FixedUpdate vs Update in Unity - Hands-On Comparison

FixedUpdate vs Update in Unity

📖 Scenario: You are making a simple Unity game where a ball moves based on player input. You want to understand how to use Update and FixedUpdate methods correctly to move the ball smoothly and with proper physics.

🎯 Goal: Build a Unity script that moves a ball using Update for input reading and FixedUpdate for applying physics movement.

📋 What You'll Learn

Create a Rigidbody variable to control the ball's physics

Use Update to read horizontal input into a variable

Use FixedUpdate to apply movement to the Rigidbody

Print messages in both Update and FixedUpdate to see when they run

💡 Why This Matters

🌍 Real World

Games often need smooth and realistic movement. Using <code>Update</code> and <code>FixedUpdate</code> correctly helps make player controls responsive and physics stable.

💼 Career

Understanding Unity's update methods is essential for game developers to create smooth gameplay and avoid bugs related to physics and input timing.

1

Setup Rigidbody variable

Create a public Rigidbody variable called rb inside the BallController class.

Unity

using UnityEngine;

public class BallController : MonoBehaviour
{
    // Create a public Rigidbody variable called rb
    // Your code here
}

Hint

Use public Rigidbody rb; inside the class.

2

Read input in Update

Add a private float variable called horizontalInput. In the Update method, set horizontalInput to Input.GetAxis("Horizontal"). Also, add a Debug.Log statement inside Update that prints "Update called".

Unity

using UnityEngine;

public class BallController : MonoBehaviour
{
    public Rigidbody rb;

    // Add private float horizontalInput
    // Add Update method to set horizontalInput and print message
    // Your code here
}

Hint

Use Input.GetAxis("Horizontal") to read left/right input.

3

Move Rigidbody in FixedUpdate

Add a FixedUpdate method. Inside it, create a Vector3 called movement with horizontalInput as the x value and 0 for y and z. Then call rb.MovePosition(rb.position + movement * 5f * Time.fixedDeltaTime). Also add a Debug.Log statement that prints "FixedUpdate called".

Unity

using UnityEngine;

public class BallController : MonoBehaviour
{
    public Rigidbody rb;
    private float horizontalInput;

    void Update()
    {
        horizontalInput = Input.GetAxis("Horizontal");
        Debug.Log("Update called");
    }

    // Add FixedUpdate method to move rb and print message
    // Your code here
}

Hint

Use rb.MovePosition inside FixedUpdate to move the ball smoothly with physics.

4

Print final messages

Add a print statement in Update that prints the current horizontalInput value. Add a print statement in FixedUpdate that prints the current rb.position.

Unity

using UnityEngine;

public class BallController : MonoBehaviour
{
    public Rigidbody rb;
    private float horizontalInput;

    void Update()
    {
        horizontalInput = Input.GetAxis("Horizontal");
        Debug.Log("Update called");
        // Print horizontalInput value
        // Your code here
    }

    void FixedUpdate()
    {
        Vector3 movement = new Vector3(horizontalInput, 0f, 0f);
        rb.MovePosition(rb.position + movement * 5f * Time.fixedDeltaTime);
        Debug.Log("FixedUpdate called");
        // Print rb.position
        // Your code here
    }
}

Hint

Use print with f-strings to show values in console.

Practice

1. What is the main difference between Update and FixedUpdate in Unity?

easy

A. Both run at the same time but handle different components.

B. Update runs every frame; FixedUpdate runs at fixed time intervals.

C. FixedUpdate runs every frame; Update runs at fixed time intervals.

D. Update is only for physics; FixedUpdate is for animations.

Solution

1. Step 1: Understand the timing of Update

   Update is called once per frame, so its timing depends on the frame rate.

2. Step 2: Understand the timing of FixedUpdate

   FixedUpdate is called at fixed intervals, independent of frame rate, ideal for physics calculations.

