Transcript document

Motion
Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Seb walks 200 metres in 40 seconds. What is his speed?
2) Lucy covers 2km in 1,000 seconds. What is her speed?
3) How long would it take Freddie to run 100 metres if he runs
at 10m/s?
4) Farley travels at 50m/s for 20s. How far does he go?
5) Hannah drives her car at 85mph (about 40m/s). How long
does it take her to drive 20km?
Vector vs Scalar
• A quantity with a magnitude only is called a
scalar. Eg distance, speed, mass
• A quantity with a magnitude and a direction
is called a vector. Eg velocity, weight,
acceleration
• What would the following quantities be?
Force, Time, Momentum, Energy
Displacement
• Displacement is defined as the
change in position of a body.
• s = final position – initial position
• “As the crow flies”
• It is a vector quantity with a
magnitude (m) and a direction
Displacement-time graphs
2) Horizontal line =
40
4) Diagonal line
downwards =
30
Displacement
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
40
30
Displacement
(metres)
20
10
0
Time/s
20
40
60
80
100
1) What is the velocity during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the velocity during the last 40 seconds?
4) When was the object travelling the fastest?
Speed vs. Velocity
Speed is simply how fast you are travelling…
This car is travelling at a
speed of 20m/s
Scalar
Velocity is “speed in a given direction”…
s
v
t
This car is travelling at a
velocity of 20m/s east
Vector
Mohit “The Jogger” Punjabi
Mohit runs 100m north in a time of 20 s,
then turns and walks 50m south to go
home and study in a further 25 s.
a) Calculate Mohit’s average speed whilst running
b) What is his average velocity whilst running?
c) What is his average speed for the 150m?
d) What is his average velocity for the whole activity?
e) What is Mohit’s average velocity in kmh-1?
Answers
a)
b)
c)
d)
e)
5.0 ms-1
5.0 ms-1 north
3.3 ms-1
1.1 ms-1 north
4.0 kmh-1
Murdoch College “Olympics” record
holders
Calculate the following record breaking speeds in ms-1
Event
Record
Holder
Distance
Time
Dash to the asian
food van
Peter Lui
160 m
16 secs
Fastest hidden pencil
case
Bryce Coultham
80 cm
1.2 s
Fastest walk to the
soccer fields
Jae Sung
1.3 km
12 mins
Fastest leap to the
front desk
Jacky
6m
2.2 s
Most dribble written
in a test
Oliver
8.4 m of writing
55 mins
James M
12 km
1 hr
Most distance
covered in any one
Physics class
Average
Speed (ms-1)
Murdoch College “Olympics” record
holders
Calculate the following record breaking speeds in ms-1
Event
Record
Holder
Distance
Time
Average
Speed (ms-1)
100m dash to the
asian food van
Winson Wu
160 m
16 secs
10
Fastest head slapped
by female
Nick Mathwin
80 cm
1.2 s
0.667
Fastest walk to
soccer academy
Robert Varodi
1.3 km
12 mins
1.8
Fastest missed
penalty in a final
Milan
6m
2.2 s
2.72
Most dribble written
in a test
Milan
8.4 m of writing
55 mins
2.5 x 10-3
Milan
12 km
1 hr
3.33
Most distance
covered in any one
Physics class
Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
a
t
1) Clarissa runs away from Oliver and accelerates from 0 to
10m/s in 5 seconds. What is her acceleration?
2) A ball is dropped by Laurence on Ampere’s head from the
window and accelerates downwards at a rate of 10m/s2 for
4 seconds. What velocity will the ball hit Ampere at?
3) Mohit’s scooter accelerates from 10 to 20m/s with an
acceleration of 2m/s2. How long did this take?
4) Zhilin joins NASA and travels on a rocket which
accelerates from 1,000m/s to 5,000m/s in 20 seconds.
What is his acceleration?
Velocity-time graphs
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 =
T/s
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?
50
The 4 Acceleration equations
1.
2.
3.
4.
V = u + at
s = t( u + v/2)
s = ut + ½ at2
v2 = u2 + 2as
[average velocity by time]
[substitute 1 into 2]
[square both sides of 1]
Acceleration
Shaun decides he is da man and takes off from the traffic
lights at the acceleration shown below.
What is his acceleration after 1s, 2s and 3s?
Has he accelerated at constant acceleration?
0 ms-1
10 ms-1
20 ms-1
30 ms-1
t = 0s
t = 1s
t = 2s
t = 3s
Acceleration
Next traffic lights Shaun accelerates at the same rate
but does not look so cool as Nicholas in his hotted up race
car does not need to stop and races through the lights
just as they turn green traveling at 30 ms-1.
How long before Nicholas catches Shaun?
0 ms-1
10 ms-1
20 ms-1
30 ms-1
t = 0s
t = 1s
t = 2s
t = 3s
Go Sandeep
Shaun’s junk heap vs Nicholas’s Speedster
Time
Car
1s
2s
3s
4s
5s
10 m
30 m
60 m
100 m
150 m
30 m
60 m
90 m
120 m
150 m
Parachutin’ Stefan
• Stefan decides that he has had
enough of Physics and he jumps
out of a plane.
• Thankfully Rhys was looking
after him and put a parachute
on his back.
If Stefan jumps out at 1000m how long would you expect
him to take to hit the ground if their was no resistance
due to the wind?
s = ut + ½ at2
1000 = ½ 9.8 x t2
t = 14.28 s
Parachutin’ Stefan
• What Velocity will Stefan
theoretically hit at if he did
not reach terminal velocity
and forgot to open his chute?
v = u + at
v = 0 + 9.8 ms-2 x 14.28 s
v = 139.9 ms-1
Parachutin’ Stefan
•
If Stefan strikes the ground and
rebounds at 5.0 ms-1.
1. What is his change in speed?
2. What is his change in velocity?
•
Assume he was travelling at v = 140 ms-1 down.
1. Change in speed = 140 -5 = 135 ms-1
2. Change in velocity = 140 down – 5 up
= 140 + 5 = 145 ms-1 Up
Part II
Forces
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
Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
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…
Weight
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…
Balanced and unbalanced forces
1) This animal is either
________ or moving
with _____ _____…
3) This animal is getting
_______….
2) This animal is getting
_________…
4) This animal is…
Balanced and unbalanced forces
Force and acceleration
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
Force, mass and acceleration
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?
M
A
Terminal Velocity
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 VELOCITY
Terminal Velocity
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 ________ _______.
Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
Weight vs. Mass
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 field strength is 1.6N/kg. What will
Dave weigh if he stands on the moon?
Friction
1) What is friction?
2) Give 3 examples where it is annoying:
3) Give 3 examples where it is useful:
4) What effect does friction have on the surfaces?
Stopping a car…
Tiredness
Too many
drugs
Thinking
distance
Looking at
girls / boys
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
Too much
alcohol
Braking
distance
Driving too
fast
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
1. Mark 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. Annielyn 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. Joseph cuddles his cat and lifts it 1.5m in the air. If he did
75J of work how much force did he use?
5. Nicholas drives his car 1000m. If the engine was producing
a driving force of 2000N how much work did the car do?
Kinetic energy
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
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?