W7.2 Water Water Everywhere!
A liquid accelerometer is constructed with a thin container of water (green for ease of visibility) that can be moved either forward or backward along a line through the length of the container. If this accelerometer is placed on a level platform the water in the accelerometer will be horizontal. When the accelerometer is accelerated toward the left by a mass hanging over a pulley, the inertia of the water will cause it to move to the rear of the accelerometer, as seen in this MPEG video.
Now suppose that the accelerometer is held at rest on an incline, as shown in the photograph below. Due to gravity, the water will collect toward the front of the accelerometer.
How will the water distribute itself when the accelerometer is released and begins to accelerate down the incline? Will the water collect more in the front end, as in the still photograph, will it collect in the back end, as in the case of acceleration along a horizontal surface shown above, or will the water do something in between, like having its surface parallel to the incline?
When the accelerometer is released the water will:
- gather toward the front of the accelerometer.
- gather toward the rear of the accelerometer.
- become uniformly distributed in the accelerometer with its surface parallel to the incline.
Solution
Reveal

Solution
The answer is (3): the water will become uniformly distributed in the accelerometer with its surface parallel to the incline, as seen in this MPEG video.
One way to view this demonstration is to look at the two components of the gravitational force: that accelerating the cart down the incline and that keeping the cart in contact with the incline (parallel to and perpendicular to the incline). The force down the incline is responsible for the acceleration of the cart. The force perpendicular to the incline is responsible for the liquid surface assuming a configuration parallel to the incline. There is no inertial effect causing the liquid to accumulate toward the rear of the vessel, as in the case of acceleration of the cart along a horizontal surface by an external force. The component of gravity along the incline acts uniformly on both the accelerometer and the water inside. In the level case it is the rear end of the accelerometer that is pushing on the liquid and causing it to accelerate along the horizontal surface.