A Jump-and-Move Game Using Your Phone's Motion Sensor (Free)

It watches for weightlessness, not for the push-off

Smartphones contain an accelerometer that measures the acceleration on the device in real time. The counter-intuitive part is that the reading does not spike at the moment you launch yourself upward.

At rest, the sensor reads about 9.8 m/s² — the pull of gravity. The instant your feet leave the ground, the device is in free fall along with you, and the combined acceleration collapses towards zero. Then landing drives it sharply back up as the floor delivers its impact.

So a jump has a characteristic shape: steady around 9.8, a dip towards zero, a large spike. Jump Coin treats one dip-and-spike pair as a single jump. Because it is watching the time spent airborne rather than the force of the push-off, stamping hard on the floor does not register.

Turning hang time into height

Once you know how long someone was airborne, you can work out how high they went. If the total time in the air is t, the rise takes t/2 and the fall takes t/2. The rise follows the free-fall relation h = ½ g (t/2)², which tidies up to h = g·t² / 8.

Numbers make it concrete. A 0.4-second hang time is roughly 20cm; 0.5 seconds is about 31cm; 0.6 seconds is about 44cm. An extra tenth of a second nearly doubles the height, so improving a record means genuinely committing to the jump.

It is an estimate, though. A phone held in the hand moves differently from your hips or centre of mass, and there is error in exactly when landing is detected. Treat it as a comparison between jumps by the same person holding the phone the same way, rather than an absolute measurement.

How to play (Jump Coin)

The coin sound is synthesized on the fly, so there are no audio files to load and no external requests. Time limits are none, 10, 30 or 60 seconds.

• Open Jump Coin on a phone (a device with an accelerometer)
• Choose a time limit and press start. On iPhone, allow the motion-sensor permission it asks for here
• Grip the phone firmly and jump in place
• On a detected jump, a coin chimes and the estimated height and your best are shown

What happens if you open it on a PC

Most laptops and desktops have no accelerometer. In that case the permission may go through but no values arrive, so Jump Coin switches to a "tap for the coin sound" button.

That is a fallback for hearing the sound, not exercise. To actually jump, open it on a phone. On the other hand, it does mean you can verify how the app behaves from a PC while reading about it.

Staying safe

This is a game about jumping repeatedly indoors, so check your surroundings first. The real hazards are the light fitting or shelf above you, cables underfoot, and slippery flooring.

In an apartment, think about the impact travelling downstairs too. Landing thuds carry through floors more readily than most household noise. Jumping on a rug or mat and avoiding late hours makes it something you can enjoy without worrying.

Mind the phone itself as well — grip it with both hands or attach a strap. If jumps are not registering, take off from both feet rather than one so the landing impact comes through clearly.

• Check overhead, underfoot and how slippery the floor is before starting
• In an apartment, jump on a mat and consider the hour
• Grip the phone with both hands or use a strap
• If detection misses, take off from both feet