Category: Q&A

Painting a house

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5 men are hired to paint a house. If an additional man is hired, the painting can be completed 4 days earlier. Calculate the number of additional men to be hired if the painting is to be completed 18 days earlier.

The action of heat on metals and metal carbonates

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Based on the data given, suggest which 2 were the metals and which 2 were the metal carbonates. Explain your answers.

Metals: A and D
Metal carbonates: B and C

Explanation:

Metals, when heated, will be oxidized by oxygen in the air to form metal oxides. The oxygen gained to form the oxide will cause the mass of the product formed to be higher. Mass increase for solids A and D after heating, so these two are metals.
Metal carbonates, when heated, can decompose to form metal oxides and carbon dioxide. The carbon dioxide formed escape to the surroundings, causing the mass of the solid product formed to be smaller.
The mass of B decreased after heating, so it must be one of the metal carbonates.
The mass and appearance of C remained unchanged, so it must be the other metal carbonate, which is not decomposed by the heat.

Movement of electrons in discharge ball

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An electrostatic generator is used to produce sparks as shown below.

The belt carries negative charges to the dome, making it negatively charged. Before a spark is produced, the discharge ball becomes positively charged.

Describe and explain the movement of electrons in the discharge ball and in the conducting rod as the ball becomes charged.

The electrons in the discharge ball and in the conducting rod moves downwards through the conducting wire, and to the ground.
Electrostatic charging by induction occurs in the ball and rod as they are placed near to the negatively charged dome. Like charges repel, causing the negatively charged electrons in the ball and rod to move away from the dome, escaping through the conducting wire to the ground.

Movement of air particles in sound waves

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A sound wave in air is illustrated below.

Points A, B, C and D are all the centers of regions of compression in the sound wave.
Describe what happens as one complete sound wave moves past a point.

The particles in the air are disturbed by the sound waves and vibrates to and fro, in a direction parallel to the direction of the sound waves.
An air particle in the sound wave will vibrate from its rest position to the left, until it reaches its maximum displacement to the left, then vibrate towards the right, move past its rest position, until it reaches its maximum displacement to the right. Then it vibrates to the left, until it returns back to its original rest position to complete one wave motion of the sound wave.