Double Certificate GCSE Science Pack 5
Unit 2
Force and Extension
A FORCE is a push or a pull; it is measured in newtons (N)
We are most conscious of pushing forces when we have to move objects about.
The pulling force owing to gravity is always with us. We are so used to its
presence that we tend to ignore it or take it for granted. It is often only when
we are climbing up a hill or a long flight of stairs that we really notice its effect.
We pay for our food by its weight – measured, on a spring, by the pulling force
of gravity.
N
0
1
2
3
4
5
6
7
8
9
10
When the spring in the weighing machine above is extended by a mass in the
correct range its:
Extension is directly proportional to load.
In this situation, the spring is said to obey Hooke’s Law.
Demonstration of Hooke’s Law
Extension
(mm)
60
50
40
30
20
10
0
E
¥
¥
¥
¥
¥
¥
0
1
Graph showing
how a loaded
spring obeys
Hooke’s Law
until its elastic
limit (E) is
exceeded
2
3
4
5
Load (N)
6 (weight or stretching force)
Notice, that in the graph above, the direct proportion between weight and extension stops at E,
where the weight is 4 newtons. At this point the spring has reached the limit of its elasticity. Any
further extension will mean that the spring has been stretched beyond the point where it will return
to its original length. This point is called the elastic limit of the spring. Above 4 newtons load, the
spring will no longer obey Hooke’s Law and will not be accurate or useful for measuring loads. It
will also be permanently stretched.
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5
Double Certificate GCSE Science Pack 5
Unit 2
4
Questions
Tick the box next to the correct answer.
1
What happens when a load is applied to
stretch a steel spring?
■
A The extension increases when
the load is increased
B The extension increases when
the load is reduced
C The extension is equal to the
load
D The extension stays the same
when the load changes
2
■
A
■
■
A steel cable was tested for its ability to be
stretched by adding weights to the cable;
the extension of the cable was measured
after each weight was added and a graph
made of the results, as shown.
A
B
C
D
P
x
Extension
5
Force
What happened to the cable at point P?
A
B
C
D
3
Diagram A shows part of a stretched
spring carrying a weight holder of weight
1 N and a weight also of 1 N. Diagram B
shows the same spring and weight holder,
but another 1 N weight has been added.
What will be the reading on the scale if
three more 1 N weights are added?
It exceeded its elastic limit
It started stretching at this point
It started to behave elastically
It suddenly became much harder
to stretch
■
■
■
■
22.4 cm
23.6 cm
24.8 cm
33.2 cm
11 cm
B
11 cm
12
12
13
13
14
14
15
15
16
16
17
17
18
18
19
19
20
20
21
21
22
22
23
23
■
■
■
■
If a force of 12 N extends a spring by
8 mm, what is the force needed to extend
the spring by only 2 mm?
A 3N
B 10 N
C 20 N
D 96 N
■
■
■
■
An experiment which measured the
stretch of a spring with different loads
resulted in the data shown below. When
the data were plotted, all points appeared
to be on a straight line except one. Which
one is out of line?
A
B
C
D
Load (N)
Stretch (cm)
100
200
400
500
3.6
7.2
10.8
18.0
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■
■
■
6
Double Certificate GCSE Science Pack 5
Unit 14
Reflection of Light
;
;
;
;
;
;
;
;; ;
;;
;
;
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Light always travels in a straight line. We can make light change
its direction of travel in either of two ways: by reflection or by
refraction (see Unit 15).
Rough surface –
diffuse reflection
Light falling on most objects is reflected in all directions
because the surface is not smooth. We call this kind of reflected
light diffuse reflection. If the object is very smooth, like a
polished metal surface, we get regular reflection.
Smooth surface –
regular reflection
The Laws of Reflection say that:
Incident ray
1
The angle of incidence is equal to the angle of reflection.
2
The incident ray, the reflected ray and the normal all lie in the
same plane.
Normal
i
r
Reflected ray
A reflected light ray has the same velocity as the incident ray and follows the same law: v = f x λ .
The image you see in a mirror is called a “virtual”
(or imaginary) image. This is because it appears to
be behind the mirror and it cannot be projected onto
a screen. The virtual image appears:
Object
1
the same size as the object
2
laterally inverted (see below) and
3
as far behind the mirror as the object is in front
of the mirror.
Image
Plane mirror
When we see ourselves in a mirror, the image we see appears to be reversed. This is called ‘lateral
(side-to-side) reversal’. That is why the word ‘Ambulance’ is written back-to-front on the vehicle, so
that when we see it in our rearview mirror, it appears correctly and we can read it quickly and easily.
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Double Certificate GCSE Science Pack 5
Unit 14
5 Two beams, X and Y, of pure blue light
are switched on at the same time and
shone onto a screen. In the path of beam
Y is placed a tube with a vacuum, as
shown in the diagram.
Questions
Tick the box next to the correct answer.
1
Shadows have sharp edges because:
A
B
C
D
■
■
■
■
light is a transverse wave
the speed of light is so very fast
light is electromagnetic
light travels in straight lines
blue light
X
2
A flat mirror is shown reflecting light.
Which statement is correct?
a
Which beam, X or Y, reaches the screen
first?
A X
B Y
C both beams reach the screen
at the same time
D beam Y is stopped by the vacuum
b
mirror
A
B
C
D
3
4
■
■
■
■
angle a is the angle of incidence
angle b is the angle of incidence
angle a is greater than angle b
angle a is equal to angle b
6
3 x 106 m/s
3 x 108 cm/s
3 x 108 m/s
3 x 1010 m/s
■
■
■
■
Light from the letters PORT passes
through a small hole onto a screen as
shown in the diagram. How would the
word appear on the screen?
7
■
■
■
■
What is the frequency of a light beam
which has a wavelength of 10-7 mm?
A
B
C
D
A light ray reflected from a mirror has a
velocity of:
A
B
C
D
Y
vacuum
blue light
■
■
■
■
0.03 Hz
0.3 Hz
3 Hz
3 x 108 Hz
A toy car is placed on a table in front of a
mirror as shown in the diagram. If the car
is moved towards the mirror at a speed of
2 cm/s, what is the speed at which the
image moves?
2 cm/s
mirror
T
P OR
Screen
PORT
PORT
TROP
PORT
A
B
C
D
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■
■
■
A
B
C
D
1 cm/s
2 cm/s
3 cm/s
4 cm/s
■
■
■
■
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