CODEWITHSUNDEEP
pythonphysicsvpython
Bob John
Bob John

Reputation: 3888

Two masses connected by a spring...trying to simulate flight but having difficulties

We have two masses of equal mass 0.1kg connected by a stiff spring with k = 10^5 N/m. The rest length of the spring (and the initial distance between the two masses) is d0 = 0.15m

Mass 1 has initial velocity (2.77, 1.25, 0) m/s while mass 2 has initial velocity (1.25, 4, 0) m/s.

I want to write a code that simulates the flight of this system under gravity while ignoring air resistance. I also want to place a third sphere located at the center of mass of the system.

Here's what I've written so far:

from visual import *

mass1 = sphere(radius = 0.01)
mass2 = sphere(pos = (0.15, 0, 0), radius = 0.01)
COM = sphere(pos = (0.15/2, 0, 0), radius = 0.01)

mass1.m = 0.1
mass2.m = 0.1

k = 1*10**5

mass1.v = vector(2.77, 1.25, 0)
mass2.v = vector(1.25, 4, 0)

mass1.p = mass1.v*mass1.m
mass2.p = mass2.v*mass2.m

dt = 0.0001
t = 0

while 1:
    g = 9.8
    d = (mass1.pos-mass2.pos)/mag(mass1.pos-mass2.pos)

    Ft12 = mass1.m*d*k*0.15
    Ft21 = mass2.m*d*k*0.15

    Fnet = Ft12 + Ft21

    mass1.p += Fnet*dt
    mass2.p += Fnet*dt

    mass1.vA = 0.5*(mass1.v + (mass1.p/mass1.m))
    mass2.vA = 0.5*(mass1.v + (mass2.p/mass2.m))

    mass1.pos += mass1.vA*dt
    mass2.pos += mass2.vA*dt
    COM.pos = (mass1.pos-mass2.pos)/2 + mass1.pos

    t += dt

Ft12 is the force of tension from ball 1 to ball 2, and Ft21 is from 2 to 1. Of course, I wouldn't be posting this if I were getting accurate results, but I'm not even getting results that I can readily observe. The screen just goes blank very quickly and I have no chance to see what's happening. I basically want to this system to be "thrown" under the effects of gravity, in which you can imagine that the COM's trajectory will be that of a parabola. Any help will be greatly appreciated. If I'm getting the forces wrong, or I need to add some in some places, please let me know!

Thank you, everyone!

Upvotes: 2

Views: 1551

Answers (2)

MDT
MDT

Reputation: 471

You have no 'sleep' time in which you are delaying your animation. As the spheres disapear from the screen over time (I've not actually looked at the physics of it so I don't know if you want that) and because you have no delay between time steps - they appear to disapear instantly.

If you 

from time import sleep

then you can run the following loop instead

dt = 0.0001
for step in range(1000):
    g = 9.8
    d = (mass1.pos-mass2.pos)/mag(mass1.pos-mass2.pos)

    Ft12 = mass1.m*d*k*0.15
    Ft21 = mass2.m*d*k*0.15

    Fnet = Ft12 + Ft21

    mass1.p += Fnet*(dt*step)
    mass2.p += Fnet*(dt*step)

    mass1.vA = 0.5*(mass1.v + (mass1.p/mass1.m))
    mass2.vA = 0.5*(mass1.v + (mass2.p/mass2.m))

    mass1.pos += mass1.vA*(dt*step)
    mass2.pos += mass2.vA*(dt*step)
    COM.pos = (mass1.pos-mass2.pos)/2 + mass1.pos
    sleep(0.5)

I only removed the while(1) because I don't like infinite loops =P

This at least lets you see what the three spheres are doing so you can examine if they behave as you expect.

Upvotes: 2

David Heffernan
David Heffernan

Reputation: 613163

I have grave doubts about this line:

dt += 1

I guess dt is the time step. You are incrementing the time step by a second each time around the loop. Normally you would increment time:

t += dt

but you don't have a time variable yet. Once your time step gets large, all hell will break loose. For now I would simply delete the dt += 1 line.

As for the rest of the code, I've not looked at it in any detail and I can't guess what your notation signifies. There may well be more problems in the code.

Upvotes: 3

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