Lesson 14 Using Moments - Independent work
Aims
Sometimes forces can produce turning effects on a system, like a spanner turning a nut.
In this worksheet you will reinforce your understanding of the moment of a force.
Remember that the moment of a force depends on the size of the force applied and the
perpendicular distance from the line of action of the force to the pivot.
The questions are increasingly difficult and offer a natural differentiation of work.
Questions
1 The diagram shows a
screwdriver being used to open
a tin of paint. The arrow shows
the position along the
screwdriver where the force is
applied. These questions are
about basic features of
moments.
a
Mark the pivot with a cross
on the diagram above and label it.
b
The painter cannot open the tin when he applies a force in the position shown in
the diagram. Suggest what the painter could do to apply a greater moment and
open the tin.
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c
The distance between the pivot and the point at which the painter applies the
force is 0.10 m. He applies a perpendicular force on the screwdriver of 35 N. Work
out the moment about the pivot, showing all your calculations.
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2 The diagram shows two children on a
see-saw. The child on the right cannot
bring the see-saw down on his side.
These questions are about clockwise and
anticlockwise moments.
a
Mark the pivot with a cross on the
diagram above and label it.
b
The two children are at the same
distance from the pivot. Explain what is different about the children that causes
the left side of the see-saw to stay down.
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c
What should the child on the left do, if she wants her friend on the other side to
come down? Explain your answer in full.
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d
The anticlockwise moment about the pivot due to the child on the left is double
the clockwise moment due to the child on the right. State what the weight of the
child on the right is compared to the weight of the child on the left.
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Challenge Questions
1 The picture below shows a boy standing in three different positions to balance a
beam with a load on the other side of the pivot. The load is the same in each case
and its distance from the pivot doesn’t change.
a
b
In which position does the boy need to push down with the most force on the
beam to keep it balanced? Tick your choice.

Position a

Position b

Position c

He applies the same force in each position.
In which position is the clockwise moment on the beam the largest? Tick your
choice.

Position a

Position b

Position c
 The moment is the same in each position.
2. The diagram shows a revolving door being
used by three people, seen from above. The
arrows show the force applied by each
person inside the door. Work out the
resultant moment about the pivot and state
the direction of rotation of the door. (You
can assume that the door is initially at rest
and that any frictional forces can be
ignored.)
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3 The diagram shows a baby’s mobile. The dots on the arms of the mobile are 10 cm
apart. Your task is to keep the mobile balanced by adding the weights shown to the
empty boxes hanging off the mobile. You can use each weight only once. Remember
that any suspension point of a balanced system carries the total weight of the masses
hanging below it!
Weights are... 1N, 2N, 3N, 4N
Using Moments Answers
Answers to questions
1
a) A cross on the point of contact between the screwdriver and the rim of the paint tin (not the end
of the screwdriver)
b) Move his hand further from the tin to increase the distance from the pivot, hence increasing the
moment, or apply a bigger force (or both).
c) 0.10 m × 35 N  3.5 N m.
2
a) Centre of the see-saw
b) The child on the left must be heavier.
c) The child on the left should move closer to the centre of the see-saw (pivot). This would reduce
her distance from the pivot and thus decrease the anticlockwise moment, so that the clockwise
moment due to the other child might become equal to it.
d) Half, because their distance from the pivot is the same.
P3 2.1-2.3 Challenge Sheet Using Moments
Answers to questions
1 a Position a
a The moment is the same in each position, as it always balances the anticlockwise moment.
2 10 N × 2 m  2 N × 0.5 m – 35 N × 1 m  –14 N m, and it will start to accelerate anticlockwise.
3