3. Final Answer:

   Update runs every frame; FixedUpdate runs at fixed time intervals. -> Option B

4. Quick Check:

   Update = frame-based, FixedUpdate = fixed interval [OK]

Hint: Remember: Update = frame, FixedUpdate = fixed time [OK]

Common Mistakes:

• Confusing which method runs at fixed intervals
• Thinking FixedUpdate depends on frame rate
• Assuming Update is for physics only

2. Which of the following is the correct syntax to declare the FixedUpdate method in a Unity C# script?

easy

A. void FixedUpdate() { /* code here */ }

B. void fixedUpdate() { /* code here */ }

C. void Fixedupdate() { /* code here */ }

D. void Fixed_Update() { /* code here */ }

Solution

1. Step 1: Check method name capitalization

   Unity requires exact method names with correct capitalization; FixedUpdate starts with capital F and U.

2. Step 2: Verify method signature

   The method must be declared as void FixedUpdate() with no parameters.

3. Final Answer:

   void FixedUpdate() { /* code here */ } -> Option A

4. Quick Check:

   Correct capitalization and signature = void FixedUpdate() { /* code here */ } [OK]

Hint: Method names are case-sensitive; use exact spelling [OK]

Common Mistakes:

• Using wrong capitalization like fixedUpdate
• Adding underscores in method names
• Adding parameters to FixedUpdate

3. Consider this Unity C# script snippet:

void Update() {
    Debug.Log("Update called");
}

void FixedUpdate() {
    Debug.Log("FixedUpdate called");
}

If the game runs at 60 frames per second and the fixed timestep is 0.02 seconds, which output will appear more frequently in the console?

medium

A. Both will appear the same number of times.

B. "FixedUpdate called" will appear more frequently.

C. "Update called" will appear more frequently.

D. Neither will appear because Debug.Log is disabled.

Solution

1. Step 1: Calculate Update calls per second

   At 60 FPS, Update runs 60 times per second.

2. Step 2: Calculate FixedUpdate calls per second

   Fixed timestep is 0.02 seconds, so FixedUpdate runs 1/0.02 = 50 times per second.

3. Final Answer:

   "Update called" will appear more frequently. -> Option C

4. Quick Check:

   60 (Update) > 50 (FixedUpdate) calls per second [OK]

Hint: Update frequency = FPS; FixedUpdate frequency = 1/fixedDeltaTime [OK]

Common Mistakes:

• Assuming FixedUpdate runs more often than Update
• Ignoring fixed timestep value
• Confusing frame rate with fixed update rate

4. You wrote this code to move a Rigidbody smoothly:

void Update() {
    rb.AddForce(Vector3.forward * forceAmount);
}

But the movement is jittery. What is the best fix?

medium

A. Move the AddForce call into FixedUpdate.

B. Increase forceAmount in Update.

C. Call rb.MovePosition inside Update instead.

D. Disable physics in Rigidbody.

Solution

1. Step 1: Identify physics update timing

   Physics updates happen in fixed time steps, so applying forces in Update causes inconsistent timing and jitter.

2. Step 2: Move physics code to FixedUpdate

   Calling AddForce inside FixedUpdate matches physics timing and smooths movement.

3. Final Answer:

   Move the AddForce call into FixedUpdate. -> Option A

4. Quick Check:

   Physics code belongs in FixedUpdate [OK]

Hint: Put physics code in FixedUpdate to avoid jitter [OK]

Common Mistakes:

• Applying physics forces in Update
• Trying to fix jitter by increasing force
• Disabling physics instead of fixing timing

5. You want to create a smooth player movement that responds to input and uses physics. Which is the best way to organize your code?

hard

A. Apply forces in Update, ignore input.

B. Read input and apply forces both in Update.

C. Read input and apply forces both in FixedUpdate.

D. Read input in Update, apply movement forces in FixedUpdate.

Solution

1. Step 1: Understand input timing

   Input should be read every frame in Update to avoid missing user actions.

2. Step 2: Understand physics timing

   Physics forces should be applied in FixedUpdate for consistent physics simulation.

3. Step 3: Combine input and physics correctly

   Store input values in Update, then use them in FixedUpdate to apply forces smoothly.

4. Final Answer:

   Read input in Update, apply movement forces in FixedUpdate. -> Option D

5. Quick Check:

   Input in Update + physics in FixedUpdate = smooth movement [OK]

Hint: Input in Update, physics in FixedUpdate for smooth control [OK]

Common Mistakes:

• Reading input in FixedUpdate causing missed inputs
• Applying physics in Update causing jitter
• Ignoring input or physics timing differences