KS3 Physics
9K Speeding Up
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Contents
9K Speeding Up
Distance, time and speed
Balanced and unbalanced forces
Friction
Summary activities
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Distance, time and speed
To work out the speed of an object you need to know:
 the distance travelled;
 how long it took to travel that distance.
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Calculating average speed
Average speed is calculated using this equation:
average speed =
total distance
total time
d
s x t
formula triangle
Speed can be measured in different units,
e.g. m/s, km/h, km/s, miles per hour.
The units of distance and time used will give the units
to be used for speed.
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Speed formula triangle
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Speed calculation example
A boy takes 1 hour to travel from his home to the cinema,
a distance of 10 km. Calculate his average speed in km/h.
d
d (distance in km)
average speed
=
(in km/h)
t (time in h)
=
s x t
10 km
1h
= 10 km/h
Cover the quantity to be
calculated - s (speed)
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Speed calculation example – units check
Sometimes the units have to be changed in a speed
calculation. Here is the same problem but with different units:
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.
d
d (distance in m)
average speed
=
(in m/s)
t (time in s)
s x t
Cover the quantity to be
calculated - s (speed)
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=
10,000 m
3600 s
1x60x60
= 2.8 m/s
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Speed calculation – question 1
Question 1
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?
Give your answer in km.
distance (km) = speed (km/h) x time (h)
d
s x t
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= 3.6 km/h x 2 h
= 7.2 km
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Speed calculation – question 1
Question 2
How long would it take a woman to walk 10 km,
if her average speed was 5.4 km/h?
time =
d
distance
speed
10 km
=
5.4 km/h
s x t
= 1.85 hours
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Car graphing activity – instructions
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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Car graphing activity – results table layout
Time/seconds
Distance/metres
0
5
10
15
20
25
30
35
40
45
50
55
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Car graphing activity – animation
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Car graphing activity – results
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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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Car graphing activity – results graph
1200
Distance / Time graph for car
Distance / metres
1000
800
600
400
200
Time / seconds
0
0
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5
10
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Car graphing activity – results graph analysis
1200
Distance / Time graph for car
Distance / metres
1000
800
600
400
200
Time / seconds
0
0
5
10
15
20
25
30
35
40
45
50
55
The
The
has
car
of stopped.
the
is going
cartoismove.
fast
The
changing
but
graph
at –ais
the
constant
flatgraph
–shows
the is
speed.
distance
not
Thespeed
car
is
starting
The
curve
thatflat.
The
of the
slope
The
car of
from
graph
the
graph
is start
straight
isThe
point
less
incurve
steep
this
is not
part
as
of
thethe
carjourney.
begins
the
speed
is the
changing.
ischanging.
upwards
as
the
to
The
slow
graph
down.
is straight
thereofisthe
no journey.
change in speed.
car
accelerates
at the– start
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Gradient of a distance/time graph
The speed of the car can be calculated by looking at the
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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Gradient of a distance/time graph
Consider the gradient of this graph at the point shown by
the two arrows in a triangle:
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Distance / Time graph for car
Distance / metres
1000
800
600
400
200
Time / seconds
0
0
5
10
15
20
25
30
35
40
45
50
55
The car has travelled from 200m to 800m = 600m.
It took from 16s to 36s to travel this distance = 20s.
So the speed at this point = 600m/20s = 30m/s.
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Speed experiment – instructions
1. Time how long it takes you to run 100m.
2. Then calculate your average speed for the run.
average speed =
total distance
total time
3. Repeat the experiment for each member of your group.
4. What was the fastest average speed for your group?
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Speed experiment – results
Name
distance (m)
time (s)
average
speed (m/s)
100
100
100
100
100
Conclusion
The fastest member of the group with an average speed
of ________ was __________.
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Speeding up vs. slowing down
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Contents
9K Speeding Up
Distance, time and speed
Balanced and unbalanced forces
Friction
Summary activities
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What is a force?
A force is a push, pull or twist. A force cannot be seen but
you can see how a force affects an object.
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Balanced and unbalanced forces
Think of a car travelling at a constant speed of 50 mph.
