Rigidbody forces and velocity in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the velocity after applying force with ForceMode.Force?

Consider a Rigidbody with mass 2 kg initially at rest. You apply a force of (4, 0, 0) using ForceMode.Force for one fixed update frame. What will be the velocity of the Rigidbody after this frame?

Unity

Rigidbody rb = GetComponent<Rigidbody>();
rb.mass = 2f;
rb.velocity = Vector3.zero;
Vector3 force = new Vector3(4f, 0f, 0f);
rb.AddForce(force, ForceMode.Force);
// Assume fixedDeltaTime = 0.02 seconds

A Velocity will be (0.04, 0, 0)

B Velocity will be (2, 0, 0)

C Velocity will be (4, 0, 0)

D Velocity will be (0.02, 0, 0)

💡 Hint

Remember ForceMode.Force applies force over time, so velocity changes by acceleration * deltaTime.

ForceMode.Force applies a continuous force. Acceleration = Force / Mass = 4 / 2 = 2 m/s². Velocity change = acceleration * deltaTime = 2 * 0.02 = 0.04 m/s.

❓ Predict Output

intermediate

What is the velocity after applying force with ForceMode.Impulse?

A Rigidbody with mass 3 kg is at rest. You apply an impulse force of (6, 0, 0) using ForceMode.Impulse. What is the velocity immediately after applying this impulse?

Unity

Rigidbody rb = GetComponent<Rigidbody>();
rb.mass = 3f;
rb.velocity = Vector3.zero;
Vector3 impulse = new Vector3(6f, 0f, 0f);
rb.AddForce(impulse, ForceMode.Impulse);

A Velocity will be (2, 0, 0)

B Velocity will be (0, 0, 0)

C Velocity will be (6, 0, 0)

D Velocity will be (0.02, 0, 0)

💡 Hint

Impulse changes velocity immediately by impulse / mass.

Impulse changes velocity instantly: velocity = impulse / mass = 6 / 3 = 2 m/s on x-axis.

🔧 Debug

advanced

Why does velocity not change after AddForce with ForceMode.Acceleration?

You have this code snippet: Rigidbody rb = GetComponent(); rb.velocity = Vector3.zero; rb.AddForce(new Vector3(10, 0, 0), ForceMode.Acceleration); But after running, velocity remains zero. What is the most likely reason?

Unity

Rigidbody rb = GetComponent<Rigidbody>();
rb.velocity = Vector3.zero;
rb.AddForce(new Vector3(10, 0, 0), ForceMode.Acceleration);

A ForceMode.Acceleration requires mass to be zero to work.

B The Rigidbody is set to kinematic, so forces don't affect velocity.

C AddForce with ForceMode.Acceleration only works in FixedUpdate, not Update.

D Velocity is reset to zero immediately after AddForce.

💡 Hint

Check Rigidbody settings that disable physics simulation.

If Rigidbody is kinematic, physics forces do not affect velocity. AddForce has no effect.

🧠 Conceptual

advanced

How does ForceMode.VelocityChange differ from ForceMode.Impulse?

Which statement correctly describes the difference between ForceMode.VelocityChange and ForceMode.Impulse when applied to a Rigidbody?

A Both apply force ignoring mass but at different time intervals.

B VelocityChange applies a continuous force; Impulse applies a one-time force.

C Impulse ignores mass and changes velocity directly; VelocityChange applies force considering mass.

D VelocityChange ignores mass and changes velocity directly; Impulse applies force considering mass.

💡 Hint

Think about how mass affects velocity change in each mode.

ForceMode.VelocityChange changes velocity directly ignoring mass. Impulse applies force that depends on mass.

❓ Predict Output

expert

What is the final velocity after multiple AddForce calls with different ForceModes?

A Rigidbody with mass 4 kg starts at zero velocity. The following code runs in one FixedUpdate: rb.AddForce(new Vector3(8, 0, 0), ForceMode.Force); rb.AddForce(new Vector3(4, 0, 0), ForceMode.Impulse); rb.AddForce(new Vector3(2, 0, 0), ForceMode.VelocityChange); Assuming fixedDeltaTime = 0.02 seconds, what is the final velocity vector?

Unity

Rigidbody rb = GetComponent<Rigidbody>();
rb.mass = 4f;
rb.velocity = Vector3.zero;
rb.AddForce(new Vector3(8f, 0f, 0f), ForceMode.Force);
rb.AddForce(new Vector3(4f, 0f, 0f), ForceMode.Impulse);
rb.AddForce(new Vector3(2f, 0f, 0f), ForceMode.VelocityChange);

A Velocity will be (4.02, 0, 0)

B Velocity will be (3.5, 0, 0)

C Velocity will be (3.04, 0, 0)

D Velocity will be (1.5, 0, 0)

💡 Hint

Calculate velocity changes from each AddForce separately and sum them.

ForceMode.Force: acceleration = 8/4=2 m/s², velocity change = 2*0.02=0.04 ForceMode.Impulse: velocity change = 4/4=1 ForceMode.VelocityChange: velocity change = 2 (ignores mass) Total velocity = 0.04 + 1 + 2 = 3.04 m/s on x-axis.