I want to be able to detect if an iphone has been bumped into something...would it be best to listen to the gyroscope / accelerometer and judge how fast its moved or if its moved and stopped suddenly. 

In fact how would I judge if the device has moved then stopped suddenly?

This answer is great but in reverse [iOS: Accurately determining energy of a bump from accelerometer output](https://stackoverflow.com/questions/6536922/ios-accurately-determining-energy-of-a-bump-from-accelerometer-output/6642071#6642071) - it smooths out the movement, if anything I want to detect a sharp movement.

Also are both the Gyroscope and Accelerometer available for 3GS ?

**UPDATED WITH CODE**

From the Apple doc <http://developer.apple.com/library/iOS/documentation/EventHandling/Conceptual/EventHandlingiPhoneOS/MotionEvents/MotionEvents.html#//apple_ref/doc/uid/TP40009541-CH4-SW26>

**Isolating Instantaneous Motion from Acceleration Data**
If you are using accelerometer data to detect just the instant motion of a device, you need to be able to isolate sudden changes in movement from the constant effect of gravity. You can do that with a high-pass filter.

Listing 4-6 shows a simplified high-pass filter computation. The acceleration values from the previous event are stored in the accelX, accelY, and accelZ member variables of the class. This example computes the low-pass filter value and then subtracts it from the current value to obtain just the instantaneous component of motion.

Listing 4-6 Getting the instantaneous portion of movement from accelerometer data

```
define kFilteringFactor 0.1

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
    // Subtract the low-pass value from the current value to get a simplified high-pass filter
    accelX = acceleration.x - ( (acceleration.x * kFilteringFactor) + (accelX * (1.0 - kFilteringFactor)) );
    accelY = acceleration.y - ( (acceleration.y * kFilteringFactor) + (accelY * (1.0 - kFilteringFactor)) );
    accelZ = acceleration.z - ( (acceleration.z * kFilteringFactor) + (accelZ * (1.0 - kFilteringFactor)) );

   // Use the acceleration data.
}

```

How would I then 'Use the acceleration data' to work out if an instantaneous motion is detected?