Solving the Force of Reverse Thrust on Mars' Polar Lander

In summary: Voutilainen in summary, his teacher gave him a question about a deceleration and he wasn't sure how to solve it, so he did some research and found that there was a second part to the question which was about the force of reverse thrust. He used the equation Fnet = Fr - Fg to solve for the force of reverse thrust.
  • #1

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Hi guys… there was a lot of arguing in my class when my teacher gave us this one assignment question that I’m not too sure about. It goes as follows…

A Polar Lander somewhere above the surface of the Mars is landing with a deceleration of 23m/s^2. If the Lander itself had a mass of approximately 700kg, solve for the net force.

So, this is what I did:
Fnet = ma
= (700kg)(-23m/s^2)
= -16100N [down] or 16100N [up]

Ok so this is the weird part, there was a second part to the question which goes as follows…

The acceleration due to gravity of Mars is 3.6m/s^2. Using this fact solve for the force of reverse thrust that was required to provide the necessary overall upward net force found above.

… there was a diagram attached as well which helped make it easier but basically what is showed was an arrow downwards on the Lander saying it was Fg and an arrow opposite to Fg (upwards) which was the force of the reverse thrust slowing it down, named Fr.

So this is what I did:

Fg = mg
= (700kg)(3.6m/s^2)
= 2520N

And this is where all the arguing started. We weren’t too sure what formula to use to find Fr.

Fnet = Fg – Fr or is it Fnet = Fr- Fg? He said the answer is 18 620 N

This is what I did. Using Fnet = Fg - Fr:
Fr = -Fnet + Fg
= -(-16100N) + 2520N
= 18 620 N which looks correct to me.

But when I saw the answer done for us, this was the method used:

Fnet = Fr – Fg
Fr = Fnet + Fg
= (16100) + 2520 (NOTE: notice how Fnet all of a sudden became positive?)
= 18 620 N
Can you do that?... or is my way the correct way? I just don’t want to lose marks for something that was given to us but seemed wrong through my eyes.
 
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  • #2
You need to start off with your free body diagram and let that be the way you determine what signs go where. For example, I would draw the lander as a block with Fr pointing up and Fg = mg pointing down. In my coordinate system, I am specifying that up is positive. Since the lander is decelerating, it is accelerating in an upward direction. This means that the net force points upward as well. So now you can write Fnet = Fr - Fg where Fnet = ma and a is a positive number (the magnitude of the deceleration). I don't think I wrote that very clearly; let me know if it still doesn't make sense.
 
  • #3
yea i get what u are saying. so if up is positive in the diagram then the Fnet since it is slowing down(moving up in this case) will also change to positive?... so if i left it the way it was where Fg was positive would this wouldn't be wrong would it?

Fnet = Fg - Fr
Fr = -Fnet + Fg
= -(-16100N) + 2520N
= 18 620 N
 
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  • #4
It's not "wrong" provided that you understand that you have now defined down as the positive direction.
 
  • #5
alright kool... thnx alot

- Tu
 
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1. What is the purpose of studying reverse thrust on Mars' Polar Lander?

The purpose of studying reverse thrust on Mars' Polar Lander is to better understand the effects of atmospheric conditions and gravity on spacecraft landings on Mars. This information can also help improve future spacecraft designs and landing strategies for Mars missions.

2. How does reverse thrust work on Mars' Polar Lander?

Reverse thrust on Mars' Polar Lander involves firing the spacecraft's engines in the opposite direction of its motion, creating a force that slows down the descent and allows for a controlled landing on the surface of Mars.

3. What challenges are involved in solving the force of reverse thrust on Mars' Polar Lander?

Solving the force of reverse thrust on Mars' Polar Lander involves considering factors such as the composition and density of the Martian atmosphere, the strength and direction of gravity, and the capabilities of the spacecraft's engines. Additionally, uncertainties and variations in these factors can add complexity to the calculations.

4. How is the force of reverse thrust calculated for Mars' Polar Lander?

The force of reverse thrust on Mars' Polar Lander is calculated using the basic principles of Newton's laws of motion. This involves determining the mass and acceleration of the spacecraft, as well as the forces acting on it, such as gravity and atmospheric drag. Advanced simulations and modeling techniques may also be used to refine the calculations.

5. What are the potential applications of understanding reverse thrust on Mars' Polar Lander?

Understanding reverse thrust on Mars' Polar Lander can have various applications in the field of space exploration. It can help improve the accuracy and success rate of future Mars landings, as well as inform the design of spacecraft and landing systems for other planetary bodies. Additionally, this knowledge can also be applied to other areas, such as aerospace engineering and propulsion systems.

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