Forces Revision
Syllabus
Motion
Forces
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Syllabus: Movement and position
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use the following units: kilogram (kg), metre (m), metre/second
(m/s), metre/second2 (m/s2), newton (N), second (s) (P1.1).
understand and use distance–time graphs (P1.2)
recall and use the relationship between average speed, distance
moved and time
average speed = distance moved / time taken (P1.3)
recall and use the relationship between acceleration, velocity and
time
acceleration = change in velocity / time taken
a = (v-u)/t
(P1.4)
interpret velocity–time graphs (P1.5)
determine acceleration from the gradient of a velocity–time graph
and the distance travelled from the area between the graph and
the time axis (P1.6).
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Syllabus Double Award
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express a force as a push or pull of one body on another
identify various types of force (e.g. gravitational, electrostatic, etc.)
understand that friction is a force that opposes motion
recall and use the relationship between unbalanced force, mass and
acceleration:
force = mass × acceleration
F=m×a
recall and use the relationship between weight, mass and g :
weight = mass × g
W=m×g
describe the forces acting on falling objects and explain why falling
objects reach a terminal velocity
describe the factors affecting vehicle stopping distance including speed,
mass, road condition and reaction time
recall and use the relationship between the moment of a force and its
distance from the pivot:
moment = force × perpendicular distance from pivot
recall that the weight of a body acts through its centre of gravity
describe how extension varies with applied force for helical springs, metal
wires and rubber bands
recall that the initial linear region of a force – extension graph is
associated with Hooke’s law
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Syllabus Separate Science
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express a force as a push or pull of one body on another identify various types of force (e.g.
gravitational, electrostatic etc)
distinguish between vector and scalar quantities
appreciate the vector nature of a force
add forces that act along a line
understand that friction is a force that opposes motion
recall and use the relationship between unbalanced force, mass and acceleration
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force = mass × acceleration
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F=m×a
recall and use the relationship between weight, mass and g :
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weight = mass × g
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W=m×g
describe the forces acting on falling objects and explain why falling objects reach a terminal velocity
describe the factors affecting vehicle stopping distance including speed, mass, road condition and
reaction time
recall and use the relationship between the moment of a force and its distance from the pivot:
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moment = force × perpendicular distance from pivot
recall that the weight of a body acts through its centre of gravity
recall and use the principle of moments for a simple system of parallel forces acting in one plane
understand that the upward forces on a light beam supported at its ends vary with the position of a
heavy object placed on the beam
describe how extension varies with applied force for helical springs, metal wires and rubber bands
recall that the initial linear region of a force - extension graph is associated with Hooke’s law
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Motion
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Speed vs. Velocity
Speed is simply how fast you are travelling…
This car is travelling at a speed
of 20m/s
Velocity is “speed in a given direction”…
This car is travelling at a velocity
of 20m/s east
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Speed
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Definition:
 The rate of change of distance
Equation:
 speed = distance
time
Units:
 m/s
Symbol:
 v (final) or u (initial)
Mnemonic:
 Xylophones are Very Terrible
x
v
t
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Acceleration
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24/03/2016
Definition:
 How quickly velocity changes
Equation:
acceleration = (final velocity – initial velocity) ÷ time
a = (v-u)
t
Symbol: a
Units: m/s2
Mnemonic:
 A Changing Vulture
undergoes Therapy
v-u
a
t
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Distance Time Graphs
x
x
t
x
x
t
t
t
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a stationary object
An object with a fast constant velocity
An object with a slow constant velocity
an object which is accelerating
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The gradient of a distance time graph is its velocity
ie distance divided by time
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What does the graph show?
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Velocity Time Graphs
v
v
t
v
v
t
t
t
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An object with a constant velocity
An object with a fast constant acceleration
An object with a slow constant acceleration
an object which has a changing acceleration
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The gradient of a velocity time graph is its acceleration
ie velocity divided by time
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What does the graph show?
