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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.