Physics 3

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  • Created by: jenni
  • Created on: 18-05-14 20:33

Moment

The moment of a force about a pivot is f x d, where d is the perpendicular distance from the pivot to the line of action of the force:

Moment (Nm)= Force (N) x perpendicular distance from the pivot to 

the line of action of the force (M)

The centre of mass of an object is the point where its mass may be thought to be concentrated. When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension. The centre of mass of a symmetrical object is along the axis of symmetry

Principle of moments states that; sum of all clockwise moments about the point = the sum of all the anticlockwise moments about that point. 

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Circular Moment

When an object moves in a circle it continously accelerates towards the centre of the circle. This acceleration changes the direction of motion of the body, not its speed.

The resulant force causing this acceleration is called the centripetal force and is always directed towards the centre of the circle.

The centripetal force needed to make an object perform circular motion increases as:

  • The mass of the object increases
  • The speed of the object increases
  • The radius of the circle decreases
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Lenses

Real Images = formed when rays of light cross, so they can be formed on a screen.

Virtual Images = cannot be formed on a screen, they are just where rays of light appear to have come from.

Refraction is the change of direction of light as it passes one medium to another. A lens forms an image by refracting light.

In a convex or coverging lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length.

Refractive index = sin i/ sin r

The nature of an image is defined by its size relative to the object (magnified or diminished), whether its upright or inverted relative to the object and whether its real or virtual.

The nature of an image produced by a converging lens for an object is virtual, upright & magnified. This means converging lenses can be used as magnifying glasses.

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The Eye

The structure of the eye;

Cornea

Tough, transparent covering over the front part of the eye. Convex in shape. Refracts light as it enters the eye (by a fixed amount).

Iris

 Coloured part of the eye that contains muscles. These relax or contract to adjust the size of the pupil. Controls how much light enters the pupil.

Pupil

Hole in the middle of the iris. Allows light to pass through as it enters the eye.

Lens

Transparent, bi-convex, flexible disc behind the iris. It is attached to the ciliary muscles by the suspensory ligaments. Refracts light to focus it onto the retina. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.

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The Eye; continued

Ciliary muscles

Muscles connected to the lens by suspensory ligaments. Adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.

Suspensory ligaments

Connect the ciliary muscles to the lens and hold the lens in place. Slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.

Retina

The lining of the back of eye containing two types of light receptor cells. Rods are sensitive to dim light and black and white. Cones are sensitive to colour. Contains the light receptors, which trigger electrical impulses to be sent to the brain when light is detected.

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The eye; continued

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The eye; continued

Two eyes showing how light goes into the eye when looking at a distant object and a near object. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/010_bitesize_gcse_tsphysics_medical_accommodation_304.gif)

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Comparison between the eye and a camera

Cameras are devices that focus light onto a photosensitive surface using a converging lens. They have some similarities to the eye.

Like the eye, the image produced by a camera is diminished, inverted & real.

Camera parts, Function in the camera & Structure in the eye performing a similar role

Lens

To focus light onto the photosensitive surface at the back of the camera. This can either be photographic film or a CCD(charge-coupled device). Lens - which focuses light onto the retina.

Focusing screw

Allows the user to adjust the focus for nearer or more distant objects. Ciliary muscles - which stretch or slacken the suspensory ligaments to adjust the shape of the lens

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Comparison between the eye and a camera

Aperture

Allows the user to adjust the amount of light entering the camera in different light conditions. Iris - which adjusts the amount of light entering the eye through the pupil.

Shutter

Allows the user to adjust the length of time that light enters the camera, which controls the amount of light to which the photosensitive surface is exposed. Not applicable (although we do have eyelids).

Photosensitive surface

Detects and records the light which is focused onto it. This can be onphotographic film, but digital cameras use CCDs which convert light into electrical signals which can be stored.

Retina - which detects light and converts it into electrical impulses which are sent to the brain.

One key difference between a camera and the eye is that a camera does not focus light onto the photosensitive surface by adjusting the shape of the lens. Instead, the focusing screws move the lens forward or backwards in order to focus the image onto the photosensitive surface.

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Short Sight

Someone with short sight can see near objects clearly, but their far point is closer than infinity. This means that they cannot focus properly on distance objects. Short sight is caused by one of the following:

  • The eyeball being elongated - so that the distance between the lens and the retina is too great.
  • The lens being too thick and cruved - so that light is focused in front of the retina.

Short-sighted can be corrected by placing a diverging lens in front of the eye, as shown in the diagrams below.

If the distance between a lens and a retina is too great it causes short-sightedness (Myopia) (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/045_bitesize_gcse_tsphysics_medical_sight3_table.gif)                     A concave lens corrects short-sightedness, allowing the image to focus on the retina (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/046_bitesize_gcse_tsphysics_medical_sight4_table.gif)

                                                                  Concave lens cures short-sightedness

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Long Sight

Someone with long sight can see distant objects clearly, but their near point is further away than 25cm. This means they cannot focus properly on near objects. Long sight is cause by one of the following:

  • The eyeball being too short- so the distance between the lens and retina is too small
  • A loss of elasticity in the lens- meaning it cannot become fat enough to focus 

As a result, the lens focuses light behind the retina instead of onto it. Long-sightedness is corrected by putting a converging lens in front of the eye, as shown in the diagrams below.

An image passing through an eye lens and focusing behind the retina equates to long-sightedness (Hypermetropia) (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/042_bitesize_gcse_tsphysics_medical_sight1_table.gif)                  A convex lens corrects long-sightedness, allowing an image to focus on the retina (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/043_bitesize_gcse_tsphysics_medical_sight2_table.gif)

                                                        A convex lens corrects long-sightedness, allowing an image to focus on the retina

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