The moment of a force is the turning effect produced when a force is exerted on an object, causing rotation about a pivot .

A pivot is the fixed point about which an object can rotate.

Why does increasing the distance a force is applied from a pivot decrease the force needed to produce the same moment?

Since moment = force × distance, a fixed moment can be produced by a smaller force if the distance from the pivot is increased — this is why a door is easier to push open at the handle (far from the hinge) than close to the hinge.

On a horizontal beam, only forces that are ______ to the distance from the pivot will cause a moment.

On a horizontal beam, only forces that are perpendicular to the distance from the pivot will cause a moment.

True or False? The moment of a force can only be measured in newton metres (N m).

False. The moment can also be measured in newton centimetres (N cm) if the distance is measured in centimetres; in an IGCSE exam, distances should be converted to metres unless a specific unit is requested.

Give two examples of everyday situations where the moment of a force causes rotation.

Any two from: turning the handle of a spanner , opening/closing a door , using a screwdriver , turning a tap , lifting a wheelbarrow , using scissors , a child on a see-saw , or a crane moving building supplies.

Define the principle of moments .

If an object is balanced , the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.

True or False? A horizontal force applied to a horizontal beam will produce a moment about the pivot.

False. Only forces perpendicular to the distance from the pivot produce a moment; on a horizontal beam, this means only forces directed upwards or downwards cause a moment.

Define a light beam (as used in moments problems).

A light beam is one that can be treated as having no mass , so any supports need only balance the weight of objects placed on it.

The supports of a beam must exert ______ forces that balance the downward-acting weight of any object placed on the beam.

The supports of a beam must exert upward forces that balance the downward-acting weight of any object placed on the beam.

A mass is moved from the left-hand side towards the right-hand side of a beam supported at both ends. What happens to the supporting forces F 1 (left) and F 2 (right)?

F 1 (left) decreases and F 2 (right) increases as the mass moves towards the right-hand support.

A beam is supported at both ends with an object placed on it. What happens if the right-hand support is removed?

The force at the right-hand support becomes zero , so it can no longer supply the anticlockwise moment needed to balance the object's weight — the beam rotates in the clockwise direction about the left-hand support.

Define centre of gravity .

The centre of gravity of an object is the point through which the weight of the object acts.

Where is the centre of gravity located for a symmetrical object of uniform density? Give an example.

It is located at the point of symmetry — for example, the centre of gravity of a sphere is at its centre.

Define a plumb line .

A plumb line is a weighted thread that hangs vertically; it is used to mark a vertical reference line down from a suspension point when locating an object's centre of mass.

To find the centre of mass of an irregular lamina, it is suspended from a pivot and a plumb line is used to draw a vertical line; this is repeated from two more suspension points, and the centre of mass lies where the three lines ______ .

To find the centre of mass of an irregular lamina, it is suspended from a pivot and a plumb line is used to draw a vertical line; this is repeated from two more suspension points, and the centre of mass lies where the three lines cross .

True or False? The centre of mass of an irregular lamina can be located using just one suspension point.

False. The lamina must be suspended from at least three different points , with a plumb line drawn each time; the centre of mass is where the three lines intersect — a single point only shows the centre of mass lies somewhere along that one line.

IGCSEPhysicsEdexcelPhysics (Modular)Unit 1Flashcards1. Forces & MotionMoments

Moments (Edexcel IGCSE Physics (Modular): Unit 1): Flashcards

Exam code: 4XPH1

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FrontMoments
Define moment.
FrontMoments
State the equation for the moment of a force, including the units of each quantity.
BackMoments
$M = F \times d$
Where:
M = moment in newton metres (N m)
F = force in newtons (N)
d = perpendicular distance from the pivot in metres (m)

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Cards in this collection (19)

Define moment.
The moment of a force is the turning effect produced when a force is exerted on an object, causing rotation about a pivot.
Define pivot.
A pivot is the fixed point about which an object can rotate.
State the equation for the moment of a force, including the units of each quantity.
$M = F \times d$
Where:
- M = moment in newton metres (N m)
- F = force in newtons (N)
- d = perpendicular distance from the pivot in metres (m)
Why does increasing the distance a force is applied from a pivot decrease the force needed to produce the same moment?
Since moment = force × distance, a fixed moment can be produced by a smaller force if the distance from the pivot is increased — this is why a door is easier to push open at the handle (far from the hinge) than close to the hinge.
On a horizontal beam, only forces that are ______ to the distance from the pivot will cause a moment.
On a horizontal beam, only forces that are perpendicular to the distance from the pivot will cause a moment.
True or False?
The moment of a force can only be measured in newton metres (N m).
False.
The moment can also be measured in newton centimetres (N cm) if the distance is measured in centimetres; in an IGCSE exam, distances should be converted to metres unless a specific unit is requested.
Give two examples of everyday situations where the moment of a force causes rotation.
Any two from: turning the handle of a spanner, opening/closing a door, using a screwdriver, turning a tap, lifting a wheelbarrow, using scissors, a child on a see-saw, or a crane moving building supplies.
Define the principle of moments.
If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
A parent weighing 690 N sits 0.3 m from the pivot of a see-saw. A child weighing 140 N sits on the opposite side. Calculate the distance the child must sit from the pivot for the see-saw to balance.
Anticlockwise moment (parent) = 690 × 0.3 = 207 N m
For balance: clockwise moment (child) = anticlockwise moment
140 × d_child = 207
d_child = $\frac{207}{140}$ = 1.5 m (2 s.f.)
True or False?
A horizontal force applied to a horizontal beam will produce a moment about the pivot.
False.
Only forces perpendicular to the distance from the pivot produce a moment; on a horizontal beam, this means only forces directed upwards or downwards cause a moment.
Define a light beam (as used in moments problems).
A light beam is one that can be treated as having no mass, so any supports need only balance the weight of objects placed on it.
The supports of a beam must exert ______ forces that balance the downward-acting weight of any object placed on the beam.
The supports of a beam must exert upward forces that balance the downward-acting weight of any object placed on the beam.
A mass is moved from the left-hand side towards the right-hand side of a beam supported at both ends. What happens to the supporting forces F₁ (left) and F₂ (right)?
F₁ (left) decreases and F₂ (right) increases as the mass moves towards the right-hand support.
A beam is supported at both ends with an object placed on it. What happens if the right-hand support is removed?
The force at the right-hand support becomes zero, so it can no longer supply the anticlockwise moment needed to balance the object's weight — the beam rotates in the clockwise direction about the left-hand support.
Define centre of gravity.
The centre of gravity of an object is the point through which the weight of the object acts.
Where is the centre of gravity located for a symmetrical object of uniform density? Give an example.
It is located at the point of symmetry — for example, the centre of gravity of a sphere is at its centre.
Define a plumb line.
A plumb line is a weighted thread that hangs vertically; it is used to mark a vertical reference line down from a suspension point when locating an object's centre of mass.
To find the centre of mass of an irregular lamina, it is suspended from a pivot and a plumb line is used to draw a vertical line; this is repeated from two more suspension points, and the centre of mass lies where the three lines ______.
To find the centre of mass of an irregular lamina, it is suspended from a pivot and a plumb line is used to draw a vertical line; this is repeated from two more suspension points, and the centre of mass lies where the three lines cross.
True or False?
The centre of mass of an irregular lamina can be located using just one suspension point.
False.
The lamina must be suspended from at least three different points, with a plumb line drawn each time; the centre of mass is where the three lines intersect — a single point only shows the centre of mass lies somewhere along that one line.

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