| Energy & Forces - Forces & their effects - Forces & Gravity - G31 |
  Teacher's
Guide
EF-D3.2 |
ISE 5-14 Curriculum Support Materials Overview advice |
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Here's a wee activity suggested by Hamish Budge
who came across it as part of a team-building exercise but thought it would make
an ideal introduction to teaching stuff on forces. Also guaranteed to break the
ice at parties! You need a 2 m long bamboo cane. Get at least 6 pupils (or adults) to hold their 2 index fingers under the cane and try to get them to lower the cane to the ground without letting their fingers lose contact with the wood. It's surprisingly difficult even if you do know force on the finger needs to be less than weight/number of fingers. |
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1. The important point of this investigation is that if you take two objects
of similar size and shape but different weight and drop them simultaneously,
they will arrive at the ground at the same time.
This is based on a famous experiment by Galileo 400 years ago, when he noticed that objects accelerate when they fall. The story goes that he experimented by dropping an iron weight and a bag of cloth of similar size and shape but very different weight, from the tower at Pisa. He found that they both arrived at the ground at the same time. He concluded that gravity pulls them both down equally. Before trying this with the children get them to predict what will happen. Try it out several times and ask them to watch very carefully. They may feel that it is not a fair test for some reason - how can you tell that both balls are being released simultaneously, how can you tell that they both land at the same time. Perhaps the children could work out a way of releasing the balls and a way of measuring the landing more accurately than sight alone. An important point to note is that items of different size and shape will fall to the ground at different rates. This is because although gravity pulls on them equally, things with a large surface area encounter resistance from the air and are slowed down. Think of a person with a parachute and one without. Who will fall the faster? |
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Weight and Mass
This is a very confusing idea but it might help if you think about going to the Moon. On Earth, let’s say you weigh 72 kg - that is, gravity on Earth is pulling you down with a force of 720 N. Now if you go to the Moon, there is no less of you, you still contain the same amount of ‘stuff’. Your mass would still be 72 kg but your weight would be 120 N because the Moon’s gravity is only 1/6 that of Earth. Another example is in orbit where the astronauts are weightless but still have the same mass - their bodies are unchanged! |
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2. Discussions of weight and mass are may be too difficult for the children
but they can understand that a force meter measures the amount of ‘pull’.
Force meters usually look just look like spring balances but have a scale in
Newtons instead of kg.
Like spring balances, they come in various scales and if you have one which will only measure up to 10 N, don’t let strong children test their strength on the spring! Worksheet D21a (G) could be used for them to record the weight in Newtons of the objects they test. If you want to extend this further, you could use worksheet D21b (G) which asks the children to work out their weight in Newtons on various planets. |
The weight that we are familiar with on Earth is really a measure of how much
gravity is pulling on an object and scientists measure this as a force. Forces
are measured in Newtons, shortened to N. (one kg is about the same as 10 N).
Technically speaking, when you weigh yourself you are measuring how much gravity
is pulling on you and so should use Newtons. The scales we use, convert our
weight into Kilogrammes or stones and pounds which is really a measure of how
much of you there is, or your mass. (Try explaining that at the slimming club!)
