Transcript 1. Speed and Acceleration - Readington Township Public Schools

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What is speed?
Speed is a measure of how far an object moves in a given
time.
This car is travelling at 60
mph. This means the car
travels 60 miles every hour.
This jet is travelling at
350 m/s. This means the jet
travels 350 meters every
second.
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How is velocity different to speed?
The speed of an object does not depend on the direction in
which it is traveling. The velocity of an object is the speed
and direction in which it is moving.
The car is traveling
north with a velocity of
10 m/s.
As the car goes round
the corner, the speed
of the car remains
constant but the
velocity changes.
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How is speed calculated?
The speed of an object can be calculated using this equation:
distance traveled
speed =
time taken
 Distance travelled is measured in meters (m).
 Time taken is measured in seconds (s).
 Speed is measured in meters per second (m/s).
The standard unit for speed in physics is m/s, but other units
such as kilometers per hour (km/h) are more convenient
when measuring the speed of vehicles. Why is this?
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Words and units
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Using a formula triangle
A formula triangle helps you to rearrange a formula. The
formula triangle for speed (s), distance (d) and time (t) is
shown below.
Cover the quantity that you are trying to work out, which
gives the rearranged formula needed for the calculation.
So to find speed (s),
cover up s…


…which gives
the formula…
s =
d
t
x
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Calculating speed question
A train takes 100 seconds to travel 1,500 m.
What is the speed of the train?
distance
speed =
time
= 1,500
100
= 15 m/s
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Calculating speed question
A car travels at 25 m/s for
3 minutes. How far does
it travel?
distance
speed =
time
distance = speed x time
= 25 x 180
= 4,500 m = 4.5 km
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Speed, distance, time calculations
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Representing speed
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Analyzing distance–time graphs
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Calculating speed from the slope
It is difficult to calculate the
slope of ‘realistic’ graphs
because the line is curved.
Simple graphs use straight
lines only, making it easy to
calculate the slope.
distance
distance
The slope of the line in a
distance–time graph equals the speed.
time
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time
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What’s the speed?
distance (m)
What is the speed of the object between points A and B?
B
70
60
50
40
30
20
10
0
 the object has moved
60 m (70 - 10 )
 it took 3 s to move this
distance (6 - 3)
A
 speed = distance/time
= 60/3
0 1 2 3 4 5 6 7 8 9
time (s)
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= 20 m/s
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Calculating speed from graphs
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DIY distance–time graph
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What is acceleration?
The acceleration of an object is a measure of how quickly
its velocity changes.
A train accelerates in a
straight line from rest. As it
does, its velocity increases.
The brakes on this
motorcycle are causing it
to slow down. This is
negative acceleration or
deceleration.
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How is acceleration calculated?
The acceleration of an object can be calculated using this
equation:
change in speed
acceleration =
time taken
 Change in speed is measured in metres per second
(m/s).
 Time taken is measured in seconds (s).
 Acceleration is measured in metres per second per
second (m/s2).
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Acceleration problem
A racing car accelerates from rest to a speed of 60 m/s in a
time of 4 seconds. What is the acceleration of the car?
acceleration =
=
change in speed
time taken
60
4
= 15 m/s2
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Using a formula triangle
A formula triangle helps you to rearrange a formula. The
formula triangle for acceleration (a), speed (s) and time (t)
is shown below.
Cover the quantity that you are trying to work out, which
gives the rearranged formula needed for the calculation.
So to find acceleration
(a), cover up a…


…which gives
the formula…
a =
s
t
x
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Acceleration problem
A hungry cheetah spots a gazelle and decides to chase it.
The cheetah accelerates at 10 m/s2 from rest until it reaches
20 m/s. How long did this take?
change in speed
acceleration =
time taken
time taken =
=
change in speed
acceleration
20
10
= 2s
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Acceleration problems calculations
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Analyzing speed–time graphs
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Calculating acceleration from the gradient
How can the acceleration of an object be calculated from a
speed–time graph?
time
 If the gradient goes
up, the object has a
positive acceleration.
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speed
speed
This gradient of the line in a speed–time
graph represents acceleration
time
 If the gradient goes
down, the object has a
negative acceleration,
or deceleration.
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What’s the acceleration?
speed (m/s)
What is the acceleration of the object between points
A and B?
35
30
25
20
15
10
5
0
B
 the object’s speed has
increased by 20 m/s
(25 - 5)
 it took 4 s to change
speed (6 - 2)
A
0 1 2 3 4 5 6 7 8
time (s)
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 acceleration = speed/time
= 20/4
= 5 m/s2
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Calculating acceleration from graphs
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The area under a speed–time graph
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DIY speed–time graph
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Speed and safety
Why are speed limits important?
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Why have speed limits?
Speed limits are an important
part of road safety. They aim
to prevent drivers from driving
at speeds that are unsuitable
and unsafe.
The speed limit of a particular
road depends on a range of
factors, such as how straight or curved it is, and its location.
The faster a vehicle is driving, the longer it will take to stop –
the overall distance this takes is the stopping distance.
stopping distance = thinking distance + braking distance
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Stopping distances
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What affects thinking distance?
The thinking distance is the distance a vehicle travels in the
time it takes for a driver to react to a situation and apply the
brakes.
What factors will affect thinking distance?
alcohol
other drugs and
some medicines
distractions,
such as
mobile phones
tiredness
speed
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What affects braking distance?
The braking distance is the distance a vehicle takes to stop
once the driver has applied the brakes.
What factors will affect braking distance?
weather
condition of
tyres/brakes
condition of road
speed
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Factors affecting stopping distances
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How do speed cameras work?
There are several types of speed camera.,
They use different methods to calculate the
speed of a vehicle.
Gatso speed cameras use radar to detect the
speed of a vehicle, then take two photos (half a
second apart) to provide visual evidence. Lines
marked on the road indicate how far the
vehicle has travelled in that time.
Truvelo speed cameras are activated by pressure detector
cables in the road. The cables are 10 cm apart and a
computer calculates how long it takes the vehicle to pass
from one to another, and therefore the speed of the vehicle.
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Speeding and speed cameras
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Glossary (1/2)
 acceleration – A measure of how an object’s velocity
changes over time. It usually refers to an object that is
speeding up.
 braking distance – The distance it takes for a vehicle to
stop once its brakes have been applied.
 deceleration – Negative acceleration, i.e. slowing down.
 non-uniform – Speed or acceleration that is constantly
changing.
 speed – A measure of how far an object moves in a given
time.
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Glossary (2/2)
 stopping distance – The total distance it takes for a
vehicle to stop, i.e. thinking distance plus braking distance.
 thinking distance – The distance it takes for a driver to
react to a situation and apply the brakes.
 uniform – Speed or acceleration that is constant and
unchanging.
 velocity – A measure of the speed and direction of a
moving object.
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Anagrams
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What does the graph show?
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What does the graph show?
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Multiple-choice quiz
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