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CIRCULAR MOTION
With circular motion, velocity is constantly changing. Velocity is both
speed and direction of an object. If an object is travelling in a circle it is
constantly changing direction, which means it's accelerating. This means
there must be a force acting on it. This force acts towards the centre of
the circle. This force that keeps something moving in a circle is called a
centripetal force.
In the exam you can be asked to say which force is actually providing the
centripetal force in a given situation. It can be tension, or friction, or
even gravity.
A car going round a bend: 1) imagine the bend is part of a circle ­ the
centripetal force is towards the centre of the circle. 2) the force is from
friction between the car's tyres and the road.
A bucket whirling round on a rope: The centripetal force comes from
tension in the rope. Break the rope, and the bucket flies off at a tangent.
A spinning fairground ride: The centripetal force comes from tensions in
the spokes of the ride.
Centripetal depends on mass, speed and radius. The faster an object's
moving, the bigger the centripetal force has to be to keep it moving in a
circle. Likewise, the heavier the object, the bigger the centripetal force
has to be to keep it moving in a circle. And you need a larger force to
keep something moving in a smaller circle ­ it has `more turning' to do.…read more

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IMAGES
A real image is where the light from an object comes together to form an image
on a `screen' ­ like the image formed on an eye's retina. A virtual image is
when the rays are diverging, so the light from the object appears to be coming
from a completely different place. When you look in a mirror you see a virtual
image of your face ­ because the object appears to be behind the mirror. You
can get a virtual image when looking at an object through a magnifying lens ­
the virtual image looks bigger and further away than the object actually is.
To describe an image properly, you need to say 3 THINGS: 1) How big it is
compared to the object; 2) Whether it's upright or inverted (upside down); 3)
Whether it's real or virtual.
Reflection of light is what allows us to see objects. Light bounces off them into
our eyes. When light reflects from an uneven surface such as a piece of paper
the light reflects off at all different angles and you get a diffuse reflection.
When light reflects from an even surface (smooth and shiny like a mirror) then
it's all reflected at the same angle and you get a clear reaction.
Refraction of light is when the waves change direction as they enter a different
medium. This is caused entirely by the change in speed of the waves. That's
what makes ponds look shallower than they are ­ light reflects off the bottom
and speeds up when it leaves the water, making the bottom look like it's nearer
than it is.…read more

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MIRRORS (PART 1)
Ray diagrams in PLANE MIRRORS points: 1) The image is the
same size as the object; 2) It is AS FAR BEHIND the mirror as
the object is in front; 3) It's formed from diverging rays,
which means it's a virtual image.
Curved mirrors are complicated. Concave mirrors are shiny
on the inside of the curve and convex mirrors are shiny on
the outside. Light shining on a concave mirror converges,
and light on a convex mirror diverges. Uniformly curved
mirrors are like a round portion of a sphere. The centre of
the sphere is the centre of curvature, C. The centre of the
mirror's surface is called the vertex. Halfway between the
centre of curvature and the vertex is the focal point, F. Rays
parallel to the axis of the concave mirror reflect and meet
at the focal point. The centre of curvature, vertex and focal
point all lie on a line down the middle of the mirror called
the axis. The centre of curvature and focal point are in front
of a concave mirror, and behind a convex mirror.…read more

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MIRRORS (PART 2)
Ray diagram for an image in a concave mirror: 1) An incident ray parallel to the axis will
pass through the focal point when it's reflected; 2) An incident ray passing through the
focal point will be parallel to the axis when it's reflected.
The distance from the mirror affects the image. With an object at C, you get a real, upside
down image the same size as the object, in the same place. Between C and F, you get a
real, upside down image bigger than the object, and behind it. An object in front of F
makes a virtual image the right way up, bigger than F, behind the mirror. So hold it in
front of F for doing your eyeliner.
Ray diagram for an image in a convex mirror: 1) An incident ray parallel to the axis will
reflect so that the reflected ray seems to come from the focal point; 2) An incident ray
that can be extended to pass through the focal point will be parallel to the axis when it's
reflected. INSTRUCTIONS: 1) pick a point on the top of the object. Draw a ray going from
the object to the mirror parallel to the axis of the mirror. Make it a bold line when it's in
front of the mirror, and a dotted line when it's behind. 2) Draw another line going from
the top of the object to the mirror, passing through the focal point on the other side.
Make it dotted when it's behind the mirror. 3) The incident ray that's parallel to the axis is
reflected as if it starts at the focal point. Make sure the reflected ray meets the incident
ray at the mirror surface. 4) The incident ray that passes through the focal point is
reflected parallel to the axis. Make sure the reflected ray meets the incident ray at the
mirror surfcae.5) Mark where these two reflected rays meet behind the mirror. That's the
top of the image. 6) Repeat the process for a point on the bottom of the object.
The image in a convex mirror is always virtual, upright, smaller than the object and behind
the mirror, closer than F. The further away the object is from the mirror, the smaller the
image. You can see a wide area in a convex mirror, which is why they put them on dodgy
road corners.…read more

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LENSES (PART 1)
Light is refracted when it enters and leaves glass prisms. 1) The ray bends towards the
normal as it enters the denser medium, and away from the normal as it emerges into the
less dense medium. Try to visualise the shape of the wiggle in the diagram ­ that can be
easier than remembering the rule in words. 2) Note that different wavelengths of light
refract by different amounts. So white light disperses into different colours as it enters a
prism. A rectangular prism has parallel boundaries, so the rays bend one way as they
enter, and then bend back again by the same amount as they leave ­ so white light
emerges. But with a triangular prism, the boundaries aren't parallel, which means the
different wavelengths don't recombine, and you get a rainbow effect.
Different lenses produce different kinds of image. There are two main types of lens ­
converging and diverging. They have different shapes and have opposite effects on light
rays. A converging lens is convex ­ it bulges outwards. It causes parallel rays of light to
converge (move together) to a focus. A diverging lens is concave ­ it caves inwards. It
causes parallel rays of light to diverge (spread out). The axis of a lens is a line passing
through the middle of the lens. The focal point of a converging lens is where rays hitting
the lens parallel to the axis all meet. The focal point of a diverging lens is the point where
rays hitting the lens parallel to the axis appear to all come from ­ you can trace them back
until they all appear to meet up at a point behind the lens. Each lens has a focal point in
front of the lens, and one behind.
There are three rules for refraction in a converging lens: 1) An incident ray parallel to the
axis refracts through the lens and passes through the focal point on the other side. 2) An
incident ray passing through the focal point refracts through the lens and travels parallel
to the axis. 3) An incident ray passing through the centre of the lens carries on in the same
direction.
Three rules for refraction in a diverging lens: 1) An incident ray parallel to the axis refracts
through the lens, and travels in line with the focal point (so it appears to have come from
the focal point). 2) An incident ray passing towards the focal point refracts through the…read more

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