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Calculating acceleration from graphs
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Calculating Distance Travelled
Sketch the graph in your books and write the answers
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Definition
Symbo
l
Unit
How far an object has moved
v
(m/s)
B
A
How fast an object is travelling
initially
Equation
Xylophones are Very Terrible
A Changing Vulture undergoes Therapy
Distance Time Graphs
Gradient = _____________
B
1
0
5
Descriptio
n
A
C
A
Speed
v
(m/s)
1
0
0
5
1
0
1
5
1
0
D
5
D
0
5
Speed
1
0
1
5
5
v
(m/s)
1
0
5
F
1
0
1
5
Worksheet
Distance Travelled =
a
H
I
H
0
t (s)
Description
I
K
K
0
t (s)
a
G
E
0
Description
G
0
Descriptio
n
E
F
0
t (s) C
x(m)
t (s)
Velocity Time Graphs
Gradient = ___________, Area under = ____________
5
B
0
F
The graph is of a sky diver, describe what is happening:
____________________________________________
____________________________________________
____________________________________________
____________________________________________
____________________________________________
____________________________________________
a
x (m)
D
E
final velocity
Mnemonic
C
5
1
0
1
5
t (s)
Distance Travelled =
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Definition
Symbol
Unit
distance
How far an object has moved
x
m
initial velocity
How fast an object is travelling
initially
u
m/s
How fast an object is travelling
finally
v
m/s
a
m/s2
final velocity
acceleration
Change in velocity
Mnemonic
Equation
Xylophones are Very Terrible
x=vt
A Changing Vulture undergoes Therapy
a = (v-u)/t
Distance Time Graphs
Gradient = Velocity
x (m)
B
1
0
5
0
0
5
1
0
1
5
1
0
5
0
2 m/s
B
Stationary
0 m/s
t (s) C
Coming
Back
Constant v
-2 m/s
0
Descriptio
n
Speed
D
Constant v
0.5 m/s
v
(m/s)
1
0
E
Faster
constant v
1 m/s
0
5
1
0
1
5
t (s)
F
t (s)
Velocity Time Graphs
Gradient = acceleration, Area under = distance
v
(m/s)
1
0
Constant v
D
D
The graph is of a sky diver, describe what is happening:
At A he is accelerating due to his weight, as his speed
increases the air resistance increases until at B they
balance and he reaches a constant velocity called the
terminal velocity. At C he opens his parachute and the air
resistance increases causing him to decelerate at D. At E
the forces are balanced again and he falls at a new lower
terminal velocity until at F he lands
A
E
C
E
Speed
x(m)
B
A
Descriptio
n
C
A
v
(m/s)
F
G
5
0
5
5
1
0
I
H
1
5
K
0
0
5
t (s)
1
0
1
5
t (s)
Description
a
F
Constant v
0 m/s2
G
Decelerating
-2 m/s2
Distance Travelled = 125 m
Description
a
H
Constant a
1 m/s2
I
Stationary
0 m/s2
K
Constant a
1 m/s2
Distance Travelled = 75 m
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Exam Question
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Forces
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What is a Force?
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Drawing Vectors
Small Force
Big Force
Going Right
Going Right
Tiny Force
Huge Force
Going Left
Going Down
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Cardsort: Types of Forces
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Upthrust:
• upwards force on a body caused by the water being displaced around
it. This is sometimes called buoyancy.
Lift:
• upwards force on an aeroplane.
Thrust:
• forwards force created by a body.
Weight:
• Force due to gravitational attraction.
Friction:
• Force caused by rubbing of surfaces that opposes motion.
Air/Water Resistance:
• Frictional Force caused by particles colliding with the body that
opposes motion.
Magnetic:
• Force caused by magnetic attraction.
Electrostatic:
• Force caused by attraction between charges.
Reaction:
• Force created by a body as a reaction to a force being applied.
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What are the Balanced Forces?
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Newton’s First Law
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Definition:
An object will remain at rest or at a
constant velocity in a straight line
unless there is a resultant force acting
on it.
Isaac Newton
1643-1727
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Calculating resultant forces
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Newton’s Second Law
If
the forces acting on an object are
• Definition:
unbalanced
then
theacting
objecton
will
• The net
force
a body is
accelerate,
like
these wrestlers:
directly
proportional
to the mass x
the acceleration of the body and
acts in the same direction.
