Finding net force from acceleration, speed and distance

AI Thread Summary
The discussion revolves around calculating the mass of a railroad engine that stops from a speed of 16 m/s over a distance of 1.1 km, under the assumption of a constant horizontal resistive force. The student initially uses the equation vf^2 - vi^2 = 2ad to find acceleration, converting the distance to meters for accuracy. It is noted that without additional data, such as the power of the engine or the tractive force, the mass cannot be determined, as multiple masses could yield the same acceleration. The conversation also clarifies that the train is not moving at a constant velocity since it is decelerating. Ultimately, the importance of accurate unit conversion and additional data for solving the problem is emphasized.
Lis277
Messages
7
Reaction score
0

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^2-vi^2=2ad
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.
 
Physics news on Phys.org
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?
 
PS what would be the acceleration if the train stopped in 1.1m?
 
PeroK said:
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
 
PeroK said:
PS what would be the acceleration if the train stopped in 1.1m?
a=-116.36?
 
Lis277 said:
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?
 
Lis277 said:
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 ...
 
PeroK said:
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
 
Lis277 said:
acceleration would be 0.2327m/s
That looks a bit too big. How did you get that?
 
  • #10
PeroK said:
That looks a bit too big. How did you get that?
oops, made a mistake
vf^2-vi^2=2ad
vf=0
vi=16
a=?
d=1.1km-->1,100m
(0)^2-(16)^2=2(1100)a
-256=2200a
-0.11636m/s=a
 
  • #11
Lis277 said:
oops, made a mistake
vf^2-vi^2=2ad
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.
 
  • #12
PeroK said:
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.
 
  • #13
haruspex said:
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?
 
  • #14
Lis277 said:
Does the fact that the train is moving at a constant velocity make any difference?
The train is not moving at a constant velocity. It is slowing down.

According to Google, context for the problem can be found at: https://www.coursehero.com/file/24013128/N11-B2p1Forcespdf/

"A total horizontal resistive force of 76kN acts on the railway engine."
 
  • Like
Likes CWatters

Similar threads

Back
Top