Transcript Stick it in! - MrSimonPorter

Forces and motion
Speed
metres
Speed = distance travelled
time taken
Metres per second
(m/s)
seconds
Speed
kilometres
Speed = distance travelled
time taken
Kilometres per hour
(km/h)
hours
triangle
d
s
x
t
No movement
distance
time
Constant speed
distance
fast
The gradient of the
graph gives the
speed
slow
time
Getting faster (accelerating)
distance
time
A car accelerating from stop
and then hitting a wall
Let’s try a
simulation
distance
time
Speed against time graphs
speed
time
No movement
speed
time
Constant speed
speed
fast
slow
time
Getting faster? (accelerating)
speed
Constant acceleration
time
Getting faster? (accelerating)
v
speed
The gradient of this graph
gives the acceleration
a=v–u
t
(v= final speed, u = initial speed)
u
time
Getting faster? (accelerating)
speed
The area under the graph gives the
distance travelled
time
A dog falling from a tall building
(no air resistance)
speed
Area = height of building
time
Forces
• Remember a force is a push (or pull)
Forces
• Force is measured in Newtons
Forces
• There are many types of forces;
electrostatic, magnetic, upthrust, friction,
gravitational………
Which of the following is
the odd one out?
Mass
Speed
Force
Temperature
Distance
Elephant
Scalars and vectors
Scalars
Scalar quantities have a magnitude (size)
only.
For example:
1 kg
Temperature, mass, distance, speed,
energy.
Vectors
Vector quantities have a magnitude (size)
and direction.
For example:
10 N
Force, acceleration, displacement,
velocity, momentum.
Copy
please!
Scalars and Vectors
No direction
vectors
scalars
Magnitude (size)
temperature
mass
Magnitude and direction
velocity
force
speed
acceleration
Representing vectors
Vectors can be represented by arrows.
The length of the arrow indicates the
magnitude, and the direction the direction!
Adding vectors
When adding vectors (such as force or
velocity) , it is important to remember they
are vectors and their direction needs to be
taken into account.
The result of adding two vectors is called
the resultant.
Copy please!
Adding vectors
For example;
Resultant force
6N
4N
2N
An interesting example
We have constant speed but changing
velocity.
Of course a changing
velocity means it must
be accelerating! We’ll
come back to this in
year 12!
velocity
Friction opposes motion!
Newton’s 1st Law
If there is no
resultant force
acting on an
object, it will move
with constant
velocity. (Note the
constant velocity
could be zero).
Does this make
sense?
Newton’s second law
Newton’s second law concerns examples
where there is a resultant force.
I thought of this
law myself!
Newton’s 2nd law
There is a mathematical relationship
between the resultant force and
acceleration.
Resultant force (N) = mass (kg) x acceleration (m/s2)
FR = ma
It’s physics,
there’s always a
mathematical
relationship!
An example
Resultant force = 100 – 60 = 40 N
FR = ma
40 = 100a
Mass of Mr Porter and bike = 100 kg
a = 0.4 m/s2
Pushing force (100 N)
Friction (60 N)
Newton’s 3rd law
If a body A exerts a force on body B, body B will exert an equal but
opposite force on body A.
Hand (body A) exerts force on
table (body B)
Table (body B) exerts force on
hand (body A)
Gravity
Gravity is a force between ALL objects!
Gravity
Gravity
Gravity is a very weak force.
The force of gravitational attraction between
Mr Porter and his wife (when 1 metre apart) is
only around 0.0000004 Newtons!
Gravity
The size of the force depends on the mass
of the objects. The bigger they are, the
bigger the force!
Small attractive force
Bigger attractive force
Gravity
The size of the force also depends on the
distance between the objects.
Gravity
The force of gravity on something is called
its weight. Because it is a force it is
measured in Newtons.
Weight
Gravity
On the earth, Mr Porter’s weight is around
800 N.
I love physics!
800 N
Gravity
On the moon, his weight is around 130 N.
Why?
130 N
Mass
Mass is a measure of the amount of
material an object is made of. It is
measured in kilograms.
Mass
Mr Porter has a mass of around 77 kg.
This means he is made of 77 kg of blood,
bones, hair and poo!
77kg
Mass
On the moon, Mr Porter hasn’t changed
(he’s still Mr Porter!). That means he still is
made of 77 kg of blood, bones, hair and
poo!
