You have two options really, increase the speed of your physics loop so that it updates as fast as you are rendering, or, interpolate,
At 20 fps, how effective are the different ways of interpolating physics?
E.g. You could
- zero all forces between physics frames, and hold all velocities constant. The lower the acceleration on an object the nearer this would be to perfect, but without having to do anything but linear interpolation
- hold all resultant forces constant, and interpolate changes to velocities. This would presumably be less expensive than doing a full calculation?
But how are you managing collisions? Interpolating between physics frames is going to cause "bounces" if you only do the collision checks at physic frames (i.e. interpolates through a wall, then the physics kicks in and bounces it back again. At 20 fps the human eye will consciously see this very obviously. At 75 fps there will be a noticeable flicker, but most humans won't know why. If you had physics at 100 fps, theoretically no-one would even see the bounces).