Transcript Document

P4 Explaining motion
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GCSE Additional Science
Ideas that:

are important from the perspective of the
subject;

enable you to pursue the subject further;

give you a sense of what the subject is like,
and how those who practice it think.
2
Explaining motion (1)
The basic principle (or commitment):

The motion of any object can be explained
by the forces acting on the object.
(So only the idea of force should be used to
explain motion.)
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Explaining motion (2)
The procedure:
1. Identify the object whose motion you are
interested in.
2. Identify all the forces acting on this object,
noting their directions.
3. Add the forces acting on the object to find
the resultant force acting on it.
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Explaining motion (3)
The rules:
1. If there is a resultant force acting on an
object, this will cause a change in its
motion, in the direction of the force.
2. If the resultant force acting on an object is
zero, its motion will not change.
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What could be easier?
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Andy kicks a football across a level pitch. It rolls to the point X and then stops.
Think about the football when it is in the middle and still moving.
stopped
moving
X
(a) The pictures below show the forces acting on the football while it is moving.
The arrows just indicate the direction of the forces, not their size.
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
Which picture best shows the forces acting on the moving football?
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KS3 responses: football
9(a)
Valid
A
C
F
G
I
J
K
M*
N
O
Total
Frequency
13
2
22
5
2
27
70
6
3
47
197
Percent
6.6
1.0
11.2
2.5
1.0
13.7
35.5
3.0
1.5
23.9
100.0
Q9(a) forces in the horizontal plane
Valid
just backwards*
both directions
no horizontal forces
just forwards
Total
Frequency
13
144
2
38
197
Percent
6.6
73.1
1.0
19.3
100.0
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KS4 responses: football
5(a)
Valid
Missing
Total
A
C
E
F
G
I
J
K
M*
N
O
Total
no response
Frequency
7
2
1
25
9
1
27
59
10
11
98
250
3
253
Percent
2.8
.8
.4
9.9
3.6
.4
10.7
23.3
4.0
4.3
38.7
98.8
1.2
100.0
Valid Percent
2.8
.8
.4
10.0
3.6
.4
10.8
23.6
4.0
4.4
39.2
100.0
Q5(a) forces in the horizontal plane
Valid
Missing
Total
just backwards*
both directions
no horizontal forces
just forwards
Total
no response
Frequency
21
184
1
44
250
3
253
Percent
8.3
72.7
.4
17.4
98.8
1.2
100.0
Valid Percent
8.4
73.6
.4
17.6
100.0
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Structure of module P4

Recognising and identifying forces acting in simple
situations
Building on knowledge that:
- If object is stationary, forces add to zero
- If forces do not add to zero, motion of object changes

Learning to describe motion more precisely

Linking force and change of momentum

Falling and rising (GPE and KE)
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P4: part 1 (lessons 1- 4)
Key ideas:

Forces arise from interactions

Always arise in pairs

Equal in size; opposite in direction

Each acts on a different object
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P4: part 1 (lessons 1- 4)

identifying forces in basic situations (pushing, pulling,
sitting, hanging)

saying clearly which object each acts on

recognising friction as an interaction
- explaining forces in walking, vehicles moving

recognise, and explain origin of, the (normal) reaction
force
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P4: part 2 (lessons 5- 6)
Key ideas:

Average speed

Instantaneous speed

Velocity (as distinct from speed)

Using graphs to represent motion
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P4: part 3 (lessons 7- 9)
Key ideas:

momentum

force x time for which it acts = change of momentum

F (Dt) = D (mv)

Application to road safety and collisions in sport

Forces in steady motion (F = 0) (the difficult case)
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P4: part 4 (lesson 11)
Key idea:

Steady motion requires no (resultant) force
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P4: part 5 (lessons 11-12)
Key ideas:

work = force x distance (in direction of force)

work done = energy transferred

Falling and rising: loss of GPE equals gain
in KE (and vice versa)
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ICT support

Video clips (of explosions, recoil, etc.)

Powerpoints (for introducing difficult ideas)

‘Identifying forces’ animated task

Powerpoint: ‘Walking graphs’

Multimedia Motion 2

Video: Forces in cycling
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Practical work

Exploring the origins of reaction forces

Explosions: speeds of the parts

Exploring F Dt = D(mv)

Measuring work in different situations

Speed of falling object after different distances

(optional) Analysis of motion using video
recordings
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Whole class discussion

Very important in several lessons

Need to plan these carefully to get maximum
benefit

Well supported by ICT resources, and activity
sheets

Consider using written exercises for smallgroup discussion rather than as individual
tasks
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box not moving
This workman is pulling a
heavy box to try and move
it along.
But the box is not moving.
Represent forces:
In the boxes below, you are asked to
mark the forces acting on each object
involved in this situation.
 by drawing arrows
to show the direction
of each force,
 with the length of the arrow
representing the size of the force.
Label each force to indicate what it is.
(a) On this diagram, mark all the horizontal forces acting on the box.
(Ignore any vertical forces.)
(b) On this diagram, mark all the horizontal forces acting on the workman.
(Ignore any vertical forces.)
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