A moment is the turning effect of a force, caused when a force acts to make an object rotate about a pivot.

The unit of a moment may also be written in newton .......... depending on the units used for distance.

The unit of a moment may also be written in newton centimetres depending on the units used for distance.

True or False? A force must be perpendicular to the pivot to produce a moment.

False. The force is not always perpendicular; only the perpendicular distance between the pivot and the line of action of the force is used to calculate the moment.

Why is a door handle positioned on the opposite side of the door to the hinge?

Placing the handle far from the hinge (the pivot) maximises the perpendicular distance for a given force, producing a greater moment and making the door easier to push or pull.

State the principle of moments .

For a system in equilibrium , the sum of the clockwise moments about a point equals the sum of the anticlockwise moments about the same point.

A couple is a pair of equal and opposite , coplanar forces that produces rotation only , with a resultant force of zero .

What three conditions must a pair of forces satisfy to form a couple ?

They must be equal in magnitude , opposite in direction , and perpendicular to the distance between their lines of action.

Unlike the moment of a single force, the moment of a couple does not depend on a .......... .

Unlike the moment of a single force, the moment of a couple does not depend on a pivot .

Define the torque of a couple.

The torque of a couple is the moment of the couple , equal to one of the forces multiplied by the perpendicular distance between their lines of action.

True or False? A couple produces a resultant force of zero, so it has no effect on the object.

False. A couple still produces a turning effect ( torque ), even though the resultant force is zero and there is no linear acceleration.

Define centre of mass .

The centre of mass of an object is the point at which the weight of the object may be considered to act.

Where is the centre of mass located for a symmetrical object of uniform density?

At its point of symmetry (e.g. the centre of a sphere).

When is an object described as stable?

An object is stable when its centre of mass lies above its base .

How does the width of an object's base affect its stability?

A wider base gives a lower centre of mass, making the object more stable . A narrower base gives a higher centre of mass, making the object more likely to topple .

When suspended from a point, an object always settles so that its centre of mass comes to rest .......... the pivoting point.

When suspended from a point, an object always settles so that its centre of mass comes to rest below the pivoting point.

Describe how a plumb line is used to find the centre of mass of an irregular laminar shape.

The shape is suspended from a pivot and allowed to settle. A plumb line is held next to the pivot and a vertical line is drawn from it; the centre of mass must lie on this line. This is repeated from two or three different suspension points, and the centre of mass is located where all the lines cross .

True or False? The centre of gravity of an object is identical to its centre of mass, whatever the gravitational field it is in.

False. Centre of gravity is only identical to centre of mass in a uniform gravitational field. Since weight = mass × g , an object's centre of gravity depends on the gravitational field, whereas its centre of mass does not.

A system is in equilibrium when the resultant force and resultant torque (moment) acting on it are both zero .

An object in equilibrium will remain at rest, or move with .......... velocity, and will not rotate.

An object in equilibrium will remain at rest, or move with constant velocity, and will not rotate.

True or False? A system is in equilibrium if only the resultant torque acting on it is zero.

False. A system is in equilibrium only when both the resultant force and the resultant torque acting on it are zero.

In equilibrium, how are coplanar forces represented in a vector diagram?

They are represented by a closed vector triangle — the force vectors, joined head-to-tail, form a closed path .

Name four common forces that act on objects in equilibrium problems.

Weight , normal reaction force , tension (from cords or strings), and friction .

