6. From this, we can calculate the sum of moments and separately calculate the sum of upward/ downward forces.
7. Repeat the experiment by changing the position of the weights and newton balances, as well as the number of weights used.
Clockwise moments
Force (N) Perpendicular distance (m)
Anticlockwise moments
Moment Force (N) Perpendicular distance (m)
Moment
Upward forces (N) Downward forces (N)
Result
By changing the point at which you calculate distance, the sum of moments and upward/downward forces should always be zero.
Conclusion
Any system in static equilibrium has to obey two things: upward forces = downward forces and the sum of moments about a point = zero. This can be found using the following formulae:
Fd(clockwise) = Fd(anticlockwise)
F = forces applied (N), d = perpendicular distance from chosen point (m)
F(up) = F(down) F = forces applied (N)
Errors
The following errors should be considered when studying this experiment:
• newton balances not at right angles to the metre rule • parallax in metre rule measurements.
LEAVING CERTIFICATE PHYSICS 417
Precautions
• Use a spirit level to make sure the metre rule is horizontal.
• Find the average of your displacement readings and verify them with your lab partners. Also make sure that the threads holding the weights are acting vertically down across the metre rule graduations, for accurate reading.
NOTE
The weight of the metre rule must be factored into the experiment. This weight will act through the centre of gravity of the metre rule.
Possible exam questions
1. Why should the metre rule be horizontal? 2.
3. 4.
How would you find the centre of gravity of the metre rule?
Why must the balances be perpendicular to the metre rule?
What quantity contains the highest percentage error?