The engine provides sufficient force to just overcome all
the frictional forces that are acting to decrease the speed.
50 mph
500 N
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500 N
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Balanced and unbalanced forces
A crosswind acting on the car produces a sideways force.
50 mph
crosswind
The crosswind causes the direction of the car to change
– this happens because the sideways forces on the car
are not balanced.
If the car turns right so that the wind is now behind the
car, what will happen to the speed?
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Balanced and unbalanced forces
The air resistance will decrease because the car has a “tail
wind” (it is being blown from behind). This means the forces
acting on the car are no longer balanced.
>50 mph
500 N
400 N
The car will increase in speed (accelerate) until the forces are
balanced again.
60 mph
500 N
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500 N
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Balanced and unbalanced forces – summary
If the forces on an object are balanced:
 If the object is stationary, it will remain stationary.
 If the object is moving, it will continue to move
at the same speed and in a straight line.
In other words, the object will continue to do what it is
already doing without any change.
If the forces are unbalanced, two things can happen:
 The speed can change. This is called acceleration.
 The direction of motion can change.
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Resultant force
The sum effect of more than one force is called the
resultant force.
The resultant force is calculated by working out the
difference between opposing forces.
A resultant force of 100 N
is accelerating the car.
100 N
500 N
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400 N
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Resultant force – question 1
1. What is the resultant force on the block?
5N 5N
Resultant force = 20N –10N
= 10N down
The block will accelerate
downwards.
20N
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10N
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Resultant force – question 2
2. What is the resultant force on the block?
5N
5N
5N
Resultant force = 5N – 0N
= 5N right
5N
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The vertical forces are equal in size
and opposite in direction so there is no
resultant force in the vertical direction.
The block will accelerate to the right.
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Resultant forces – question 3
3. What is the resultant force on the block?
3N
7N
10N
17N
20N
13N
Resultant force = (20N +10N) – 13N
= 17N right
10N
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The vertical forces are equal in size and
opposite in direction so there is no
resultant force in the vertical direction.
The block will accelerate to the right.
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Resultant force activity
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Opposite forces
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Contents
9K Speeding Up
Distance, time and speed
Balanced and unbalanced forces
Friction
Summary activities
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Friction
Friction always tries to slow moving objects down
– it opposes motion.
Friction is created whenever two touching objects or
surfaces move past each other.
Friction also occurs when things move through air.
This is called air resistance or drag.
NOTE:
The size of the frictional force equals the applied force
unless the applied force is bigger than the maximum
value of the frictional force. If this is the case then the
frictional force remains at the maximum possible value.
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Sources of friction
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Air resistance and drag
Air resistance is a type of friction caused when objects
move through the air.
Cars are designed so that they are streamlined. The flow of
air around the body is made as smooth as possible so that
air resistance is minimized.
Air resistance depends on:
 the size of the car;
 the shape of the car;
400 N
 the speed of the car.
300 N
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Other sources of friction in cars
One of the most important
sources of friction in cars
is that between the tyres
and the road.
When the car brakes, the maximum possible amount of
friction is desirable so that the car does not skid.
The friction between the tyres and road is affected by the:
 inflation pressure of the tyres;
 road surface;
 surface condition caused by the weather (rain, ice, etc).
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Effects of frictional forces
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Contents
9K Speeding Up
Distance, time and speed
Balanced and unbalanced forces
Friction
Summary activities
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Glossary
 acceleration – A change in speed.
 air resistance – A frictional force that acts against an object
moving through air.
 balanced forces – Forces acting on an object that do





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not change its speed or direction.
drag – A frictional force, such as air resistance or water
resistance, which slows down a moving object.
friction – A force that occurs when two things rub against
each other and so slows down a moving object.
speed – How quickly an object is moving. It equals the
distance moved divided by the time taken, often measured in
‘metres per second’ (m/s).
streamlined – A smooth shape which reduces drag.
unbalanced forces – Forces acting on an object that
change its speed or direction.
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Match the descriptions
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Anagrams
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Multiple-choice quiz
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