Transcript in m/s - Wildern VLE

P5 Space 4 Reflection
Revision Seminar
Centripetal force
Consider a ball of Pleistocene attached to some string:
The ball is kept in its path by the
tension in the string – an example of a
CENTRIPETAL FORCE. This force also
produces the change in velocity due to
the direction constantly changing.
This force is INCREASED if you increase the mass of the
object, its speed or decrease the radius of the circle.
Other examples
of centripetal
forces:
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Orbits
Electrons
Momentum
Any object that has both mass and
velocity has MOMENTUM. Momentum
(symbol “p”) is simply given by the formula:
P
Momentum = Mass x Velocity
(in kgm/s)
(in kg)
(in m/s)
M
What is the momentum of the following?
1) A 1kg football travelling at 10m/s
2) A 1000kg Ford Capri travelling at 30m/s
3) A 20g pen being thrown across the room at 5m/s
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4) A 70kg bungi-jumper falling at 40m/s
V
Conservation of Momentum
In any collision or explosion momentum is conserved (provided that there
are no external forces have an effect). Example question:
Two cars are racing around Teville Gate. Car A collides with the back of
car B and the cars stick together. What speed do they move at after the
collision?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Mass = 1800kg
Speed = ??m/s
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
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…V = 36.7m/s
More questions…
1) A white snooker ball moving at 5m/s strikes a red ball and pots it.
Both balls have the same mass. If the white ball continued in the same
direction at 2m/s what was the velocity of the red ball?
2) A car of mass 1000kg heading up the M1 at 50m/s collides with a
stationary truck of mass 8000kg and sticks to it. What velocity does
the wreckage move forward at?
3) A defender running away from a goalkeeper at 5m/s is hit in the back
of his head by the goal kick. The ball stops dead and the player’s speed
increases to 5.5m/s. If the ball had a mass of 500g and the player had
a mass of 70kg how fast was the ball moving?
4) A gun has a recoil speed of 2m/s when firing. If the gun has a mass of
2kg and the bullet has a mass of 10g what speed does the bullet come
out at? (Hint: total momentum before is zero and the gun has a
negative velocity afterwards)
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A past exam question…
June 2000
Two lorries are travelling in the same direction along a motorway.
Lorry A
Lorry B
Mass = 20,000kg
Mass = 30,000kg
Speed = 14m/s
Speed = 20m/s
1) Calculate the momentum of Lorry A as it travels along the motorway.
2) Calculate the momentum of Lorry B as it travels along the motorway.
(3 marks)
3) Lorry B collides with Lorry A and they stick together. Calculate the
common speed of the lorries immediately after the collision.
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(3 marks)
Newton’s Laws of Motion
These are my three laws of
motion (summarised):
1) If an unbalanced force acts on an
object that object will either accelerate
or change direction:
F
2) That force is given by F=ma
3) When a force acts on an object
there is an equal force acting in
the opposite direction (“Action and
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reaction are equal and opposite”)
M
A
Newton’s
nd
2
Law
Instead of F=ma Newton actually said that the force acting on
an object is that object’s rate of change of momentum. In
other words…
mv
Force = Change in momentum (in kgm/s)
(in N)
Time (in s)
F
T
For example, David Beckham takes a free kick by kicking a stationary
football with a force of 40N. If the ball has a mass of 0.5kg and his
foot is in contact with the ball for 0.1s calculate:
1) The change in momentum of the ball,
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2) The speed the ball moves away with
Example questions
1) A golfer strikes a golf ball with a force of 80N. If the ball
has a mass of 200g and the club is in contact with it for
0.2s calculate a) the change in momentum of the golf ball,
b) its speed.
2) A tennis player strikes a serve with a force of 30N. If the
ball has a mass of 250g and the racket is in contact with it
for 0.15s calculate the ball’s change in momentum and its
speed.
3) A footballer takes a goal kick by kicking a 0.4kg football
away at 10m/s. If his foot was in contact with the ball for
0.1 seconds calculate the force he applied to the ball.
4) A golfer strikes a 200g golf ball away at 50m/s. If he
applied a force of 50N calculate how long his club was in
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contact with the ball for.
