---
title: "Recoil & Explosion Designer Interactive Lab | Sandbox Physics"
description: "Can internal releases steer the centre of mass? Internal releases cannot steer the centre of mass."
canonical_url: "https://sandboxphysics.com/labs/recoil-explosion/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/recoil-explosion/"
last_modified: "2026-11-04"
---

M035 · Closed-system momentum

# Recoil & Explosion Designer

**Can internal releases steer the centre of mass?** Internal releases cannot steer the centre of mass. A payload gains momentum and the remaining carrier takes the opposite change; an off-centre release also turns it. With no external horizontal force, all the parts together keep the original centre-of-mass velocity, whether they separate together or one at a time.

Arrange up to four payloads on a frictionless carrier. Change each release direction, energy and timing; compare recoil and spin with the unchanged centre-of-mass motion.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

Model assumptions

The carrier and payloads are uniform disks in one horizontal plane. Gravity is balanced by ideal frictionless supports. Arms, guides, feet and springs are massless. Attached parts form one rigid body. Releases are instantaneous internal impulses along each nozzle; equal reaction impulses include carrier rotation. Simultaneous releases share one impulse-progress parameter and each source supplies its stated positive work. Zero-energy unlocking retains the local rigid-body velocity and spin. Free disks then coast without drag. The record ends before the first free-body recontact; subsequent impacts are not modeled. Rotation is positive counterclockwise in the top view; gold denotes the mass-weighted centre-of-mass projection, not another body.

Physics tutorial

## The carrier turns, the centre of mass does not

**Background** A spring pushes a payload outward and the remaining carrier back. If the push misses the remaining group’s centre of mass, the carrier also starts to spin.

**Why it matters** An explosion can change every individual trajectory without changing the motion of the whole system’s centre of mass. Include every fragment when locating that centre.

Start with the essentials

- **Focus question**: Can changing internal release directions make the gold centre-of-mass marker turn?
- **One-sentence intuition**: Internal impulses occur in opposite pairs. They cancel in total momentum; their angular effects also balance when orbital motion and disk spin are both counted.

### Core mathematical model

#### Centre of mass from actual positions

$$
\mathbf R=\frac{\sum_i m_i\mathbf r_i}{\sum_i m_i},\qquad \mathbf P=\sum_i m_i\mathbf v_i=M_{\mathrm{tot}}\dot{\mathbf R}
$$

The gold marker is evaluated from the carrier and every payload, including those already far away. Its projected trail remains straight when the initial drift is nonzero.

#### An internal impulse and its reaction

$$
\Delta\mathbf p_i=J_i\mathbf n_i,\qquad \Delta\mathbf P_{\mathrm{rem}}=-J_i\mathbf n_i,\qquad \Delta L_{\mathrm{rem}}=-\mathbf r_i\times J_i\mathbf n_i
$$

The lever arm is measured from the remaining rigid group’s centre of mass. Opposite radial shots can cancel recoil; an off-centre nozzle can turn the carrier.

#### No horizontal external force

$$
\dot{\mathbf P}=\mathbf0,\qquad \mathbf R(t)=\mathbf R(0)+\frac{\mathbf P(0)}{M_{\mathrm{tot}}}t
$$

The support balances gravity but supplies no horizontal force or torque. Changing timing does not change this straight-line law.

#### Energy and angular momentum

$$
E=\sum_i\left(\frac12m_iv_i^2+\frac12I_i\omega_i^2\right)+U_{\mathrm{spring}},\qquad L=\sum_i\left(\mathbf r_i\times m_i\mathbf v_i+I_i\omega_i\right)
$$

Uniform disk inertia is used for each physical disk. The energy plot separates the remaining rigid group’s translation and rotation, and includes the intrinsic spin of each free disk.

### Common difficulties

#### The carrier is only one part

**Typical misconception** If the carrier bends away, the centre of mass must bend too.

**Better mental model** The payloads carry the compensating momentum. The mass-weighted sum of all trajectories retains the original centre-of-mass motion.

#### Unlocking need not add energy

**Typical misconception** A zero-energy payload must stop when released.

**Better mental model** Before release, a spinning assembly already gives each payload a tangential velocity and spin. Unlocking preserves those instantaneous velocities.

### Run the experiment

1. 01

   #### Turn the carrier

   Watch Off-centre recoil. Drag a payload or its aiming handle and replay the result.

   **What to observe:** The carrier recoils and turns after each release; the gold trail keeps its direction.

2. 02

   #### Program multiple releases

   Select each payload and set its mass and energy. Open the release program to choose separate times or add a fourth payload.

   **What to observe:** Simultaneous sources act on the same remaining carrier. Separating them in time changes the later launcher orientation and fragment paths.

3. 03

   #### Release without a spring push

   Choose Unlock a spinning assembly, then compare its trails with Opposite simultaneous shots.

   **What to observe:** The unlocked disks leave tangentially with no added spring energy. The opposite-shot preset instead converts spring energy into balanced outward motion.

Physical conclusion

Track all the mass, not just the carrier. Internal energy can become translation and rotation while the centre of mass continues at its original velocity.

Motion & dynamics · Starter path

## From a falling ball to an orbit

Start by measuring gravity and splitting motion into components. Then add contact forces and collisions, and follow the same laws all the way to an orbit.

[Start here → **Free Fall & Measuring Gravity** Capture a few drops and fit the camera positions to estimate gravitational acceleration. Then add air damping and watch the fit drift from the truth. Observe · 14 min](https://sandboxphysics.com/labs/free-fall/)

1. [1 Free Fall & Measuring Gravity](https://sandboxphysics.com/labs/free-fall/)
2. [2 Projectile Flight Designer](https://sandboxphysics.com/labs/projectile-flight/)
3. [3 Inclined Plane & Friction](https://sandboxphysics.com/labs/inclined-plane/)
4. [4 One-Dimensional Collision Bench](https://sandboxphysics.com/labs/collision-bench/)
5. [5 Mass–Spring Oscillator](https://sandboxphysics.com/labs/mass-spring/)
6. [6 How Orbits Take Shape](https://sandboxphysics.com/labs/orbit/)

[View the full path →](https://sandboxphysics.com/topics/mechanics/#starter-path)