocket, you can then use equations of motion to determine the maximum height and distance the rocket can reach before it falls back to Earth.

What are some real-world applications of the Newton's Second Law Rocket Problem?

The Newton's Second Law Rocket Problem has many practical applications, such as in the design and testing of rockets and spacecraft. It can also be used in the study of aerodynamics and in the development of new propulsion systems for space exploration.

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