# Setting gravity in RigidBodyDynamics.jl

**URL:** <https://discourse.julialang.org/t/setting-gravity-in-rigidbodydynamics-jl/16748>\
**Category:** Modelling & Simulations\
**Created:** [October 24, 2018, 4:41pm UTC](https://discourse.julialang.org/t/setting-gravity-in-rigidbodydynamics-jl/16748 "2018-10-24T16:41:10Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![HiroIshida](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hiroishida/32/7424_2.png) [@HiroIshida](https://discourse.julialang.org/u/HiroIshida)\
**Post date:** [October 24, 2018, 4:41pm UTC](https://discourse.julialang.org/t/setting-gravity-in-rigidbodydynamics-jl/16748/1 "2018-10-24T16:41:10Z")

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I have just started to use [RigidBodyDynamics](https://github.com/JuliaRobotics/RigidBodyDynamics.jl). According to the [tutorial](https://github.com/JuliaRobotics/RigidBodyDynamics.jl/blob/master/notebooks/Quickstart%20-%20double%20pendulum/Quickstart%20-%20double%20pendulum.ipynb), the proper way to set gravity seems `m = Mechanism(world; gravity = SVector(0, 0, g)`. But when constructing a mechanism using `parse_urdf`, what is the proper way to set gravity? My trial was like

> m = parse\_urdf(myurdf.urdf)  
> m.gravitational\_acceleration = FreeVector3D(default\_frame(m.tree.root), SVector(0, 0, -9.8))

but I guess mine is wrong and there is a better way.

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**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [October 24, 2018, 5:17pm UTC](https://discourse.julialang.org/t/setting-gravity-in-rigidbodydynamics-jl/16748/2 "2018-10-24T17:17:49Z")

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Yeah, that’s a bit of an oversight. An additional keyword argument for that should be added to `parse_urdf`. For now, I’d do

```julia
urdf = "bla.urdf"
m = parse_urdf(urdf)
m.gravitational_acceleration = FreeVector3D(root_frame(m), 0, 0, -9.81)

```

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**Author:** ![HiroIshida](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hiroishida/32/7424_2.png) [@HiroIshida](https://discourse.julialang.org/u/HiroIshida)\
**Post date:** [October 25, 2018, 6:30am UTC](https://discourse.julialang.org/t/setting-gravity-in-rigidbodydynamics-jl/16748/3 "2018-10-25T06:30:59Z")

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I understand. Thanks for your answer and great software!