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.