Practical 2 - Determination of unknown masses using the principal of moments Flashcards
What are the apparatus for this practical?
-Nail to pivot ruler at the 70cm mark
-Metre long ruler
-Clamp and stand
-200g mass and hanger
-Loops of thread
At what point do we nail the ruler to create its pivot?
70cm mark
What does a risk assessment apply to?
People AND equipment
What are three hazards of this practical?
-Pointy nail
-Stand + clamp
-Loose weight
What’s the risk with a pointy nail?
Eye injury
What’s the control measure for a pointy nail?
Clamp the pointy end facing away
What’s the risk with the stand and clamp?
Toppling/mass falling over -injury or damage equipment
What’s the control measure for the stand and clamp?
Use a G-clamp
What’s the risk with the loose weight?
Sliding off ruler
What’s the control measure for the loose weight sliding off the ruler?
Use both hands
What’s the first stage in completing this practical?
-Loop the 200g (1.96N) over the metre rule uni it’s horizontal + note down the distance (l) from the point
-Can now calculate the mass (or weight) of the metre rule using the principal of moments
Which principle do we use to do our calculations in this practical?
The principle of moments
How do we know that 200g = 1.96N?
W = mg
W = 0.2 x 9.81
W = 1.96N
What’s the equation used to calculate the weight of the metre rule?
0.20 x metre rule weight = l x 1.96
Why do we use 0.20 in these calculations?
It’s the distance from the weight force (in the centre - 0.5m) to the pivot (0.7m)
What’s the second stage of completing this practical?
-Replace the 200g mass with an unknown weight (W) and adjust the position until it balances again
-Measure the distance, d, of the unknown weight from the pivot - can now calculate the unknown weight again using the principle of moments
What’s the equation used to calculate the unknown weight?
0.2 x metre rule weight = d x unknown weight
How do we convert the unknown weight into a mass?
Divide it by 9.81
With what do we check the real weight at the end?
Top pan balance
What’s the uncertainty of our measurements on a 1m ruler with mm measurements?
+-0.001m
How do we calculate the % uncertainty of our answer?
% u = add the uncertainties
——————————— x final answer of the mass
100
What’s the relationship between the unknown mass and the uncertainty?
Lighter unknown mass = Higher uncertainty
Name two factors that could lead to additional uncertainty in the measurements
-uncertainty of the 200g mass
-beam may not have been exactly level - could use a spirit or a laser level to improve this
What could we use the ensure that the beam is definitely level?
A spirit or even a laser level
How do we get the absolute uncertainty over a spread of data?
Range
———
2
