# Practical exercise 4: MAKE A WATER CLOCK

## Tools

• PET bottle (preferably with a permanent cross-section),

• thin long nail,

• matches, large photo bowl, marker,

• stopwatch, watch or mobile phone.

## Process

1. Make a water clock. Make a small hole in the bottom of the PET bottle with a heated nail (beware of safety!). At the same time, stick a paper meter along the entire length of the bottle so that the zero is at the height of the hole in the bottle.

2. Pour water into the bottle (hold the hole with your thumb so that the water does not flow out) and place it over the photo bowl. Then measure the rate of drop of the water level in the bottle according to the scale: first measure the times when the water level is at the individual divisions of the scale (first test how fast the water flows out, and then choose such large divisions that you can comfortably measure the gradual drop time).

3. Record the measured values in a table (see below).

 Measurement number Time Level height

4. Draw a graph of the water level dependence on time. Draw an appropriate curve connecting the marked points in the graph.

5. Read the values corresponding to the same time intervals from the graph and mark the (uneven) time scale on the bottle with a marker.