13/04/2015
OCR Additional Science
P3 Forces for Transport
AGAC
Speed vs. Velocity
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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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Distance, Speed
and Time
Speed = distance (in m)
(in m/s)
time (in s)
d
S
t
Dave walks 20 metres in 4 seconds. What is his speed?
5 m/s
Laura covers 12m in 3 seconds. What is her speed?
4 m/s
How long would it take to run 100 metres if you run at
10m/s 10 s
Steve travels at 50m/s for 20s. How far does he go?
1000 m
Susan drives her car at 40m/s. How long does it take her
to drive 200m? 5 s
Distance-time graphs
2) Horizontal line =
40
Not moving
4) Diagonal line
downwards =
Constant speed…
back to start
30
Distance
(metres)
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20
10
0
1) Diagonal line =
Constant speed
Time/s
2
4
6
8
10
3) Steeper diagonal line =
Faster constant speed
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40
Distance
(metres)
d
sxt
30
20
10
0
Time/s
2
4
6
8
What is the speed during the first 2 seconds?
10
10/2= 5m/s
40m
What is the speed during the last 4 seconds? 40/4= 10m/s
4 to 6 seconds
When was the object travelling the fastest?
How far is the object from the start after 6 seconds?
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Distance-time graph for non-uniform motion
40
Distance
(metres)
Object is
accelerating
up to here
30
Object is now
decelerating
20
10
0
Time/s
20
40
60
80
100
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Acceleration
v-u
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
a
t
1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her
acceleration? 10/5= 2m/s2
2) A ball is dropped and accelerates downwards at a rate of 10m/s2
for 12 seconds. How much will the ball’s velocity increase by?
10x12= 120m/s
1) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2.
How long did this take? 10/2= 5s
2) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds.
What is its acceleration?
4000/2= 2000m/s2
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Velocity-time graphs
1) Upwards line =
constant80
acceleration
Velocity
4) Downward line =
constant deceleration
60
m/s 40
20
0
10
20
30
40
50
2) Horizontal line =
3) Upwards line =
constant velocity
constant acceleration
T/s
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v
axt
80
½ x 10 x 20=
60
Velocity
10 x 40=
m/s 40
clue: it’s the area
under the graph!
½ x 20 x 60=
20
½ x 10x40 =
0
T/s
10
10 x 40=
20
30
40
50
1) How fast was the object going after 10 seconds?
40m/s
2) What is the acceleration from 20 to 30 seconds?
2m/s2
-3m/s2
3) What was the deceleration from 30 to 50s?
4) How far did the object travel altogether?
200m +
400m +
400m + 100m + 600m =
1700m
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Speed-time graph for non-uniform motion
40
Distance
(metres)
Object’s
acceleration
is increasing
30
Object’s
acceleration
is decreasing
20
10
0
Time/s
20
40
60
80
100
What is a force?
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A force is a “push” or a “pull”. Some common examples:
________ – pulls
things downwards
_____ – acts against
anything moving
___ ________ (drag) – acts
against anything moving through air
______ – keeps things afloat
Words – upthrust Air resistance friction weight
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Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
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Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
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Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Air Resistance
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Air resistance is a force that opposes motion through air. The
quicker you travel, the bigger the air resistance:
The bigger the arrow, the bigger the force
The same applies to a body falling through a liquid (called
“drag” or “upthrust”).
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Examples of Air Resistance
Balanced and unbalanced forces 1
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In each of these
situations the
car is not moving
before each force is
applied
car accelerates
car accelerates
car does not move
car accelerates
Balanced and unbalanced forces 2
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Stage 1
Car starting off
Stage 2…
continuing
from stage 1
(moving)
car accelerates
car still
accelerates
but not as
much as in
Stage 1
Next slide for stage 3
Balanced and unbalanced forces 2
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Stage 3…
continuing from
Stage 2 (moving)
Car moves at a
steady speed
Now see the next slide
To see how this can be
Put into a graph …..
Stage 4…
continuing from
Stage 3 (moving)
Car
decelerates
Balanced and unbalanced forces
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Velocity
constant
speed
Stage 1
Stage 2
Stage 3
Stage 4
Time
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Balanced and unbalanced forces
1) This animal is either
stationary
________ or moving
with _____
_____…
constant
speed
3) This animal is getting
slower
_______….
2) This animal is getting
faster
_________…
4) This animal is…
stationary
Force and acceleration
13/04/2015
If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
A
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Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2.
What mass has the car? 3000=
1.5
1000 = 2m/s2
500
F
2000Kg
3) A car accelerates at a rate of
5m/s2. If it has a mass of 500kg
how much driving force is the
500 x 5 =
engine applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How 10 =
much does it accelerate by? 20
M
2500N
0.5 m/s2
A
The following factors
INCREASE distances
Stopping a car…
Tiredness
Stopping distance
=
Thinking distance
Poor
visibility
+
Braking distance
Wet roads
Icy roads
Tyres/brakes
worn out
Too much
alcohol
Thinking
distance
Too many
drugs
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Braking
distance
Driving too
fast
Car Safety Features
Air bags
Rigid cage
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Seat belts
Paddle controls
Crumple zone
Work Done
Each Worker needs to
do the same amount of
work in lifting these
boxes on to the shelves
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4
3
2
A
A
B
B
C
C
1
Work Done
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4
3
Worker A
A
A
B
B
C
C
2
lifts 4 boxes on to the 1st shelf
A
A
A
A
1
Work Done
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4
3
Worker B
lifts 2 boxes on to the 2nd shelf B
A
A
B
B
C
C
A
A
A
B
A
2
1
Work Done
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Worker C
lifts 1 box on to the 4th shelf C
4
3
A
A
B
B
C
C
A
A
B
B
A
A
2
1
Work Done
No. of boxes x height
1x4=4
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C
4
“box” = weight or force
“height” = distance moved
3
No. of boxes x height
2x2=4
A
A
B
B
C
C
B
B
No. of boxes x height
4x1=4
A
A
A
A
2
1
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Work done
When any object is moved around work will need to be
done on it to get it to move (obviously).
