How fast? Measuring speed
What is speed
• Speed is the quantity that tells us how fast
something is moving.
• For example:
• The car’s speed was 50 km per hour when it
hit the wall.
• We give its value including the unit, written as
km/h. In science, we usually measure speed in
metres per second- m/s.
Distance and time
• How can we measure a runner's speed?
• We need to find how many metres a runner
travels in each second.
• We measure distance and time, and we calculate
• Average speed=distance travelled/time taken
Or simply
Speed=distance/time
• We have to say average speed because the
runner's speed may be changing as they run
• You can measure the distance between the two
points using a tape measure.
• You can measure the time taken using a
stopwatch.
• Task:
• 1. A car travels 100m in 5s. What is its average
speed?
• 2. A red car travels 400m in 20s, a blue car travels
660m in 30 s.
• Which car has the greater average speed?
Speed check
• Speed camera may be used to check that drivers
are not going too fast.
• A speed camera stands at the side of the road. It
takes a photograph of any car which is speeding.
1.There are two detector strips in the road- distance
2. Camera detects the car, and camera measured
the time.
And we know the speed- mini computer calculates
Light gates
• We can use light gates in laboratory.
• This is similar to speed camera, it measures
the speed of moving object.
• We have two beams , and when the trolley
breaks it, it sends the pulse of electricity to
the timer.
• Light gates are useful because they can
measure very shot time intervals- less than a
second.
Speed calculation
• Calculate the skidoo’s speed?
• IT takes about 5 minutes to travel 8 km along
the track.
• Distance travelled=8km=8000m
• Time taken=5 minutes=300s
• Speed=distance/time=8000m/300s=26.7m/s
How far?
The faster you run and the longer you run for;
the farther you will get.
Distance travelled=average speed x time taken
Example:
A bus is traveling along a road. Its speed is
25m/s. How far will it travel in one minute?
Distance travelled=25m/s x 60s=1500m
How long?
•
•
•
•
•
Time taken= distance travelled/average speed
Example:
An aircraft flies at an average speed of
250m/s.
How long will it take to fly between two
airports which are 750 km away.
• Time taken= 750000m/250m/s=3000s
Distance/ time graphs
Distance is shown on the y-axis and time on the
x- axis
1) If an object is moving at a steady speed, the
graph will be a straight line sloping upwards.
2) For two moving objects. If one graph line
slopes upwards more steeply, it is because of
grater distance in each second.
3) Graph with object which is speeding up.
Distance time graphs
• Drawing distance/ time graphs
Section of journey
Time/ s
Distance/ m
Start
0
0
A
50
500
B
150
900
C
200
900
D
250
1000
E
300
2000
• Examples
• Page 130, 131, 132, 133, 134