This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

## Sunday, October 11, 2009

### Simple Machines - The Movable Pulley

One of the functions of simple machines is to lighten our workload. A small effort can move a greater load. What is sacrificed is that the distance moved by the effort is greater than the distance moved by the load.

As an example, we will take a look at the movable pulley. So how does a movable pulley help to lighten our work?

Remember that in order to be able to move a load that is greater than the effort, the distance moved by the effort has to be greater than the distance moved by the load.

With reference to the diagram above:-

Pulley A - The distance from the pulley to the ceiling is 2m. This means that the cable attached to the pulley is also 2m.

Pulley B – The distance moved by the effort is 2 m. However, the load moves only 1 m from the original position of the pulley. This is confirmed by …. 2-m cable divided by 2 = 1m, which means that the distance from the pulley to the ceiling is 1m. Hence, the distance from the pulley now, is 1 m from where it was originally.

Since the effort moved twice the distance moved by the load, only ½ the effort is needed to raise the load. Hence if the load weighs 10 kg, an effort of slightly over 5 kg only is needed to move the load.

keentolearn said...

Hi Mr Song

A supplier transports ice blocks from a factory to a market. The ice cubes are covered with thick cloth. What is the main reason for covering the ice blocks with thick cloth? Explain

How much information should we put in the answer so as to secure A* marks?

Observer said...

Main reason - To prevent the ice blocks from gaining heat. [1/2 the full mark]

Explain - The thick cloth is a poor conductor of heat. [1/2 the full mark]

If time permits, always give the complete answer so as not to lose marks unnecessarily even if you are not asked to explain.

Ans: The thick cloth is a poor conductor of heat. It prevents the ice blocks from gaining heat (from the atmosphere and melting too quickly).

( ) optional

keentolearn said...

Hi observer

Observer said...

Key concepts:

Heat flows from hotter object to cooler object (until both reach the same temperature)

Heat gains causes object to change from solid to liquid (and liquid to gas)

Poor conductor of heat – Heat passes less quickly.

Key words:
Heat gain
Poor conductor of heat

Based on key concepts – may want to include Melting (as part of key words)

keentolearn said...

Hi observer

Anonymous said...

For the case of the moveable pulley shown, the effort is moving against gravity. How much does gravity plays a part here?

Thanks
Jan

Excel Eduservice said...

The gravity on the load definitely has a part to play. Technically, gravity acting on the cable too, has a part to play. However, at PSLE level, you are to ignore the gravitational force on the cable.

The only force other than the load you have to consider, is friction.

Tilted Earth said...

Dear Mr. Song

Which gearing should a cyclist change to if he or she wants to travel fast on a flat road? How about the gearing if one cycles up/down a slope?

Is this part of the syllabus?

Excel Eduservice said...

So far, I have not come across a Q like that. However, if you want to travel fast on a flat road, the prime mover (the gear where the effort is acting on) has to have more teeth than the gear that is attached to the wheel. In this way, effort moves less than the load.

For uphill, you will want to use less effort. Hence, the prime mover must have less teeth than the load. This way, it means effort has to move a greater distance than the load, which means less effort is needed to move the load.

For downhill, it does not matter because the bicycle gears are designed such that if you do not pedal the bike, it goes into ‘free gear’. This is unlike motor vehicles where lower gears do have an effect in slowing down the vehicle.

PS – PSLE Qs are not as complex as the above.

Tilted Earth said...

Dear Mr Song