Replicating Physics-Based Interactions for Characters

In this lesson, you will learn how to synchronize character-driven physical impulses—like kicking a ball or shoving a physics-enabled crate—so that every client experiences the same outcome.

The Synchronization Challenge

In Unreal Engine, physics simulation typically happens locally on each machine. If a player walks into a physics object, their local client calculates the collision, while the server calculates its own version. Without coordination, the "physics discrepancy" results in the object flying in different directions on different screens, or worse, desyncing entirely.

Authoritative Interaction Patterns

To maintain consistency, you must ensure the Server is the final arbiter of physics forces. When a character interacts with a physical object, you should avoid simply letting the character's movement component collide with the object locally. Instead, use an authoritative workflow:

1. Client-Side Trigger: The character detects an interaction (e.g., overlapping a physics object).
2. Server RPC: The client sends an RPC to the server, identifying the target object and the force vector.
3. Server Execution: The server validates the request and applies the AddImpulse or AddForce to the UPrimitiveComponent of the object.

// Executed on the Server
void AMyCharacter::Server_InteractWithPhysicsObject_Implementation(UPrimitiveComponent* TargetComp, FVector Impulse)
{
    if (TargetComp && TargetComp->IsSimulatingPhysics())
    {
        TargetComp->AddImpulse(Impulse, NAME_None, true);
    }
}

Ensuring Stability

By applying the impulse on the server, the server’s physics simulation updates the object's transform. Because the object is set to Replicates and Replicate Movement is enabled, the server will naturally broadcast the new position and velocity to all connected clients.

To prevent visual jitter for the player initiating the interaction, you can apply a "predicted" impulse locally on the client before the server confirms it, provided you reconcile the state if the server’s physics result differs significantly. However, for most gameplay objects, letting the server-authoritative broadcast handle the movement is the safest way to ensure all players see the exact same physical outcome.

Key Takeaway: Always apply physics forces to interactable objects via Server RPCs to ensure the server remains the authoritative source of truth for all clients.