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Tutorial

My dear students,

Refer unit 11 in your science text book.

Write down this note in your science exercise book.

Answer all the questions.

The activities will be done in the school laboratory after the vacation.

Turning effect of a force /moment of the force.

e.g.

Opening a tap

Unlocking a door using a key

Closing a window

Contents

11.1 Moments

11.2 Couple of forces

When a force is applied on an object, if it starts to rotate/turn around an axis or a

point, it is known as the moment of force / turning effect of a force.

Tutorial 07

Taxila Central College – Horana

Grade 10

Unit 11 – Turning effect of a force

Part II

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So the tendency of an object to rotate due to the action of a force is known as the turning

effect of a force / moment of the force.

Pivot /Fulcrum

The point around which the object rotates is called as pivot /fulcrum.

The axis of rotation

The axis through which the object rotates is known as

the axis of rotation.

Fulcrum/pivot

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Investigation of the factors which affect for the rotation of an object.

Activity

Observation

Mark the four points A, B, C and D

on the same level of a door

attached to its frame by hinges.

Attach a Newton balance to the

point A with the aid of a rubber

sucker clamp and apply a

perpendicular force on the door to

open it.

Measure the force necessary to

slightly move the door using the

Newton balance.

Repeat this procedure for the

Perpendicular

distance from

the axis of

rotation to the

force.

Newton

balance

reading when

the door just

start to move

Value of

forcex

Perpendicular

Distance

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Questions

I. At what point was the force required to the door rotates the door a minimum?

………………………………

II. At what point was the force required to the door rotates the door a maximum?

………………………………

III. The amount of force required to open the door …………………….., when the perpendicular

distance from the rotational axis to the line of action of the force decreases.

IV. The amount of force required to open the door …………………….., when the perpendicular

distance from the rotational axis to the line of action of the force increases.

V. In which instance it is easier to open the when a force is applied to open the door?

……………………………………..

VI. What is meant by moment of force?

……………………………………………………………………………………………………………………………………………………

……………………………………………………………………………………………………………………………………………………

……………………………………………………………………………………………………………………………………………………

The expression for the moment of force

Moment of force= Force (F) x

The perpendicular distance from the

rotational axis to the line of action of the

force (m)

M = F d

Perpendicular

distance

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VII. What is the unit of moment of force?

………………………………………………………………………………………………………………………………

Investigation of the dependence of the moment on the magnitude of the force

(The activities will be done in the school laboratory after the vacation.)

Activity 02 (Text book -Page 191)

Drill the holes at the points O, A, B, C and D as shown in the above diagram and note the

distance from the point O to each point.

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Observation

Stage Force(N)

Initial set up

After tightening the screw by quarter of a turn

Initial set up after tightening the screw by half of a turn

Conclusion

…………………………………………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………………………………

Investigation of the dependence of the moment on the perpendicular distance

to the force from the point of suspension.

Activity 03 (Text book -Page 193)

Observation

Stage Force(N)

Distance to the balance(x)

Initial set up

After tightening the screw by quarter of a turn

Initial set up after tightening the screw by half of a turn

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Conclusion

…………………………………………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………………………………

Question

Identify the following instances where the moment of force can be seen in our day to day life.

………………………………………………………………….. …………………………………………………………………

……………………………………………………………………… ……………………………………………………………………

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Direction of the moment of force of object under the action of the

moment of force

Classification of moment of force by considering the direction of rotation

1. Clockwise moment

2. Anti - clockwise moment

The clockwise moment acts downwards on the right.

The anti-clockwise moment acts downwards on the left.

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Direction of the moment of force and equilibrium of object under the action of

the moments of force

F1 and F2 are two forces.

Therefore

Clockwise moment = F1 x d1

Anti - clockwise moment = F2 x d2

Question

1. Write down an expression for the resultant moment, If the two forces are applied in the same

time,

2. If the opposite moments are equal, what can you say about the object?

………………………………………………………………………………………………………………………………………………………..

Resultant moment =

M = F d

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Calculation of moment of force

Example 01

The figure given below shows a uniform rod AB of length 1 m is suspended and balanced at its centre.

If a weight of 20 N is suspended from B, let’s calculate the clockwise moment by using the values given

in the figure.

F1 = 20 N

d1 =0.5 m

Then

Clockwise moment = F1 x d1

=20 N x 0.5 m =10 Nm

Pay your attention for

the unit.

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Let’s calculate the weight that should be suspended from the point C to balance the rod.

Assume that the weight suspended from C is x,

Anti - clockwise moment = F2 x d2

= x x 0.4 m

If the rod balances, there is no resultant moment and the rod is at equilibrium.

Anti - clockwise moment = Clockwise moment

x x 0.4 m = 20 N x 0.5 m

x = 25 N

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Exercise

2.)

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The couple of forces

Activity

Two coplanar forces of equal magnitudes acting in opposite directions, along two

lines of action that are spaced apart are known as a couple of forces.

Observation

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

Conclusion

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

…………………………………………………………………

When a couple of force is applied on an object, it starts

rotate.

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Question

Complete the following sentences.

I. When a couple of forces is applied on an object, the resultant of the two forces

is……………………………..because the two forces have ……………………magnitude.

II. The two forces act in ………………………..direction.

III. So the linear displacement in an object if a couple of forces is applied on it is

……………………………………..

IV. What is the definition for the moment of a couple of force?

V. Write down an expression for the moment of a couple of force.

…………………………………………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………………………………

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Application of couple of forces

Mark the couple of forces in the following instances.