Why physics simulate realistic behavior in Unity - Visual Breakdown

Concept Flow - Why physics simulate realistic behavior

Start Simulation

↓

Apply Physics Laws

↓

Calculate Forces & Collisions

↓

Update Object Positions & Velocities

↓

Render Updated Scene

↓

Repeat Each Frame

The physics engine applies real-world laws step-by-step each frame to update object movement and collisions, creating realistic behavior.

Execution Sample

Unity

void FixedUpdate() {
  Rigidbody rb = GetComponent<Rigidbody>();
  rb.AddForce(Vector3.down * 9.81f);
}

This code applies gravity force to an object every physics frame to simulate falling.

Execution Table

Step	Action	Force Applied	Velocity Before	Velocity After	Position Before	Position After
1	Apply gravity force	Vector3(0, -9.81, 0)	Vector3(0, 0, 0)	Vector3(0, -0.0981, 0)	Vector3(0, 10, 0)	Vector3(0, 9.9951, 0)
2	Apply gravity force	Vector3(0, -9.81, 0)	Vector3(0, -0.0981, 0)	Vector3(0, -0.1962, 0)	Vector3(0, 9.9951, 0)	Vector3(0, 9.9853, 0)
3	Apply gravity force	Vector3(0, -9.81, 0)	Vector3(0, -0.1962, 0)	Vector3(0, -0.2943, 0)	Vector3(0, 9.9853, 0)	Vector3(0, 9.9709, 0)
4	Apply gravity force	Vector3(0, -9.81, 0)	Vector3(0, -0.2943, 0)	Vector3(0, -0.3924, 0)	Vector3(0, 9.9709, 0)	Vector3(0, 9.9517, 0)
5	Apply gravity force	Vector3(0, -9.81, 0)	Vector3(0, -0.3924, 0)	Vector3(0, -0.4905, 0)	Vector3(0, 9.9517, 0)	Vector3(0, 9.9278, 0)
Exit	Simulation continues each frame	-	-	-	-	-

💡 Simulation repeats every physics frame to continuously update object motion realistically.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Velocity	Vector3(0,0,0)	Vector3(0,-0.0981,0)	Vector3(0,-0.1962,0)	Vector3(0,-0.2943,0)	Vector3(0,-0.3924,0)	Vector3(0,-0.4905,0)	Vector3(0,-0.4905,0)
Position	Vector3(0,10,0)	Vector3(0,9.9951,0)	Vector3(0,9.9853,0)	Vector3(0,9.9709,0)	Vector3(0,9.9517,0)	Vector3(0,9.9278,0)	Vector3(0,9.9278,0)
Force	Vector3(0,0,0)	Vector3(0,-9.81,0)	Vector3(0,-9.81,0)	Vector3(0,-9.81,0)	Vector3(0,-9.81,0)	Vector3(0,-9.81,0)	Vector3(0,-9.81,0)

Key Moments - 3 Insights

Why does the velocity increase downward each step?

Because gravity force is applied every physics frame, it adds downward acceleration, increasing velocity as shown in execution_table rows 1-5.

Why does the position change slightly each step even though velocity is small?

Position updates by adding velocity times time step, so small velocity changes cause small position changes each frame (see execution_table position columns).

Why does the simulation repeat every frame instead of once?

Physics simulates continuous motion by updating forces, velocity, and position every fixed frame, creating smooth realistic movement (exit note explains this).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the velocity after applying force?

A Vector3(0, -0.1962, 0)

B Vector3(0, -0.2943, 0)

C Vector3(0, -0.0981, 0)

D Vector3(0, 0, 0)

💡 Hint

Check the 'Velocity After' column in execution_table row 3.

At which step does the position first become less than 9.995?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Position After' column in execution_table rows 2, 3, and 4.

If the gravity force was doubled, how would velocity change after step 1?

A Velocity would be about Vector3(0, -0.0981, 0)

B Velocity would stay the same

C Velocity would be about Vector3(0, -0.1962, 0)

D Velocity would be zero

💡 Hint

Velocity changes by force times time step; doubling force doubles velocity change (see variable_tracker Velocity values).

Concept Snapshot

Unity physics simulates realistic behavior by applying real-world forces like gravity every fixed frame.
Each frame updates velocity and position based on forces.
This continuous update creates smooth, believable motion.
Use Rigidbody.AddForce() to apply forces.
Physics runs in FixedUpdate() for consistent timing.

Full Transcript

In Unity, physics simulates realistic behavior by applying forces such as gravity every fixed frame. The physics engine calculates how these forces change an object's velocity and position step-by-step. For example, applying gravity adds a downward force each frame, increasing velocity downward and moving the object lower. This process repeats every physics frame, updating the scene continuously to create smooth and believable motion. The code example shows applying gravity force in FixedUpdate, which runs at fixed intervals for stable physics simulation. The execution table traces velocity and position changes over five steps, showing how the object falls gradually. Key points include understanding why velocity increases each step due to gravity, why position changes slightly each frame, and why the simulation repeats continuously. The visual quiz tests understanding of velocity and position values at specific steps and the effect of changing force magnitude. This step-by-step trace helps beginners see how physics simulation works in Unity to create realistic behavior.