A LevelPhysicsOCRFlashcards3. Forces & MotionMoments

Moments (OCR A Level Physics): Flashcards

Exam code: H556

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Cards in this collection (26)

Define moment.
A moment is the turning effect of a force, caused when a force acts to make an object rotate about a pivot.
What is the equation for the moment of a force?
$Moment = Force \times perpendicular distance from the pivot$
The SI unit of the moment is the newton metre (N m).
The unit of a moment may also be written in newton .......... depending on the units used for distance.
The unit of a moment may also be written in newton centimetres depending on the units used for distance.
True or False?
A force must be perpendicular to the pivot to produce a moment.
False.
The force is not always perpendicular; only the perpendicular distance between the pivot and the line of action of the force is used to calculate the moment.
Why is a door handle positioned on the opposite side of the door to the hinge?
Placing the handle far from the hinge (the pivot) maximises the perpendicular distance for a given force, producing a greater moment and making the door easier to push or pull.
State the principle of moments.
For a system in equilibrium, the sum of the clockwise moments about a point equals the sum of the anticlockwise moments about the same point.
A beam in equilibrium has one force, F₂, producing a clockwise moment about a pivot, and two forces, F₁ and F₃, producing anticlockwise moments about the same pivot. How are these related?
$F_{2} \times d_{2} = (F_{1} \times d_{1}) + (F_{3} \times d_{3})$
Define a couple.
A couple is a pair of equal and opposite, coplanar forces that produces rotation only, with a resultant force of zero.
What three conditions must a pair of forces satisfy to form a couple?
They must be equal in magnitude, opposite in direction, and perpendicular to the distance between their lines of action.
Why does a couple produce a turning effect but not linear acceleration?
A couple has a resultant force of zero, so by Newton's second law ( $F = m a$ ), the object experiences no linear acceleration, only rotation.
Unlike the moment of a single force, the moment of a couple does not depend on a ...........
Unlike the moment of a single force, the moment of a couple does not depend on a pivot.
Define the torque of a couple.
The torque of a couple is the moment of the couple, equal to one of the forces multiplied by the perpendicular distance between their lines of action.
Write the equation for the torque of a couple, stating the unit of each quantity.
$τ = F d$
- $τ$ is the torque of the couple in $N m$
- $F$ is one of the forces in $N$
- $d$ is the perpendicular distance between the forces in $m$
True or False?
A couple produces a resultant force of zero, so it has no effect on the object.
False.
A couple still produces a turning effect (torque), even though the resultant force is zero and there is no linear acceleration.
Define centre of mass.
The centre of mass of an object is the point at which the weight of the object may be considered to act.
Where is the centre of mass located for a symmetrical object of uniform density?
At its point of symmetry (e.g. the centre of a sphere).
When is an object described as stable?
An object is stable when its centre of mass lies above its base.
How does the width of an object's base affect its stability?
A wider base gives a lower centre of mass, making the object more stable. A narrower base gives a higher centre of mass, making the object more likely to topple.
When suspended from a point, an object always settles so that its centre of mass comes to rest .......... the pivoting point.
When suspended from a point, an object always settles so that its centre of mass comes to rest below the pivoting point.
Describe how a plumb line is used to find the centre of mass of an irregular laminar shape.
The shape is suspended from a pivot and allowed to settle. A plumb line is held next to the pivot and a vertical line is drawn from it; the centre of mass must lie on this line. This is repeated from two or three different suspension points, and the centre of mass is located where all the lines cross.
True or False?
The centre of gravity of an object is identical to its centre of mass, whatever the gravitational field it is in.
False.
Centre of gravity is only identical to centre of mass in a uniform gravitational field. Since weight = mass × g, an object's centre of gravity depends on the gravitational field, whereas its centre of mass does not.
Define equilibrium.
A system is in equilibrium when the resultant force and resultant torque (moment) acting on it are both zero.
An object in equilibrium will remain at rest, or move with .......... velocity, and will not rotate.
An object in equilibrium will remain at rest, or move with constant velocity, and will not rotate.
True or False?
A system is in equilibrium if only the resultant torque acting on it is zero.
False.
A system is in equilibrium only when both the resultant force and the resultant torque acting on it are zero.
In equilibrium, how are coplanar forces represented in a vector diagram?
They are represented by a closed vector triangle — the force vectors, joined head-to-tail, form a closed path.
Name four common forces that act on objects in equilibrium problems.
Weight, normal reaction force, tension (from cords or strings), and friction.

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