# Fix acceleration

Dear Lammps user

Can I ask if there is a command in Lammps that can fix the acceleration of a particle on a constant value? (the particle is interacting with its neighbors, however, it is forced to move by a constant acceleration).

Thanks
Fatima

You’ll probably want to check out fix addforce, and any other fixes that sound similarly useful (e.g., fix drag). Only you know exactly what you need, so you should look through as many as possible to be certain it does what you want.

Just apply middle school level physics of hard bodies: Acceleration is force divided by mass ( \vec{a} = \frac{\vec{F}}{m}). Thus you can compute the force a particle should have for a constant acceleration by multiplying it with the mass: \vec{F} = \vec{a} \cdot m and then use the computed force vector with fix setforce after defining a group that only contains this one particle. The previously computed force field forces would be irrelevant in this case and ignored.