Class exercise 4a

1. Does the shape of an object affect whether it floats or sinks? If a piece of plasticine is folded into a different shape, what changes aside from its shape?

A: Specifically, the shape of an object does not affect whether it floats or sinks, but the volume of the object that changes as its shape changes does so. For example, a lump of plasticine does not float. But if it is moulded into a cup, its volume increases, thus decreasing its density and allowing it to float.

However, if you submerge the plasticine cup, it will sink, therefore it is not the shape that allows it to float, but the increase in volume.

2. A piece of metal sinks in water, but a can (made of metal) floats in water. Why do you think this is so? Do you think the material determines whether the object sinks or floats?

A: The overall density of the can is less than that of water, as its volume is significantly greater than its mass due to the large volume of air inside it. However, the density of the piece of metal is much higher than the can’s as its volume is smaller than the can’s even though their masses are similar. Therefore, the material does not determine whether the object sinks or floats.

3. Why do some drink cans float, and some drink cans sink? Fill in the table below that will help you determine the factors that affect the density of an object.


(use the weighing balance)

(read off the can)










Coke Zero





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Class exercise 4b – Making a density ladder

Aim: To make a density ladder, which can be used to estimate the density of some objects


  • Boiling tubes
  • One cup each of: glycerine, honey, oil, coloured water
  • Droppers for each cup
  • Rubber gloves
  • Lego block

From left to right: Oil, honey, glycerine and blue food colouring (to colour the water)


1. A density ladder can be formed by carefully adding the four liquids into a test-tube. Predict the sequence that the four liquid layers will form, starting from the bottom layer.

A: Honey, glycerine, coloured water, oil.

2. Add the liquid into the test-tube, according to the sequence you have predicted. Do not shake the test tube. Can you see four distinct layers? If not, test on another sequence, until you can see four distinctive layers of liquid forming in the test-tube. Write down the sequence (starting from the bottom layer).

A: Honey, glycerine, coloured water, oil. Our prediction is correct!


Another version of the density ladder that we could take home

Another version of the density ladder that we could take home

We shook the test tube anyway... It came out like this. (It was a separate one.)

We shook the test tube anyway… It came out like this. (This is a separate one.)

3. Given that the densities of the four liquids are: 0.9 g/cm3; 1.0 g/cm3; 1.3 g/cm3; 1.4 g/cm3, match the density with the liquid. Explain your choices.

Liquid Density
Oil 0.9 g/cm3
Coloured water 1.0 g/cm3
Glycerine 1.3 g/cm3
Honey 1.4 g/cm3

A: Less dense liquids will float on top of denser liquids, therefore the densest liquid would be at the bottom, while the least dense liquid would be at the top.

4. Try dropping in the lego block. What do you observe?

A: It floats between the layer of coloured water and the layer of oil.

5. Try dropping the lego block into each liquid. What do you observe?

A: It floats in honey, glycerine and coloured water, but not in oil.

6. Estimate the density of the lego block. Give reasons for your estimation.

A: Its density is 0.95 g/cm3. As it floats in coloured water but not in oil, its density should be between 0.9 g/cm3 and 1.0 g/cm3.

Lego block suspended in honey

Lego block suspended in honey

Lego block sinking in oil

Lego block sinking in oil

Lego block floating in test tube of glycerine

Lego block floating in test tube of glycerine

Class exercise 4a – Exploring Density

On why watermelons, bowling balls and coke cans float.

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Is the shape of the plasticine important?

Yes Plasticine shaped into boats/cups float. Lumps of plasticine sink. The pieces of plasticine shaped into cups/boats take up more space = have more volume compared to the lumps of plasticine, even though both have the same mass. Therefore, the density of the cups/boats is smaller compared to the lumps, and they are able to float.

Drink cans

The cans all have the same shape, but do they have the same volume?

Yes All the drink cans are labeled with the same volume – 330 ml. However, some cans float while some sink. The volumes of the cans are the same, however the cans have different amounts of drink inside them. Therefore their masses are different, which affects the density of each can and thus determines whether they can float.

Coke cans

What is the difference between a can of Coke and a can of Coke Light?

 Coke Light contains artificial sweeteners while normal Coke contains sugar.  Both types of drink cans float.

 – Not sure –


Does the skin of the fruit affect the density of the fruit?

Yes. When a whole orange is placed in the fish tank, it floats. However, when a peeled orange is placed in the tank, it sinks. The skin of the orange contains numerous air spaces which reduces the overall volume of the orange, thus reducing its density. However, when the orange peel is removed, the ratio of its mass to its volume decreases, thus its density increases and the peeled orange sinks.

Bowling ball

Does a bowling ball float or sink in water?

It will sink. The bowling ball floats in the fish tank. The density of the bowling ball is actually smaller than that of water.

Class exercise 2 – Pendulum

Aim: To find out the effect of changing the length of a pendulum on the period of oscillation

Hypothesis: The longer the length of the pendulum, the longer the period of oscillation


  • Retort stand, split cork
  • Strings, scissors, ruler
  • Pendulum bob
  • Stopwatch



–       Base of retort stand should be facing away from pendulum

–       Excess string should not be in the way of the pendulum’s swing

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1.  Tie the pendulum bob on one end of a thread.

