Transcript Forces of Transport

Distance-time graphs
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2) Horizontal line =
40
4) Diagonal line
downwards =
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
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40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
100
1) What is the speed during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the speed during the last 40 seconds?
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4) When was the object travelling the fastest?
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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
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80
100
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Acceleration
V-U
Acceleration = change in speed (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?
2) A ball is dropped and accelerates downwards at a rate of
10m/s2 for 12 seconds. How much will the ball’s speed
increase by?
3) A car accelerates from 10 to 20m/s with an acceleration of
2m/s2. How long did this take?
4) A rocket accelerates© George
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1,000m/s
to 5,000m/s in 2
School 2010
seconds. What is its acceleration?
Speed-time graphs
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1) Upwards line =
80
Velocity
m/s
4) Downward line =
60
40
20
0
10
2) Horizontal line =
20
30
40
50
3) Upwards line =
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T/s
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
1) How fast was the object going after 10 seconds?
2) What is the acceleration from 20 to 30 seconds?
3) What was the deceleration from 30 to 50s?
4) How far did the object
travel
altogether?
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School 2010
50
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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
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80
100
Introduction to Forces
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A force is a “push” or a “pull”. Some common examples:
Weight (mg) – pulls
things towards the
centre of the Earth
Air resistance/drag – a contact
force that acts against anything
moving through air or liquid
_____ – a contact force
that acts against anything
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Upthrust
– keeps things afloat
moving
Balanced and unbalanced forces
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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.
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Weight
Balanced and unbalanced forces
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Reaction
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Weight
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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 same applies to a body falling through a liquid (called
“drag” or “upthrust”).
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Examples of Air Resistance
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Balanced and unbalanced forces
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Balanced and unbalanced forces
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1) This animal is either
________ or moving
with _____ _____…
3) This animal is getting
_______….
2) This animal is getting
_________…
4) This animal is…
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Force and acceleration
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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
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1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate
by?
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M
A
Stopping a car…
Tiredness
Thinking
distance
Too many
drugs
(reaction time)
Too much
alcohol
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
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Braking
distance
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Driving too
fast
Car Safety Features
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Work done
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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
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D
Example questions
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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?
2. Alicia lifts a laptop 2m into the air with a force of 10N.
How much work does she do?
3. Martin does 200J of work by pushing a wheelbarrow with a
force of 50N. How far did he push it?
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?
5. Carl drives his car 1000m. If the engine was producing a
driving force of 2000N
how
much
work did the car do?
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Work and Power
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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
W = Work done (in joules)
P = Power (in watts)
T = Time (in seconds)
P
T
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Some example questions
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1) What is the power rating of a light bulb that transfers 120
joules of energy in 2 seconds?
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
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?
4) How much energy does a 150W light bulb transfer in a) one
second, b) one minute?
5) Simon’s brain needs energy supplied to it at a rate of 40W.
How much energy does it need during a physics lesson?
6) Ollie’s brain, being more intelligent, only needs energy at a
rate of about 20W. How much energy would his brain use
in a normal day?
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Kinetic energy
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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
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Example questions
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1) Nicole drives her car at a speed of 30m/s. If the
combined mass of her and the car is 1000kg what is her
kinetic energy?
2) Shanie rides her bike at a speed of 10m/s. If the
combined mass of Shanie and her bike is 80kg what is her
kinetic energy?
3) Dan is running and has a kinetic energy of 750J. If his
mass is 60kg how fast is he running?
4) George is walking to town. If he has a kinetic energy of
150J and he’s walking at a pace of 2m/s what is his mass?
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Terminal Speed
Consider a skydiver:
1) At the start of his jump the air
resistance is _______ so he
_______ downwards.
2) As his speed increases his air
resistance will _______
3) Eventually the air resistance will be
big enough to _______ the
skydiver’s weight. At this point
the forces are balanced so his
speed becomes ________ - this is
called TERMINAL SPEED
Words – increase, small,
constant, balance, accelerates
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Terminal Speed
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
causing him to start _____ ____.
5) Because he is slowing down his air
resistance will _______ again until
it balances his _________. The
skydiver has now reached a new,
lower ________ _______.
Words – slowing down, decrease,
increases, terminal speed,
weight
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Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
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Diver hits the ground
Gravitational Potential Energy
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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)
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(metres)
Some example questions…
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How much gravitational potential energy have the following objects
gained?:
1.
A brick that has a mass of 1kg 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.
How much GPE have the following objects lost?:
1.
A 200g football dropping out of the air after being kicked up 30m,
2. A 500gN egg falling 10m out of a bird nest,
3. A 1,000kg car falling off its 2m ramp.
4. Nathan when falling 1.5m to the ground after being hit by a van
(Nathan’s mass is around©80kg).
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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
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kinetic energy
Weight vs. Mass
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Earth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
W
Weight = Mass x Gravitational Field Strength
(in N)
(in kg)
(in N/kg)
M
g
1) What is the weight on Earth of a book with mass 2kg?
2) What is the weight on Earth of an apple with mass 100g?
3) Dave weighs 700N. What is his mass?
4) On the moon the gravitational
© George field
Spencer strength
School 2010 is 1.6N/kg. What will
Dave weigh if he stands on the moon?