Solution to Extended Learning 3 of Class 7 Chapter 9 Motion and Time

3. Make a model of a sand clock which can measure a time interval of 2 minutes (Fig. 9.17).

Answer:

You can make a model of the sand clock in the following way:

Aim: To make a working sand clock which measures a time interval of 2 minutes.

Materials Required: Two plastic cans with lids, sharp knife, sand, stopwatch.

Procedure:

(i) Take off the lids of the two plastic cans and fill a quarter of one with sand.

(ii) Take the knife and slowly carve out a small hole of the same size in each of the lids. Use caution while using the knife.

(iii) Fasten the lids back on the plastic cans.

(iv) Now place the plastic can containing the sand on top of the empty can, such that the holes are aligned.

(v) At the same time take a stopwatch and start noting how long it takes for the sand from the upper can to flow completely into the lower can. You can also look at the watch in your home while doing this.

(vi) If the sand takes less than 2 minutes to flow from the upper can to the lower can, add more sand and repeat the activity until the time interval is exactly 2 minutes. If it takes more than 2 minutes to flow, remove some sand and repeat the same activity until the time interval is exactly 2 minutes. Note: After the sand completely empties out from the upper can to the lower can, you will need to invert the setup to continue. Now the lower can containing the sand will have to be placed on top of the other one which is now empty.

**Make Your Own Sand Clock:** The two plastic cans each with a small hole in the lid are shown in Figs (a) and (b). Place can (b) on top of can (a), keeping the holes aligned as seen in (c). Watch how long it takes for the entire sand to flow out. How many tries does it take you to achieve a time interval of 2 minutes?

Observations:

You will see the sand flowing out from the upper can to the lower can as shown in Figure (c).
It will probably take you a few tries to achieve an exact time interval of 2 minutes.

Conclusions:

The above setup can be used as a sand clock to measure in this case a time interval of 2 minutes.
The time interval measured will depend on the size of each hole and the amount of sand in the upper can. The size of holes being fixed, you will have to vary the amount of sand in the upper can to achieve an exact time interval of 2 minutes.

“Make a model of a sand clock which can measure a time interval of 2 minutes (Fig. 9.17).” – Solved.