“TURNING EFFECT OF FORCES”

Combined Science 5129BENGKEL KECEMERLANGAN AKADEMIK 2015

MOMENTS

LEARNING OUTCOMES

MOMENT“The turning effect a force on an object.”

Opening a Door

Playing on a Sea-Saw

Turning a Tap

MOMENTA moment of force will rotate an object in a

clockwise or anti-clockwise direction about a fixed position (pivot or centre of rotation).

Clockwise Moment

Anticlockwise Moment

MOMENT

The SI unit of Moment of a force is the Newton-metre (Nm)

Moment of a force is a vector quantity.

MOMENT FORCEPERPENDICULAR DISTANCE FROM FORCE TO PIVOT

X=

Force

⦜ distance to pivot

Force

⦜ distance to pivot

ANTICLOCKWISEMOMENT

CLOCKWISEMOMENT

PRINCIPLES OF MOMENTS

“When a body is in equilibrium, the sum of clockwise moments about the pivot is equal to the sum of the

anticlockwise moments about that same pivot.”

TOTAL CLOCKWISE MOMENTS

TOTAL ANTICLOCKWISE MOMENTS

=

TOTAL ANTICLOCKWISE MOMENTS

TOTAL CLOCKWISE MOMENTS

=

Force A

⦜distance for FA

⦜distance for FB

Force B Force D

⦜distance for FD

Force C

⦜distance for FC

TWO ANTICLOCKWISE MOMENTS created by Force A and Force B

TWO CLOCKWISE MOMENTS created by Force C and Force D

CONSIDER THE FOLLOWING EQUILIBRIUM SCENARIO

(FA x ⦜dA) + (FB x ⦜dB) = (FC x ⦜dC) + (FD x ⦜dD)

PRINCIPLES OF MOMENTSIdentify the pivot first.

Identify & label the clockwise forces and its ⦜ distances to the pivot.

Identify & label the anticlockwise forces and its ⦜ distances to the pivot.

Use brackets ( ) for each moments.

Be careful with your maths!

EXAMPLE EXERCISE 1Fig. 12.1 shows a spanner being used.

A moment of 30 Nm is needed to tighten the nut.

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Calculate the force applied to the spanner.

EXAMPLE EXERCISE 2(a) State the units of the moment of a force.

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(b) Fig.4.1 shows a spanner being used to undo a bolt.

The force needed to undo the bolt is smaller when a spanner with a longer handle is used. Explain why.

EXAMPLE EXERCISE 3

A boy on a diving board is shown in Fig.6.1.

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The boy weighs 500 N and stands 1.5 m from the pivot.

The clockwise moment of the boy’s weight about the pivot is equal to the anti-clockwise moment of the

force F about the pivot.

(a) Force F is 0.30 m from the pivot. Calculate force F.

EXAMPLE EXERCISE 4A uniform beam is pivoted at its midpoint. An object is placed on the beam as shown. Which force and position will balance the system?

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A

B

C

D

20 N acting downwards, 40 cm to the right of the pivot

20 N acting upwards, 40 cm to the right of the pivot

50 N acting downwards, 10 cm to the left of the pivot

50 N acting upwards, 10 cm to the left of the pivot

EXAMPLE EXERCISE 5A metre rule is pivoted at its centre of gravity.

A weight of 8.0 N is suspended from the rule at a distance of 0.20 m from the pivot, as shown in Fig.3.1.

The metre rule is held horizontally by means of a stretched spring that is 0.40 m from the pivot.

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(a) State the principles of moments.

EXAMPLE EXERCISE 5A metre rule is pivoted at its centre of gravity.

A weight of 8.0 N is suspended from the rule at a distance of 0.20 m from the pivot, as shown in Fig.3.1.

The metre rule is held horizontally by means of a stretched spring that is 0.40 m from the pivot.

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(b) Calculate:

(i) the moment of the 8.0 N weight about the pivot

EXAMPLE EXERCISE 5A metre rule is pivoted at its centre of gravity.

A weight of 8.0 N is suspended from the rule at a distance of 0.20 m from the pivot, as shown in Fig.3.1.

The metre rule is held horizontally by means of a stretched spring that is 0.40 m from the pivot.

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(b) Calculate:

(ii) the force exerted on the metre rule by the spring.

EXAMPLE EXERCISE 6A bar is placed on a pivot and blocks of m1 and m2 are placed on

the bar, as shown in Fig.17.1.

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The bar is horizontal. The distances d1 and d2 of the blocks from the pivot are shown in Fig.

17.1.

The masses and their distances from the pivot may be changed so that the bar stays horizontal, tips clockwise or tips anticlockwise.

EXAMPLE EXERCISE 6Fig.17.2 shows the bar tipping anticlockwise.

5129/22/O/N/12 Q17

Different masses m1 and m2 and distance d1 and d2 are shown in Table.17.1.

Complete Table 17.1 by stating whether the bar is horizontal, tips clockwise or tips anticlockwise. The first line has been completed for

you.

EXAMPLE EXERCISE 65129/22/O/N/12 Q17

Complete Table 17.1 by stating whether the bar is horizontal, tips clockwise or tips anticlockwise.

The first line has been completed for you.

PRACTISE QUESTION

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Complete the following questions in pairs.

Remember to raise your hands if you need help with the question.