77kg
Mass and weight
Mass is a measure of the amount of
material an object is made of. It is
measured in kilograms.
Weight is the force of gravity on an object.
It is measured in Newtons.
Calculating weight
To calculate the weight of an object you
multiply the object’s mass by the
gravitational field strength wherever you are.
Weight (N) = mass (kg) x gravitational field strength
(N/kg)
Gravity = air resistance
Terminal velocity
Air resistance
As the dog falls faster and air resistance
increases, eventually the air resistance
becomes as big as (equal to) the force of
gravity.
The dog stops getting faster (accelerating)
and falls at constant velocity.
gravity
This velocity is called the terminal velocity.
Falling without air resistance
Can you copy the
words please?
Without air resistance objects fall
faster and faster and faster…….
Where
did I
come
from?
They get faster by 10 m/s every
second (10 m/s2)
This number is called “g”, the
acceleration due to gravity.
gravity
Falling without air resistance?
distance
time
Falling without air resistance?
speed
Gradient = acceleration = 9.8 m.s-2
time
Falling with air resistance?
distance
time
Falling with air resistance?
Terminal speed
speed
time
Stopping distances
The distance a car takes to stop is called
the stopping distance.
Two parts
The stopping distance can be thought of in
two parts
Stopping distances
Thinking distance is the distance traveled whilst
the driver is thinking (related to the driver’s
reaction time).
Thinking distance
This is affected
by the mental
state of the driver
(and the speed of
the car)
Braking distance
This is the distance traveled by the car
once the brakes have been applied.
Braking distance
This affected by the speed and mass of
the car
Braking distance
It is also affected by the road conditions
Braking distance
And by the condition of the car’s tyres.
Typical Stopping distances
YouTube - Top Gear 13-5: RWD Braking Challenge
YouTube - Think! - Slow Down (Extended) (UK)
Momentum
• What makes an object hard to stop?
• Is it harder to stop a bullet, or a truck
travelling along the highway?
• Are they both as difficult to stop as each
other?
Momentum
• Momentum is a useful quantity to
consider when thinking about
"unstoppability". It is also useful when
considering collisions and explosions. It is
defined as
Momentum (kgm/s) = Mass (kg) x Velocity (m/s)
p = mv
An easy example
•
A lorry has a mass of 10 000 kg and a
velocity of 3 m/s. What is its momentum?
Momentum = Mass x velocity
= 10 000 x 3
= 30 000 kgm/s
Law of conservation of
momentum
• The law of conservation of linear
momentum says that
“in an isolated system, momentum
remains constant”.
We can use this to calculate what happens after a collision (and in fact during an
“explosion”).
Conservation of momentum
• In a collision between two objects,
momentum is conserved (total
momentum stays the same). i.e.
Total momentum before the collision = Total momentum after
Momentum is not energy!
A harder example!
• A car of mass 1000 kg travelling at 5 m/s
hits a stationary truck of mass 2000 kg.
After the collision they stick together. What
is their joint velocity after the collision?
A harder example!
Before
2000kg
1000kg
5 m/s
Momentum before = 1000x5 + 2000x0 = 5000 kgm/s
After
Combined mass = 3000 kg
V m/s
Momentum after = 3000v
A harder example
The law of conservation of momentum tells us that
momentum before equals momentum after, so
Momentum before = momentum after
5000 = 3000v
V = 5000/3000 = 1.67 m/s
Momentum is a vector
• Momentum is a vector, so if velocities are
in opposite directions we must take this
into account in our calculations
An even harder example!
Snoopy (mass 10kg) running at 4.5
m/s jumps onto a skateboard of
mass 4 kg travelling in the opposite
direction at 7 m/s. What is the
velocity of Snoopy and skateboard
after Snoopy has jumped on?
I love
physics
Because they are in opposite directions, we make
one velocity negative
An even harder example!
10kg
-4.5 m/s
7 m/s
4kg
Momentum before = 10 x -4.5 + 4 x 7 = -45 + 28 = -17
14kg
v m/s
Momentum after = 14v
An even harder example!
