The greater the “mechanical advantage” in a pulley system, the heavier the loads you can lift.

Also, in another type of pulley system consists of two **pulley wheels** each on a shaft, connected by a **belt**. This transmits rotary motion and force from the input, or driver shaft, to the output, or driven shaft.

If the pulley wheels are different sizes, the smaller one will spin faster than the larger one.

Let’s try this question:

Refer : http://www.bbc.co.uk/schools/gcsebitesize/design/systemscontrol/mechanismsrev7.shtml to learn more about velocity ratio, output speed and torque (turning force).

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Try this question:

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After watching the video, let’s try this question.

The longitude of Rockhampton is approximately 150° E and the longitude of Dhaka is 90° E. What is the time in Dhaka when it is 6am in Rockhampton?

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After watching the video, let’s try this question:

Sound travels at a speed of about 330 metres per second in air. Light travels at 300 000 kilometres per second in air.

You see the lightning (before / after) you hear the thunder during a thunderstorm, because light travels (slower / faster) than sound.

The time difference between seeing the lightning and hearing the thunder lets you calculate how far away you are from the storm.

During a thunderstorm, Jason saw lightning and nine seconds later heard thunder. Approximately how far away was the storm? ____________ km.

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Recap:

- A solar eclipse happens when the ________’s shadow falls somewhere on the surface of ________.
- A lunar eclipse happens when the ________’s shadow falls across the ________.

- Total eclipses occur with the darkest part of the shadow called ________.

- In the lighter part of the shadow, the ________, we get partial eclipses.

- Given the moon orbits earth every 29 days or so, why we don’t get eclipses every month?

____________________________________________________________ - The sun and the moon
**appear to be**________ size**in our sky**. - In actual, the sun is ________ times bigger than the moon.
- The sun is ________ times farther away than moon.
- The moon has an ________ orbit.
- The moon’s orbit is tilted ________ degrees from the earth’s orbit.
- There are ________ points where the moon’s orbit crosses the sun’s plane, called nodes.
- As the earth moves along its orbit, those points line up with the sun about ________ a year.

- If the moon is between the sun and earth at that time, we get a ________ eclipse. If it’s behind the earth at that time, we get a ________ eclipse.
- Because the moon has an ________ orbit, so its size varies about ________ % throughout a month.
- At its closest distance (biggest in size in our sky), we can get ________ solar eclipses.
- But when it’s farthest away (smallest in size), we get ________ solar eclipse.
- The moon is moving away from earth by about ________ inches every year.
- A ________ solar eclipse is the only time we can see the sun’s ________ (sun’s outer atmosphere).

- The totality of solar eclipse lasts no longer than ________ minutes, where as lunar eclipse can last ________ minutes.
- During a total lunar eclipse, the moon often turns ________. If we didn’t have an atmosphere, the moon would go completely dark, but the gas molecules in our air scatter the ________ bluer wavelengths of light, while the ________ , redder wavelengths pass through to the moon.

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Yes, we can solve this question by counting, but there is another way to solve this type of problem.

First, find the smallest 3-digit number divisible by 21:

- the smallest 3-digit number is 100
- divide 100 by 21 gives 4.76, which
to 5rounds up- so the smallest 3-digit multiple of 21 will be 5 x 21 = 105.
Next, find out the largest 3-digit number divisible by 21:

- the largest 3-digit number is 999
- divide 999 by 21 gives 47.57, which
to 47rounds down- so the largest 3-digit multiple of 21 will be 47 x 21 = 987.

Method 1:The 3-digit multiples of 21 form the arithmetic sequence5 x 21, 6 x 21, …, 46 x 21, 47 x 21

Hence, the number of 3-digit numbers which are multiples of 21 is 47 – 5 + 1 = 43

Method 2:Find out how many intervals by 21 are between these two numbers.(987 – 105) ÷ 21 = 42

Hence, the number of multiples of 21 from 105 to 987 inclusively is 42 + 1 = 43.

Now, let’s try this:

*How many 3-digit numbers are multiple of 23?*

Answer: 39

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