Physics Mock Revision

• The centre of mass of an object is the point where its whole mass is concentrated.
• A suspended object will swing until its centre of mass  is vertically below the point of suspension.

To find the centre of mass of any flat shape you can:

• Suspend the shape and a plumb line from the same point and wait until they stop swinging.
• Draw a line along the plumb line.
• Repeat these steps, however suspend the shape from a different pivot point.
• The centre of mass is where the two drawn lines cross.

In simple shapes like squares, rectangles and circles you can find the centre of mass from drawing the lines of symmetry and seeing where the lines cross. (Eg. In a square the centre of mass is directly in the middle)

A moment is the turning effect of a force.

MOMENT(Nm) = FORCE(N) x perpendicular DISTANCE(m)

A spanner is a good example of moments.

• The force on a spanner causes a "turning effect" or Moment on the nut. The larger the force the larger the moment.
• Using a longer spanner but the same force can exert a larger moment because the distance from the pivot is greater.
• To get the maximum moment (turning effect) you need to push at right angles to the spanner. Pushing at any other angle would result in a smaller moment as the perpendicular distance between the line of action and pivot is smaller.

Eg. If a spanner is 0.1m away from the pivot and a force of 10 N is apllied the moment would be 1Nm (10x0.1), however if the spanner was 0.2m away from the pivot and the force applied was still 10N the moment would be 2Nm (10x0.2)

The principle of moments states that for an object in equilibrium:

The sum of all the clockwise moments about any point

                                          =

The sum of all the anticlockwise moments about any point

• A seesaw is a good example of showing this:

Eg. Your younger brother weighs 300N. He sits 2m from the pivot of a seesaw. If you weigh 700N, where should you sit to balance the seesaw?

anticlockwise moment = clockwise moment

                       300 x 2 = 700 x y

                                y = 0.86m (600/700)

• Unstable objects tip over easily and stable objects don't.
• The centre of mass of an object determines whether or not the object is stable.

1. The most stable objects have a wide base and a low centre of mass.

2. An object will tip over if its centre of mass moves beyond the edge of its base.

3. Due to moments, if the weight does not act in line with the pivot it'll cause a resultant moment, which will either right the object or cause it to tip.

For an object moving in a circle at constant speed, in any instant:

• The object's velocity is directed along a tangent to a circle
• The velocity changes direction as it moves around
• The change of velocity is towards the centre of the circle

Therefore the object always accelerates continuously towards the centre of the circle and as it always acts towards the centre, it is called Centripetal Acceleration.

Tension, friction or gravity can provide a centripetal acceleration:

• A car going round a roundabout has a centripetal force which acts towards the centre of the roundabout, the force is from friction between the road and the tyres.
• A bucket whirling around on a rope, the centripetal force comes from the tension in therope - break the rope and the bucket flies off at a tangent.
• A spinning fairground ride, the centripetal force comes from tension in the spokes of the ride.

Centripetal force depends on mass, speed and radius:

• The faster an object is moving the bigger the centripetal force has to be to keep it moving.
• The heavier the object the bigger the centripetal force has to be to keep it moving.
• You need a larger centripetal force to keep something moving in a small circle (it has more turning to do)

Example: Two cars are driving at the same speed around the same circular track. One has a mass of 900kg, the other has a mass of 1200kg. Which car has the bigger centripetal force?

The speed and radius of the circle are the same and the only difference is the mass of the two cars. This means that there is no working out to do and you can say:

The 1200kg car (the heavier one) must have the larger centripetal force.  

X Rays are electromagnetic waves which have short wavelengths.

To make an X Ray photograph:

• X Rays from an X Ray tube are directed at a patient and a lightproof cassette containing a photographic or a flat panel detector is placed on the other side of the patient.
• X Rays pass through the patients body.
• X Rays pass through soft tissue but are absorbed by bone, teeth and thin metal objects, this then casts an image which allows radiographers to identify broken bones.
• X Rays can also be used to create images with CT scanners and to destroy tumours at or near the body's surface.
• Soft tissue organs can be filled with a contrast medium which absorbs X Rays and can be seen on X Ray images.
• Lead can stop X Rays reaching certain parts of the body. For example pregnant women wear lead vests when having an X Ray to protect the baby from potentially harmful X Rays.

Dangers:

• X Radiation ionises substances it passes through
• High doses can kill living cells
• Low doses can cause cell mutation and cancerous growth.
• There is no evidence of a safe limit below which living cells would not be in some way damaged.

• An ultrasound scanner consists of: A transducer placed on the body's surface, a control system and a display screen
• Each pulse from the transducer: Is partially reflected from the different tissue boundaries or returns to the transducer as a sequence of reflected pulses from the boundaries, arriving back at different times.

• Lenses are usually made of glass or plastic and they all change the direction of light rays by refraction.
• Light is refractedwhen it enters and leaves a glass prism:

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.

• In a triangular prism white light enters the prism and emerges in a 'rainbow effect'. Whereas with a rectangular prism you get white light emerging as well as entering. This is because a rectangular prism has parallel boundaries, so the rays bend one way as they eneter and then bend back by the same amount as they leave. In a triangular prism the boundaries are not parallel which means the different wavelengths do not recombine.

• There are two main type of lense: Converging and diverging. They have opposit effect on light rays.

Converging (Convex)

• It causes pararllel rays of light to converge (move together) to a focus
• The focal point of a converging lens is where rays hitting the lens parallel to the axis all meet.

Diverging (Concave)

• It causes parallel rays of light to diverge (spread out)
• The focal point of a diverging lens is the point where rays hitting the lens parallel to the axis appear to come from - you can trace them back until they all seem to meet up at a point behind the lens.

Converging

• 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.

Diverging

• 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 lens and travels parallel to the axis
• 3) An incident ray passing through the centre of the lens carries on in the same direction.

Converging Lens

• An object positioned at 2F will produce a real, upside down image the same size as the object.
• Between F and 2F will make a real, upside down image bigger than the object and beyond 2F.
• Closer than F will make a virtual, upright image bigger than the object on the same size as the lens.

Diverging Lens

• A diverging lens will always produce a virtual image.
• The image will always be upright, virtual and on the same side of the lens as the object - no matter where the object is positioned.

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