• Equation:
• F =mx a
• N kg
m/s2
• Mnemonic:
• Fear my anger
Isaac Newton
1643-1727
F
m
a
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Gravitational Field Strength
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Definition
• Acceleration due to a gravitational field. Can be
measured in N/kg or m/s2.
• Is 10 N/kg on Earth.
Symbol:
• g
Equation:
• W =mx g
W
• N kg
m/s2
Mnemonic:
• Wow my goodness
m
g
• Wazzup my G
Worksheet
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Direction of Friction
• Friction always acts in the opposite direction to the way
an object is moving.
• What is the direction of friction for the following moving
objects?
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Terminal velocity of a skydiver
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Velocity–time graph of skydiver
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Stopping Distance
Thinking Distance
Braking Distance
Stopping Distance = = Thinking Distance + + Braking Distance
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Factors affecting stopping distances
Copy into your book and add:
mass of the car
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Stopping distances
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Moments
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a moment is a turning force about a point.
moment about a point = force x distance
If an object is balanced the sum of the moments on it must add up to
zero.
 Moments clockwise = moments anticlockwise
“Give me a place to stand
and I will move the Earth”
Archimedes
(287 – 212 BC)
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Principle of Moments
Separate Science
Drag and drop any of the masses onto the “see – saw” and try to get it to
balance. The masses are in kilograms and the distance in metres.
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Centre of Gravity
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The weight of an object acts through the centre of
gravity.
If you balance an object the moments on it are zero.
Therefore the centre of gravity must be at the pivot
point when balanced
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Extension of a metal
Load (N)
Plastic region
Elastic limit
Load  extension
Extension
(cm)
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Force
Description
upwards force on a body caused by the water being displaced around it..
upwards force on an aeroplane.
forwards force created by a body.
Force due to gravitational attraction
Force caused by rubbing of surfaces that opposes motion
Frictional Force caused by particles colliding with the body that opposes
motion
Force caused by magnetic attraction.
• Definition:
• A force is a
_____, _____ or
_____ that can
cause a change in
motion, shape, or
direction.
• Symbol:
• F (_______)
• W (_______)
• Units:
• N (_______)
Force caused by attraction between charges
Force created by a body as a reaction to a force being applied.
Thinking Distance
Braking Distance
Mnemonic
Equation
Units
Fear my Anger
Wow my Goodness
Moment = _______ x ________
Stopping distance = ______________ +
______________
Loa
d
(N)
______
region
______
limit
________________
________________
Worksheet
Extension
(cm)
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Force
Description
Upthrust
upwards force on a body caused by the water being displaced around it..
Lift
upwards force on an aeroplane.
Thrust
forwards force created by a body.
Weight
Force due to gravitational attraction
Friction
Force caused by rubbing of surfaces that opposes motion
Air/Water Resistance
Frictional Force caused by particles colliding with the body that opposes
motion
Magnetic
Force caused by magnetic attraction.
Electrostatic
Force caused by attraction between charges
Reaction
Force created by a body as a reaction to a force being applied.
• Definition:
• A force is a push,
pull or twist that
can cause a
change in motion,
shape, or
direction.
• Symbol:
• F (force)
• W (weight)
• Units:
• N (newton)
Mnemonic
Equation
Units
Fear my Anger
F=ma
N = kg m/s2
Tyre Conditions
Wow my Goodness
W=mg
N = kg N/kg
Alcohol
Brake Conditions
Moment = force x distance
Nm = N m
Drugs and Medicines
Road Conditions
Stopping distance = braking distance + thinking distance
m
Distractions (mobiles)
Weather Conditions
Thinking Distance
Braking Distance
Speed of Car
Speed of Car
Tiredness
Find the Resultant Force:
Mass of Car
Loa
d
(N)
Plastic
region
Elastic
limit
Load  extension
Hooke’s Law is Obeyed
Extension
(cm)
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Crossword
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