Momentum before = Momentum after
-17 = 14v
V = -17/14 = -1.21 m/s
The negative sign tells us that the velocity is
from left to right (we choose this as our
“negative direction”)
Impulse
Ft = mv – mu
The quantity Ft is called the impulse, and of
course mv – mu is the change in momentum (v =
final velocity and u = initial velocity)
Impulse = Change in momentum
Units
Impulse is measured in Ns
or kgm/s
5 m/s
Impulse
-3 m/s
Note; For a ball bouncing off a wall, don’t
forget the initial and final velocity are in
different directions, so you will have to make
one of them negative.
In this case mv – mu = 5m - -3m = 8m
Example
• Jack punches Chris in the face. If Chris’s head (mass 10 kg) was
initially at rest and moves away from Jack’s fist at 3 m/s, and the fist
was in contact with the face for 0.2 seconds, what was the force of
the punch?
• m = 10kg, t = 0.2, u = 0, v = 3
• Ft = mv – mu
• 0.2F = 10x3 – 10x0
• 0.2F = 30
• F = 30/0.2 = 150N
The turning effect of a force
depends on two things;
The size of the force
Obviously!
The turning effect of a force
depends on two things;
The distance from the pivot (axis of rotation)
Not quite
so obvious!
Axis of rotation
Turning effect of a force – moment
of a force
Moment (Nm) = Force (N) x distance from pivot (m)
Note the unit is Nm, not N/m!
A simple example!
Moment = Force x distance from pivot
Moment = 50 N x 0.15 m
50 N
Moment = 7.5 Nm
0.15 m
nut
spanner
(wrench)
If the see-saw balances, the turning
effect anticlockwise must equal the
turning effect clockwise
1.2 m
2.2 m
pivot
?N
110 N
Anticlockwise moment = clockwise moment
Anticlockwise moment = clockwise moment
? X 1.2 = 110 x 2.2
? X 1.2 = 242
? = 242/1.2
? = 201.7 N
1.2 m
2.2 m
pivot
?N
110 N
Anticlockwise moment = clockwise moment
Principal of Moments
Rotational equilibrium
is when the sum of
the anticlockwise
moments equal the
sum of the clockwise
moments.
YouTube - Alan Partridge's
Apache office
COPY PLEASE!
Centre of gravity
The centre of gravity of an object is the
point where the object’s weight seems to
act.
I think he
wants you
to copy this
Complex shapes
How do you find the
centre of gravity of
complex shapes?
Complex shape man
Finding the centre of mass
i. Place a compass or needle through any part of
the card.
ii. Make sure that the card “hangs loose”.
iii. Hang a plumb line on the needle.
iv. After it has stopped moving, carefully draw a
line where the plumb line is.
v. Place the needle in any other part of the card.
vi. Repeat steps ii to iv.
vii. Where the two drawn lines cross is where the
centre of mass is.
viii. Physics is the most interesting subject.
Hooke’s law
Elastic limit
Force
(N)
The extension of a spring is
proportional to the force
applied (until the elastic limit
is reached)
Extension (cm)
Steel, glass and wood!
Force
Even though they don’t
stretch much, they obey
Hooke’s law for the first part
of the graph
Extention
Rubber
Force
Extension
The Solar System
Main points
• Know the names of the planets!
• My very easy method just speeds up naming
planets
• They orbit in ellipses with the sun at one foci
• Inner planets small and rocky
• Outer planets large and mainly gas
• Asteroid belt between Mars and Jupiter
Comets
• Giant dirty snow balls (ice and dust)
(diameter 100m - 50 km?)
• Very elliptical orbits
• Short period (T < 200 yrs) and long period
(could be thousands of years)
• Oort cloud
• Tail(s) always point away from the sun
• Evaporate as they get closer to the sun
Orbital motion
• Space objects
• use the relationship between orbital
speed, orbital radius and time period
• orbital speed = 2× π ×orbital radius/(time
period)
• v = 2× π × r
T
My address
11507 Meadow Lake Drive
Houston
Texas 77077
USA
Earth
Solar System
My address
11507 Meadow Lake Drive
Houston
Texas 77077
USA
Earth
Solar System
Milky way
Local group
Universe
Galaxies
• A large collection of stars held together by
their mutual gravity.
• Dwarf galaxies might have only a few
million stars, many galaxies have
hundreds of billions.
• The Universe has around 100 billion
galaxies
Orbital speed
• Speed = distance/time
• v = (2πr)/T
• r = radius of orbit
• T = Period (time for one orbit)