Energy loss in collisions
In the “Forces” module we looked at how to calculate an object’s kinetic
energy:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
in m/s
We’ve also said that in a collision momentum is conserved (unless an
external force acts). The same cannot usually be said for kinetic energy…
For example, consider the following collision. How much kinetic energy is
lost?
Before
Speed = 50m/s
Speed = 20m/s
Mass = 1000kg
Mass = 800kg
After
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Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Speed = 30m/s
Energy loss in collisions
Consider a head-on collision where the cars stick together. How much
kinetic energy is lost in this example? Where does all the energy go?
Before
Speed = 50m/s
Speed = 30m/s
After
Speed = 10m/s
In this example more kinetic energy was lost. We say
it was a “less elastic collision”. An “elastic collision” is
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one where the kinetic energy is conserved.
Diffraction
More diffraction if the size of the gap is similar to the wavelength
More diffraction if wavelength is increased (or frequency decreased)
Diffraction depends on frequency…
A high frequency (short wavelength)
wave doesn’t get diffracted much – the
house won’t be able to receive it…
Diffraction depends on frequency…
A low frequency (long wavelength) wave
will get diffracted more, so the house
can receive it…
Transmitting information
As we said before, different types of electromagnetic radiation can be
used to send different types of information, e.g. an optical fibre:
Optical fibres have two main advantages: they can send more information
compared to electrical cables of the same diameter and with less signal
weakening.
Another example is radio waves:
Ionosphere
The longer
wavelength waves are transmitted by being reflected off the
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ionosphere (an electrically charged layer in the Earth’s atmosphere).
Transmitting information
Sounds can be transmitted long distances by converting them into
electrical signals first:
A microphone converts ______ waves into electrical
signals with the same amplitude and ________. These
waves are then transmitted through cables where they
are regularly _________ to reduce signal loss.
Radio waves can also be used as “carrier waves”:
Signal
____ wave
______ wave
Transmitter
Wave is
demodulated
back into a ____
Words – signal, frequency, modulated, carrier, sound, amplified
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Refraction through a glass block:
Wave slows down and bends
towards the normal due to
entering a more dense medium
Wave slows down but is
not bent, due to entering
along the normal
Wave speeds up and bends
away from the normal due to
entering a less dense medium
Finding the Critical Angle…
1) Ray gets refracted
3) Ray still gets refracted (just!)
THE CRITICAL
ANGLE
2) Ray still gets refracted
4) Ray gets
internally reflected
Uses of Total Internal Reflection
Optical fibres:
An optical fibre is a long, thin, _______ rod made of
glass or plastic. Light is _______ reflected from one
end to the other, making it possible to send ____
chunks of information
Optical fibres can be used for _________ or
ENDOSCOPES (medical devices that are used to see
inside the body)
Words to use – communications, internally, large, transparent
Dispersion of Light
This happens because the light is refracted:
RED LIGHT is
refracted THE
LEAST
VIOLET LIGHT is
refracted THE MOST
Lenses
Lenses use the idea of refraction:
When light enters a
MORE DENSE medium
it slows down…
A prism uses this idea
to split light. This
happens because purple
light is refracted more
than red light
Another example:
The lens in the eye is used to focus what we see:
Converging and diverging
lenses
CONVERGING (Convex)
Thickest at the centre
DIVERGING (Concave)
Thinnest at the centre
Ray diagrams
The rays of light are
refracted INWARDS and
meet at the focus, F.
F
F
The image formed is REAL –
in other words, it can be
seen on a screen
The rays of light are
refracted OUTWARDS.
A VIRTUAL image is formed
– in other words, the image
doesn’t actually exist
Ray diagrams
To draw ray diagrams follow these two rules:
1) Draw a ray from the TOP of the object PARALLEL to
the axis and then going through F:
F
2) Draw a ray from the TOP
of the object going through
the CENTRE of the lens
(which will be undeviated)
This image is
REAL, UPSIDE
DOWN and
SMALLER than
the object
Practical uses of lenses
Cameras use the lens arrangement where the
object is beyond 2F – this is why they can’t be
used at very short range.
Magnifying glasses use the arrangement where
the object is between F and the lens – this is
why they don’t work when you move them away
from the object.