Show Me The Physics

Speeding Up or Slowing Down?

Predict first: You slide a phone forward, then it slows to a stop. While it is slowing down, which way does its acceleration point: forward, backward, or is it zero?
  1. Keep the phone or tablet still for a second so it can calibrate (flat on a table is easiest).
  2. Move it in a straight line: slide it along the table, or hold it and move it. Start from rest, speed up, slow down, stop.
  3. Auto mode picks the direction you push; you can also lock it to left–right, top–bottom, or toward/away.
  4. Watch the velocity arrow and the acceleration arrow. Try to complete all four cases.

New: Circle mode. Hold the phone out and spin to see centripetal acceleration (uses the gyroscope).

Works best with short slides (about 1–3 seconds) that start and end at rest. A lab cart works great too. Without a motion sensor it plays a demo.

Show Me The PhysicsSpeeding Up or Slowing Down?
velocity vacceleration a
Calibrating…Lay the phone flat and keep it still.
VELOCITY0.00m/s
ACCEL.0.00m/s²
POSITION0.00m
The rule: if v and a point the same way, the object speeds up. If they point in opposite directions, it slows down. The direction of acceleration does not tell you which way something is moving.

The four cases 0 / 4

Waiting for motion sensor…

Works on a table or held in your hands: move the device in a straight line, let it stop, and repeat. Forward (+) is shown on the track. For left–right motion it is toward the right edge of the screen, so the on-screen phone moves the same way as the real one. Dots below the track are dropped every 0.1 s (a motion diagram): they spread out when speeding up and bunch together when slowing down. Velocity is calculated from the acceleration, so it is reset to zero each time the phone comes to rest.