FixedUpdate vs Update in Unity - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Update and FixedUpdate call counts

Consider a Unity script that counts how many times Update and FixedUpdate are called during 1 second of game time. What is the expected output?

Unity

using UnityEngine;

public class CallCounter : MonoBehaviour
{
    private int updateCount = 0;
    private int fixedUpdateCount = 0;
    private float timer = 0f;

    void Update()
    {
        updateCount++;
        timer += Time.deltaTime;
        if (timer >= 1f)
        {
            Debug.Log($"Update calls: {updateCount}, FixedUpdate calls: {fixedUpdateCount}");
            updateCount = 0;
            fixedUpdateCount = 0;
            timer = 0f;
        }
    }

    void FixedUpdate()
    {
        fixedUpdateCount++;
    }
}

A Update calls: ~50, FixedUpdate calls: ~60

B Update calls: ~100, FixedUpdate calls: ~100

C Update calls: ~60, FixedUpdate calls: ~50

D Update calls: ~30, FixedUpdate calls: ~30

💡 Hint

Think about the default frame rate and fixed timestep in Unity.

By default, Unity runs Update roughly once per frame (about 60 FPS) and FixedUpdate at a fixed timestep of 0.02 seconds (50 times per second). So in 1 second, FixedUpdate is called about 50 times and Update about 60 times.

🧠 Conceptual

intermediate

When to use FixedUpdate instead of Update?

Which scenario is best suited for using FixedUpdate instead of Update in Unity?

A Applying physics forces to a Rigidbody

B Reading player input for movement

C Updating UI elements every frame

D Playing animations smoothly

💡 Hint

Think about what kind of operations need consistent timing.

FixedUpdate runs at fixed time intervals and is synced with the physics engine. It is best for applying physics forces to Rigidbody objects to ensure consistent physics simulation.

❓ Predict Output

advanced

Effect of Time.deltaTime in Update vs FixedUpdate

What will be the output of the following code snippet if the game runs at 60 FPS?

Unity

using UnityEngine;

public class TimeTest : MonoBehaviour
{
    private float updateSum = 0f;
    private float fixedUpdateSum = 0f;
    private int updateCount = 0;
    private int fixedUpdateCount = 0;

    void Update()
    {
        updateSum += Time.deltaTime;
        updateCount++;
        if (updateCount == 60)
        {
            Debug.Log($"Update sum: {updateSum}");
        }
    }

    void FixedUpdate()
    {
        fixedUpdateSum += Time.fixedDeltaTime;
        fixedUpdateCount++;
        if (fixedUpdateCount == 50)
        {
            Debug.Log($"FixedUpdate sum: {fixedUpdateSum}");
        }
    }
}

A Update sum: ~0, FixedUpdate sum: ~0

B Update sum: ~0.5, FixedUpdate sum: ~1

C Update sum: ~1, FixedUpdate sum: ~0.5

D Update sum: ~1, FixedUpdate sum: ~1

💡 Hint

Consider what Time.deltaTime and Time.fixedDeltaTime represent.

Time.deltaTime is the time between frames, about 1/60 seconds at 60 FPS. Summing it over 60 frames gives about 1 second. Time.fixedDeltaTime is fixed at 0.02 seconds, so summing over 50 FixedUpdate calls also gives about 1 second.

🔧 Debug

advanced

Why does Rigidbody not move smoothly?

A developer moves a Rigidbody by changing its position inside Update(). The movement looks jittery. Why?

Unity

void Update()
{
    transform.position += Vector3.forward * speed * Time.deltaTime;
}

A Because Rigidbody physics should be updated in FixedUpdate, not Update

B Because Time.deltaTime is not accurate in Update

C Because transform.position cannot be changed directly

D Because speed variable is not initialized

💡 Hint

Think about how Unity physics and Rigidbody work.

Rigidbody physics updates happen in FixedUpdate. Moving Rigidbody by changing transform.position in Update causes physics and rendering to be out of sync, resulting in jittery movement.

🧠 Conceptual

expert

Why might FixedUpdate be called multiple times per frame?

In Unity, sometimes FixedUpdate is called more than once during a single frame. Why does this happen?

A Because the fixed timestep is longer than the frame time, so FixedUpdate is skipped

B Because the frame rate is lower than the fixed timestep rate, so FixedUpdate runs multiple times to catch up

C Because the frame rate is higher than the fixed timestep rate

D Because FixedUpdate is called randomly by Unity

💡 Hint

Think about how Unity keeps physics simulation in sync with real time.

If the frame rate is lower than the fixed timestep rate, Unity calls FixedUpdate multiple times per frame to keep physics simulation consistent and not fall behind real time.

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