2. Clamp the other end firmly between two pieces of split cork to make a pendulum.

3. Measure the length of the string L between the middle of the bob and the edge of the split cork clamping the thread, for L = 80.0 cm.

4. Give the pendulum bob a slight displacement and allow it to oscillate steadily. An example of a complete oscillation is when the bob moves from A to B and back to A (see diagram 2).

  • When displacing the pendulum, make sure it is in a plane parallel to you (not swinging diagonally, orbiting etc.)
  • Place your eyes at the same level as the pendulum to reduce parallax error

5. Time N steady oscillations, t1, using a stopwatch. Repeat the measurement a second time and record it as t2. Vary N from 10 to 20 oscillations depending on L.

6. The time taken for 1 complete oscillation is called the period, T. Calculate the average time
taken < t >, period T, and the value of .

7. Repeat the experiment for various readings of L between 50.0 cm to 100.0 cm.

8. Tabulate your results in the table given.


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  1. Plot the graph of T / s against  √L/cm^1/2 , and determine the gradient of the graph. Show your working clearly.


Gradient = 0.2/0.6

= 1/3

2. Why was it necessary to measure the time taken for 20 oscillations instead of measuring the period directly?

>> To increase the accuracy of the measurement of the period by taking the average of 20 periods and thus reducing errors such as human reaction time in operating the stopwatch.

3. What were the controlled variables? How did you ensure that they did not affect the experiment?

–       Angle at which the pendulum is released: I ensured that the pendulum was released parallel to the corner of a drawer behind it in order to fix the angle at which it was released

–       Environmental factors (e.g. the wind created by the fans in the laboratory): Although I did not take this variable into account, I could have switched off the fans and closed the doors to create a controlled environment.


The longer the length of the pendulum, the longer the period of the oscillation. Therefore, my hypothesis was correct.

Class exercise 3b – Volume of irregular objects

Aim: To determine the volume of a stone and ping pong ball using the displacement method


  • Ping pong ball
  • Sinker (a stone)
  • String and Masking Tape
  • Displacement can
  • Measuring cylinder


1. Elevate the displacement can.  Place the measuring cylinder on the bench to collect water that would overflow from the spout. (In our case, we placed the measuring cylinder in the sink and the displacement can on the bench)

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2. Fill up the displacement can until water overflows from the spout.  Wait till the water stops flowing out from the displacement can, and pour away all the water in the measuring cylinder.

3. Sketch the experimental set-up:

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4. Tie the stone with a string, and gently lower the stone into the displacement can.

5. Note the volume of water collected in the measuring cylinder, V.

6. Use the same method to determine the volume of the ping pong ball:

  1. Tie the ping pong ball and the stone together using the same piece of string
  2. Prepare a separate, larger container (We used another displacement can) to collect the displaced water as it exceeds the capacity of the measuring cylinder
  3. Lower the two objects into the displacement can, ensuring that the ping pong ball is trapped below the stone when in the can (so that it does not float and its entire volume can be measured).
  4. Pour the displaced water into separate measuring cylinders to measure its volume.
Our first (failed) attempt. We later wrapped the ping pong ball and the sinker together to stop it from floating.

Our first (failed) attempt. We later bound the ping pong ball and the sinker together with the string, with the ball at the bottom, to stop it from floating.


1st attempt 2nd attempt
Volume of sinker, V / cm3 25.5 25.5
Volume of ping pong ball and sinker / cm3 56 63.5
Volume of ping pong ball / cm3 30.5 38
Average volume of ping pong ball / cm3



– List three possible causes of inaccuracy in recording the volume of the water contained in the measuring cylinder.

  1. Wetness of stone
  2. Volume of string
  3. Masking tape on stone (left over from previous experiments)
  4. Water left in measuring cylinder after 1st measurement
  5. Water left in the spout of the displacement can

– Suggest how the possible causes mentioned can be minimized so that measurements can be more accurate.

  1. Dry the stone with a cloth after each attempt
  2. Lower the entire length of string into the water when measuring the volumes of the ball and stone. Also, use the same string for measuring both volumes.
  3. Use a new, dry measuring cylinder for each measurement.

Class exercise 3 – Area of irregular objects

Aim: To estimate area of irregular shapes

1) Place your left (or right) hand firmly on the piece of graph paper. Trace the outline of your palm and fingers onto the paper. Determine the approximate area covered by your hand.

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Number of grids counted = 117

Area of each grid = 1 cm2

Approximate area of hand = 117 cm2

2) This is an estimation of the area of your and. Suggest how the accuracy of your estimation can be improved.

-> Use a smaller grid to estimate the area and keep my fingers closed when I trace the outline of my hand. This reduces the area of my hand left out of the measurement due to the grid not being ≥50% covered by the outline.

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Number of grids counted = 302

Area of each grid = 0.36 cm2

Approximate area of hand = 108.72 cm2