We can work out the amount of work done in moving an
object using the formula:
Work done = Force x distance moved
in J
in N
W
in m
F
D
Example questions
13/04/2015
1. Bori pushes a book 5m along the table with a force of 5N.
He gets tired and decides to call it a day. How much work
did he do?
25J
2. Alicia lifts a laptop 2m into the air with a force of 10N.
How much work does she do?
20J
3. Martin does 200J of work by pushing a wheelbarrow with a
force of 50N. How far did he push it?
4m
4. Chris cuddles his cat and lifts it 1.5m in the air. If he did
75J of work how much force did he use?
50N
5. Carl drives his car 1000m. If the engine was producing a
driving force of 2000N how much work did the car do?
2 000 000J
Work and Power
13/04/2015
The POWER RATING of an appliance is simply how much work
it does (i.e. how much energy it transfers) every second.
In other words, 1 Watt = 1 Joule per second
W = Work done (in joules)
W
P = Power (in watts)
T = Time (in seconds)
P
T
Some example questions
13/04/2015
1) What is the power rating of a light bulb that transfers 120
joules of energy in 2 seconds?
60W
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
2000W
3) Isobel runs up the stairs in 5 seconds. If she transfers
1,000,000J of energy in this time what is his power rating?
200 000W
Kinetic energy
13/04/2015
Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using
the formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
Example questions
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KE =1/2mv2
1) Emily drives her car at a speed of 2m/s. If the combined
mass of her and the car is 1000kg what is her kinetic
energy?
KE =1/2 mv2
KE = 0.5 x 1000 x (2)2
KE = 2000 J
2) Max rides his bike at a speed of 10m/s. If the combined
mass of Max and his bike is 80kg what is his kinetic
energy?
2
KE =1/2 mv
KE = 0.5 x 80 x (10)2
KE = 4000 J
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
small
resistance is _______
so he
accelerates
_______ downwards.
2) As his speed increases his air
resistance will increase
_______
3) Eventually the air resistance will be
equal
big enough to _______
the
skydiver’s weight. At this point
the forces are balanced so his
constant - this is
speed becomes ________
called TERMINAL VELOCITY
Words – increase, small,
constant, equal, accelerates
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Terminal Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
increases
causing him to start slowing
_____ down
____.
5) Because he is slowing down his air
decrease again until
resistance will _______
weight
it balances his _________.
The
skydiver has now reached a new,
terminal _______.
velocity
lower ________
Words:
terminal velocity, weight,
decrease, increases, slowing down
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Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
No air resistance !
Time
New, lower terminal
velocity reached
Diver hits the ground
Gravitational Potential Energy
13/04/2015
To work out how much gravitational potential energy
(GPE) an object gains when it is lifted up we would use
the simple equation…
GPE = Mass x Acceleration of free-fall x Change in height
(Joules) (newtons)
(=10N/kg)
(metres)
13/04/2015
Gravitational Potential Energy
To work out how much gravitational potential energy
(GPE) an object gains when it is lifted up we would use
the simple equation…
GPE = Mass x gravitational field strength x Change in height
(Joules)
(kg)
On Earth g = 10N/kg.
This means that every
kilogram on Earth
experiences a downwards
weight of 10N
(N/kg)
(metres)
GPE
m
g
Now let us
consider
an earlier
problem….
h
Work Done
In this example the
work done
produces an
increase in potential
energy
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GPE= mxgxh
1x10x4=40J
C
3
GPE= mxgxh
2x10x2 = 40J
A
B
C
4
GPE= mxgxh
4x10x1 = 40J
A
A
B
B
A
A
Each box has a mass of
1Kg and the shelves are
1m apart
Each worker has transferred 40J into GPE for the boxes
2
1
Example questions
13/04/2015
GPE
mxgxh
How much gravitational potential energy have the following objects
gained?:
1.
A brick that has a mass of 1kg and is lifted to the top of a house (10m),
2. A 1,000kg car lifted by a ramp up to a height of 2m,
3. A 70kg person lifted up 50m by a ski lift.
1. GPE = 1x10x10= 100J
3. GPE=70x10x50= 35000J
2. GPE=1000x10x2= 20 000J
How much GPE have the following objects lost?:
4. A 0.2kg football dropping out of the air after being kicked up 30m,
5. A 50g egg falling from a nest 10m above the ground
6. A 1,000kg car falling off its 2m ramp.
4. 60J
5. 5J
6. 20 000J
Roller Coasters
1) Electrical energy is
transferred into gravitational
potential energy
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3) Kinetic energy is
transferred back
into gravitational
potential energy
2) Gravitational potential
energy is transferred into
kinetic energy