# Finding net force from acceleration, speed and distance

Lis277

## Homework Statement

The power driving the railroad engine is switched off. The engine stops, from its speed of 16m/s^-1, without braking in a distance of 1.1km. A student hypothesis that the horizontal resistive force is constant. Based on the hypothesis, calculate the mass of the railway engine.

## Homework Equations

Teacher didn't provide any relevant equations but i assume F=ma and vf^2-vi^2=2ad are important

## The Attempt at a Solution

First i tried to find
acceleration

vf=0
vi=16
a=?
d=1.1
(0)^2-(16)^2=2(1.1)a
-256=2.2a
-116.36=a

I think the next step would be to find net force to plug all my variables into F=ma to find mass. I know net force is the total force on an object but I'm not sure how to find net force from my given variables.

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Does it seem logical to you that an object that starts at 16 m/s and stops after 1.1 km can only have one possible mass?

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PS what would be the acceleration if the train stopped in 1.1m?

Lis277
Does it seem logical to you that an object that starts at 16 m/s and stops after 1.1 km can only have one possible mass?
I would assume it only has one mass

Lis277
PS what would be the acceleration if the train stopped in 1.1m?
a=-116.36?

Lis277
a=-116.36?
since acceleration is normally in the units of meters and i was given a distance in km, do i have to convert to m before plugging the distance in the equation to find acceleration?

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since acceleration is normally in the units of meters and i was given a distance in km, do i have to convert to m before plugging the distance in the equation to find acceleration?
What do you think?

Many distances could be 1.1 something. Inches, feet, yards, miles, light years, megaparsecs, angstrom units ...

Lis277
What do you think?

Many distances could be 1.1 something. Inches, feet, yards, miles, light years, megaparsecs, angstrom units ...
acceleration would be 0.2327m/s

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acceleration would be 0.2327m/s
That looks a bit too big. How did you get that?

Lis277
That looks a bit too big. How did you get that?
vf=0
vi=16
a=?
d=1.1km-->1,100m
(0)^2-(16)^2=2(1100)a
-256=2200a
-0.11636m/s=a

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vf=0
vi=16
a=?
d=1.1km-->1,100m
(0)^2-(16)^2=2(1100)a
-256=2200a
-0.11636m/s=a

Strictly speaking you should only really specify two significant digits and acceleration is ##m/s^2##.

On the other point, you can't know the mass without further data, as any vehicle could have this acceleration.

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you can't know the mass without further data
I suspect that the intended additional data is either the power being developed by the engine before it was switched off, or maybe the tractive force. @Lis277 , please check you were not told either of these.

Lis277
I suspect that the intended additional data is either the power being developed by the engine before it was switched off, or maybe the tractive force. @Lis277 , please check you were not told either of these.
Does the fact that the train is moving at a constant velocity make any difference?

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