Here’s question on Mass that was posted by a parent on our

Your child needs to know how to link the concepts from Mass and Energy to tackle these questions accurately.

## Thought Process

The four balls are of the same size and volume but are made of different materials.

As copper and iron are denser than styrofoam and paper, the mass of the copper (W) and iron (Z) ball will be larger than the styrofoam (X) and paper (Y) ball.

Hence, this suggests that Statement B, “Ball W has a greater mass than Ball X is correct.

The four balls in the diagram share the same cross-sectional area, therefore the air resistance acting on the balls are the same.

As such, the four balls will reach the sand at the same time.

Therefore, this shows that Statement C, “All the balls will reach the sand at the same time” is correct.

Based on this line of thought, we can also safely eliminate Statement A, “Ball Y will reach the sand first” and Statement D, “Ball W and Z will reach the sand before Ball X and Y“.

### Note:

The concept of “Density” is not tested at the PSLE level.

However, students are required to recognise that different materials have different densities.

Therefore, different objects with the same volume, may weigh more than others because of different densities.