Lesson 13 · The force accumulator - Build a Physics Engine

The force accumulator

Several forces act on a body at once, and last frame's forces must not linger into this one. Today you sum multiple forces and clear the accumulator each step.

The goal

Accumulate multiple applied forces into one net force and reset it to zero on demand.

Start here - the target

TO DO

Scenario: Summing and clearing forces

Givena body of mass 1

WhenApplyForce({3, 0}) and ApplyForce({0, 4}) are both called

Thenthe net Force is {3, 4}, and integrating one step from rest gives velocity {3, 4}, position {3, 4}

Andafter ClearForces the Force is {0, 0}, so a following step with no new force leaves the velocity unchanged

Background

Real motion comes from several forces at once - gravity pulling down, a contact pushing up, maybe a spring or the wind. Because ApplyForce accumulates, calling it repeatedly builds up the net force on the body: {3, 0} plus {0, 4} gives {3, 4}, and dividing by mass gives the acceleration to integrate. This is the force accumulator pattern, and it is how the world will let many independent effects act on one body in a single frame.

The essential companion is clearing the accumulator after each step. Forces are applied fresh every frame; if you never reset Force, last frame’s gravity would add to this frame’s and the body would accelerate without bound. So a step is always three beats: derive the acceleration from the accumulated force, integrate, then zero the accumulator so the next frame starts clean. Pin that a cleared body with no new force coasts at constant velocity.

Make it work

// ApplyForce already adds into b.Force; clearing zeroes it
func (b *Body) ClearForces() { b.Force = Vec2{} }
// a full step: derive acceleration, integrate, then clear the accumulator
// b.Acceleration = b.accelerationFromForce(); b.Integrate(dt); b.ClearForces()

CheckpointDONE

Forces sum into a net force each step and the accumulator clears cleanly. Commit and stop here.