1 Equilibrium of Concurrent, Coplanar Force Systems EF 202 - Week 5.
Investigation of the laws of equilibrium for a set of coplanar forces
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Transcript of Investigation of the laws of equilibrium for a set of coplanar forces
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Investigation of the laws of equilibrium for a set of
coplanar forces
The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan
T.H.
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Apparatus
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Method
• Find the centre of gravity of the metre stick• Find the mass and hence the weight of the
metre stick• Set up the apparatus as in the photo• Use a spirit level to ensure the apparatus
is horizontal• Record each weight with its position
(include the weight of the metre stick acting at its centre of gravity.)
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Balance the metre stick on a straight sharp edge e.g. a protractor and record its centre of gravity. Here, the centre of gravity is at the 502 mm mark.
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Why isn’t the centre of gravity at the 500 mm mark?
• One of the ends of the metre stick might be slightly damaged.
• The material from which the metre stick is made might not be uniform.
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Find the mass of the metre stick
• The mass is 117.8 g
• What is the weight of the metre stick?
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Set up the apparatus
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Use a spirit level to check that the metre stick is horizontal
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Why does the metre stick have to be horizontal?
• The weights act vertically downwards.
• We calculate the moments by multiplying each weight by the perpendicular distance from a reference point.
• If the metre stick is horizontal, these distances can be read directly.
• Otherwise, we would have to use trigonometry.
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Record the forces and their positions
Note that there are five downward forces (including the weight of the metre stick),
and two upward forces.
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How do these results verify the two laws of equilibrium?
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First Law: When a body is in equilibrium, the vector sum of the forces acting on it is zero.
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Second Law: When a body is in equilibrium, the sum of the moments about any point is
zero.
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The sum of the clockwise and the anticlockwise moments, to two significant
figures, is zero.
This verifies the second law.
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Sources of Error
• Take the readings from the metre stick at eye level to avoid the error of parallax.
• Replacing the spring-based Newton balances with digital Newton balances would increase the accuracy of the upward force measurements.