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Distance (d ) – describes how far an object has travelled from a starting point.

Units for distance are metres (m) or kilometres (km)

Eg. Christchurch is 360km from Dunedin

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Time ( t ) – describes how long an object takes to reach its end point.

Units for time are seconds (s), minutes (min) and hours (hr)

Eg. It takes 17.5 min

to get to school.

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Speed (v) – used to describe how fast an object is moving.

Speed is often referred to as velocity (v).

Units for speed are measured as distance per unit of time

m/s km/hr km/s.

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Instantaneous speed (v) – how fast an object is moving at a given point in time.

Average speed (vav) – how fast an object is moving over the entire journey.

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To work out the speed of an object you need to know:

d and

t

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The units for speed depends on the units for distance and time.

distance travelled

time takenaverage speed =

d

vav t

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A boy takes 1 hour to travel from his home to the cinema, a distance of 10 km. Calculate his average speed in km/hr.

Vav =

=

= 10 km/hr

d

vav tCover the quantity to be calculated.

dt

10 km

1 h

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A boy takes 1 hour to travel from his home to the cinema, a distance of 10 km. Calculate his average speed in m/s.

Vav =

=

= 2.8 m/s

d

vav tCover the quantity to be calculated.

dt

10 000 m

3 600 s

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A family set off from home and walk at an average speed of 3.6 km/h. How far will they travel in two hours?

d = Vav x t

= 3.6 km/h x 2 h

= 7.2 km

d

vav tCover the quantity to be calculated.

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How long would it take a woman to walk 10 km, if her average speed was 5.4 km/h?

t =

=

= 1.85 h

d

vav tCover the quantity to be calculated.

dVav

10 km

5.4 km/h

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Go to website.

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This graphing experiment shows an animation of a car travelling along a straight road.

1. Copy the results table shown on the next slide and complete it as the movie is played.

2. Record the distance the car has travelled every five seconds.

3. Plot a graph of your results.

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Time/seconds Distance/metres

0

5

10

15

20

25

30

35

40

45

50

55

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Time/seconds Distance/metres

0 0

5 16

10 76

15 186

20 234

25 484

30 634

35 784

40 904

45 974

50 994

55 994

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0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Dis

tance

( m

etre

s)

Time (seconds)

Distance / Time graph for car

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0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Dis

tance

( m

etre

s)

Time (seconds)

Distance / Time graph for car

The car is going fast but at a constant speed.The graph is straight in this part of the journey.

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The speed of the car can be calculated by looking at the slope (gradient) of the distance/time graph.

Speed is “distance travelled” divided by “time taken”.

These values can be read off the distance/time graph at different points, and this is the same as the gradient of the graph.

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0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Dis

tance

( m

etre

s)

Time (seconds)

Distance / Time graph for car

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Acceleration (a) – shows the change in speed over a period of time.

Acceleration can be both positive (acceleration) and negative (deceleration).

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The units for acceleration depends on the units for speed and time.

final speed - initial speed

time takenacceleration =

∆V

a ∆t

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A car speeds up from a stop light to a speed of 15.3 m/s in just 4 seconds. Calculate the acceleration of the car.

a =

=

= 3.8 m/s/s or m/s2

∆V

a ∆tCover the quantity to be calculated.

∆V∆ t

15.3 m/s

4 s

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As she climbs a hill, a cyclist slows down from 7 m/s to 3 m/s in 10 seconds. What is her acceleration?.

a =

=

= - 0.4 m/s2

∆V

a ∆tCover the quantity to be calculated.

∆V∆ t- 4 m/s

10 s

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While traveling along a highway a truck goes from 100 km/hr to 60 km/hr in 8 seconds. What is the truck’s acceleration?.

a =

=

= - 1.4 m/s2

∆V

a ∆tCover the quantity to be calculated.

∆V∆ t

8 s

- 40 km/h- 11.1 m/s

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The slope of a speed-time graph gives an objects acceleration.

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The area under the graph gives us the distance travelled

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