1© Manhattan Press (H.K.) Ltd.

• Position and distancePosition and distance

Position, distance and Position, distance and displacementdisplacement

• Position and displacementPosition and displacement• TimeTime

• Scalar and vector quantitiesScalar and vector quantities

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study motions of bodies

How? How?

Why?Why?

Position and distance

1. position

2. direction

3. time taken

4. other related physical quantities

MechanicsMechanics

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Position and distance

• position — denotes both the direction & distance from a given reference point

• unit of distance: metre (m) / kilometre (km)

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1 km north

Shopping centre X from Chris:direction: northdistance: 1 km

1 km east

Shopping centre X from Tracy:

direction: east

distance: 1 km

Chris

Tracy X

Y

N

1 km

1 km

1 km 1 km

position of Tracy

position of Chris

Position of shopping centre X

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direction = south-east

distance = km

　 　 　 = 1.41 km

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The position of church Y from Chris:

Chris

Tracy

Y

N

1 km

1 km

1 km 1 km

Position of church Y

X

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direction = south-east

distance = km

　 　 　 = 2.83 km

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The position of church Y from Tracy:

Chris

Tracy

X

Y

N

1 km

1 km

1 km 1 km

Position of church Y

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destination

starting point

• Distance from destination = 200 m

• Distance from starting point (complete 1 loop) = 400 m

The reference point

200 m

200 m

Different reference point Position changes

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Position and displacement

• Distance alone is not enough

Distance = 5 m, represents A or B?

Ｏ ＢＡ5m 5m

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Position and displacement

• represents change in position

How can we solve it

Displacement

• a physical quantity which has both

magnitude & direction

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Position and displacement

Displacement of A and B from O:

Ｏ ＢＡ

direction direction

negative (–) positive (+)

5 m 5 m

Magnitude = 5 m

Direction:

• Left ()

• Right(+)5 m +5 m

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Position and displacement

magnitude of displacement = shortest distance between starting point & end point

• independent of the travelled path

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Position and displacement

distance

length

independent of the travelled path

length

depend on the travelled path

displacementdistance

displacementThinking 1Thinking 1

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Time

Measure the duration of an event

Unit: second (s), minute (min), hour (h)

a sundial

an atomic clock

a watcha quartz clock

a stopwatch

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time, distance

& temperature

Scalar and vector quantities

magnitude only

scalar quantity vector quantity

magnitude & direction

displacement