Teacher notes ( 1 of 2 )
Balancing act
Curriculum links
England and Wales (Key Stage 3 Science Programme of Study)
key concepts
1.1a and b, 1.4a
key processes
2.1a and c, 2.2a and b
range and content
3.1b
curriculum opportunities
4a, c, k
Northern Ireland (Science Statutory Requirements)
knowledge, understanding and skills
develop: enquiry skills; critical thinking; practical skills
research information
learn about: forces
objective 1 – develop as individuals
mutual understanding: team work
objective 3 – as contributors to the economy/
environment
identify skills used in: mechanical engineering
Scotland (SQA Science Outcomes)
third level
SCN 313L
fourth level
SCN 423L
fourth level
SCN 439W Phys
Introducing the activity
The principle of levers was discovered by Archimedes; it is fundamental to structural and mechanical engineering. For a
system to be in equilibrium, the overturning moment must be resisted by an equal moment acting in an opposing direction.
You might introduce the activity with a simple demonstration of gravity, equal and opposite forces and finally rotational
movement resulting from unbalanced forces:
•
Hold a ruler horizontally and then let go. Ask what happened.
•
Place the ruler on a table. Ask why it doesn’t fall through.
•
Move the ruler so that about one third is hanging over the edge of the table. Ask why the table is still providing enough
opposite force to hold the ruler in place.
•
Move it so that it just tips off the table. Ask pupils to watch carefully, and notice how the ruler moves. Guide them to realise
that one end rises off the table while the other falls. The ruler rotates, with the table edge acting as a pivot. (If the left end
goes up and the right end down, rotation is clockwise.)
•
Move the ruler to the other end of the table and ask students which direction it rotated this time.
The rotation is caused by unbalanced forces. It happens when gravity is pulling on more of the overhanging ruler, than is
supported by the table.
This could be extended by balancing the ruler between two clamp stands and hanging a mass from the centre. Students may
be able to say that the stands are providing the equal and opposite force and it is divided equally between them.
You might follow up your explanation by asking how a bridge stays up. [For a bridge supported on two piers (such as the
Foyle Bridge in Londonderry), each pier is, among other things, providing an equal and opposite force to the weight of the
deck to prevent it rotating, in exactly the same way as your demonstration.]
In modern theatres, as at the Waterfront, the structure that holds up the lighting is called a bridge; the moments of force from
the lights [and lights can be up to 60 kg each] will be counteracted in a similar way to a road bridge.
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Teacher notes ( 2 of 2 )
The practical activity
Pupils should work at least in pairs, for there to be enough hands to hold the equipment
steady when adding loads.
Part 1: In this activity, pupils investigate the basic mechanism of a balance: the ‘clockwise’ moment must be the same as the
‘anticlockwise’ moment.
•
Measuring in grams and centimetres, pupils will find that a large load on one side of the fulcrum can be balanced by a smaller
load on the other, provided that the smaller one is further from the fulcrum.
•
The right-hand column in their table should be an opportunity for finding an equality in load x distance on either side. If
results are not giving numbers that can be readily inspected, pupils can plot a graph of (mass x distance) LHS against (mass x
distance) RHS which should then give a best-fit to show y = x.
•
It is important that the masses are placed centrally on the length markings (see the note in the equipment list) otherwise
noticeable errors will creep in.
Part 2: This can show pupils that moments are the product of the load and its perpendicular distance from the pivot.
•
Pupils may suggest that the mass of the string has an effect; this is true and therefore they must keep the string intact when
they shorten the suspended length.
Part 3: Deter pupils from having masses at more than one place on the left-hand side as this would make it less easy to spot that
moments can be summed, to achieve equilibrium.
•
There will be a number of ways in which they may describe
•
S(load x distance)LHS = S(load x distance)RHS
•
In engineering terms, for equilibrium, S(force x lever arm) = 0 because moments will be positive or negative, depending on
their direction, and should sum to zero.
•
Some pupils will be able to identify a pattern and will see that they can then predict the load position for equilibrium. Others
will need help to see the pattern. This is an opportunity to reinforce the importance of the pattern for predicting, so that it
can be used in practical situations such as the lighting rig.
•
Faster pupils might look at the extension questions.
Possible extensions
•
This activity presents an opportunity to reinforce (or introduce) the connection between the terms: mass, load, weight and
force.
•
In the practical, the ruler tips up, not because of the force of the load, but because the force is at a distance from the pivot.
A moment is properly measured in N m.
•
Pupils can place the masses on a newtonmeter and record the respective weights or, if already familiar with the concept,
calculate each force using 10 N kg-1 as the gravitational field strength. Distances can be converted from centimetres to metres
and the table, as given on the pupils’ worksheet, rewritten to show force and distance in newtons and metres respectively.
[You may need to deal with confusion between a newtonmeter and a newton metre.]
•
Torque is another term for the moment of a force. They are the same. However, moment of force is often preferred because
mechanical engineers use the term torque in relation to rotating shafts.
Equipment
• metre rule, wood rather than plastic; score the underside to form a groove across the midpoint
• triangular pivot block (fulcrum) large enough to lift the ruler well above the bench
• masses (10 x 10 g and 1 x 100 g)
• fine string or thread (about 1 m)
• newtonmeter (for extension activity)
A ready-made ‘lever kit’ is available from educational suppliers. These kits have square metal loads which will need to be weighed
so that pupils use approximately the same mass. Pupils only need to count the number of square ‘loads’ rather than using absolute
values. As the ‘loads’ are square, they can be positioned with greater precision by being placed diagonally on the beam.
Information sources
http://www.walter-fendt.de/ph11e/lever.htm (applet using newtons and metres to calculate moments)
www.tomorrowsengineers.org.uk
site for more
Visit our web
d careers
activities an
information.