I am having what seems to me a really stupid problem but I can't get it to work. In the example I have an RBD box dropping with gravity to a ground plane then I want a Magnetforce to repel the box like an explosion at frame 121. I have tried keying the Magnetic force to jump to -150000 at frame 121 from Zero at frame 120. But it seems to have no effects. If I delete all values for magnetic force then make it -150000 on frame 1 it falls and jumps immediately with no issues.

How do I have a force apply at a certain frame?

Thank you in advance!!

you have set it up correctly
however since your object is resting for a while after it settles, it goes to sleep before the magnet force is activated

so uncheck Collisions/Bullet Data/Enable Sleeping on your rbd object and it should work

Solution works perfectly. Thank you