FixedUpdate vs Update in Unity

Introduction

FixedUpdate and Update are two ways Unity runs code repeatedly. They help control when and how often your game actions happen.

Use Update to check for player input like pressing keys or clicking the mouse.

Use FixedUpdate to move objects smoothly when physics like gravity or collisions are involved.

Use Update to update animations or UI elements every frame.

Use FixedUpdate to apply forces or change velocity on objects with Rigidbody components.

Use Update for things that need to happen every frame, and FixedUpdate for physics calculations.

Syntax

Unity

void Update() {
    // code runs every frame
}

void FixedUpdate() {
    // code runs at fixed time intervals
}

Update runs once every frame, which can vary in time depending on the device.

FixedUpdate runs at a steady, fixed time step, good for physics calculations.

Examples

This checks for a spacebar press every frame.

Unity

void Update() {
    if (Input.GetKeyDown(KeyCode.Space)) {
        Debug.Log("Jump pressed");
    }
}

This applies an upward force at fixed intervals for smooth physics movement.

Unity

void FixedUpdate() {
    Rigidbody rb = GetComponent<Rigidbody>();
    rb.AddForce(Vector3.up * 10);
}

Sample Program

This script moves an object forward smoothly using FixedUpdate and logs when the W key is pressed using Update.

Unity

using UnityEngine;

public class MoveObject : MonoBehaviour {
    Rigidbody rb;

    void Start() {
        rb = GetComponent<Rigidbody>();
    }

    void Update() {
        if (Input.GetKeyDown(KeyCode.W)) {
            Debug.Log("W key pressed");
        }
    }

    void FixedUpdate() {
        rb.MovePosition(rb.position + Vector3.forward * 0.1f);
    }
}

Important Notes

Update frequency depends on frame rate, so it can be uneven.

FixedUpdate runs at consistent intervals, making it better for physics.

Never put physics code in Update to avoid jittery movement.

Summary

Update is for frame-based actions like input and animations.

FixedUpdate is for physics and smooth movement at fixed time steps.

Use each one where it fits best to keep your game running smoothly.

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