Moments (CIE IGCSE Physics)
Revision Note
Author
Katie MExpertise
Physics
Moments
- As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects to rotate
- An example of a rotation caused by a force is on one side of a pivot (a fixed point that the object can rotate around)
- This rotation can be clockwise or anticlockwise
The force will cause the object to rotate clockwise about the pivot
- A moment is defined as:
The turning effect of a force about a pivot
- The size of a moment is defined by the equation:
M = F × d
- Where:
- M = moment in newton metres (Nm)
- F = force in newtons (N)
- d = perpendicular distance of the force to the pivot in metres (m)
The moment depends on the force and perpendicular distance to the pivot
- This is why, for example, the door handle is placed on the opposite side to the hinge
- This means for a given force, the perpendicular distance from the pivot (the hinge) is larger
- This creates a larger moment (turning effect) to make it easier to open the door
- Opening a door with a handle close to the pivot would be much harder, and would require a lot more force
- Some other examples involving moments include:
- Using a crowbar to prize open something
- Turning a tap on or off
- A wheelbarrow
- Scissors
Principle of Moments (Core)
- The principle of moments states that:
If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot
- Remember that the moment = force × distance from a pivot
- The forces should be perpendicular to the distance from the pivot
- For example, on a horizontal beam, the forces which will cause a moment are those directed upwards or downwards
Worked example
A parent and child are at opposite ends of a playground see-saw. The parent weighs 690 N and the child weighs 140 N. The adult sits 0.3 m from the pivot.
Calculate the distance the child must sit from the pivot for the see-saw to be balanced.
Step 1: List the know quantities
-
- Clockwise force (child), Fchild = 140 N
- Anticlockwise force (adult), Fadult = 690 N
- Distance of adult from the pivot, dadult = 0.3 m
Step 2: Write down the relevant equation
-
- Moments are calculated using:
Moment = force × distance from pivot
-
- For the see-saw to balance, the principle of moments states that
Total clockwise moments = Total anticlockwise moments
Step 3: Calculate the total clockwise moments
-
- The clockwise moment is from the child
Momentchild = Fchild × dchild = 140 × dchild
Step 4: Calculate the total anticlockwise moments
-
- The anticlockwise moment is from the adult
Momentadult = Fadult × dadult = 690 × 0.3 = 207 Nm
Step 5: Substitute into the principle of moments equation
140 × dchild = 207
Step 6: Rearrange for the distance of the child from the pivot
dchild = 207 ÷ 140 = 1.48 m
Exam Tip
Make sure that all the distances are in the same units and you’re considering the correct forces as clockwise or anticlockwise, as seen in the diagram belowClockwise is defined as the direction the hands of a clock move (and anticlockwise as the opposite)
Principle of Moments (Extended)
EXTENDED
Diagram showing the moments acting on a balanced beam
- In the above diagram:
- Force F2 is supplying a clockwise moment;
- Forces F1 and F3 are supplying anticlockwise moments
- Hence:
F2 x d2 = (F1 x d1) + (F3 x d3)
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