Physics unit 3 - GCSE (entire syllabus)

• The turning effect of a force is known as the moment of a force
• Moment = force x perpendicular distance from pivot
• We can increase the moment by:
  • increasing the applied force
  • increasing the distance from the pivot
• For an object to be balanced (in equilibrium), the anti-clockwise moment must equal the clockwise moment

Test yourself

If there is a force of 50N and a distance of 0.1m, what is the moment?

(5n/m)

• The centre of mass of an object is the point where its mass may be thought to be concentrated
• Finding a centre of mass:
  • Dangle an object from a point on a clamp
  • Attach a piece of string to the clamp
  • Draw a straight line vertically down along the string
  • Repeat again with a different point
  • The place where all the lines intersect is the centre of mass
• An object will remain stable as long as the line of action of its weight lies inside the base
• If the line of action falls out of the base, the object will topple
• Make an object more stable by:
  • Lowering the centre of mass
  • Having a wider base
  • 

Test yourself

1) a. Will this bus topple if it continues driving, with its line of action in the same place?

1) b. How can I make the bus more stable?

(No, wider bass, lower centre of mass -have a shorter/single decked bus)

• An object moving at a constant speed around a circle has a constantly changing velocity (it's changing direction) and therefore accelerate
• The centripetal force needed increases as:- (gets quicker)
  • the mass increases
  • the radius decreases
• Force = Mass(kg) x Velocity² (m/s²)
•                         Radius
• 

Test yourself

What is the force if the Velocity is 3m/s, the mass is 4kg and the radius is 6m?

(6N)

• Gravity causes small masses to orbit larger ones in space
• Force increases with:
  • size
  • smaller distance
• The stronger the force, the shorter the orbit
• The orbits of the planets are elliptical(ellipses) with the sun centred at one of the foci.

Test yourself

Why is earth's orbit longer than mercury's?

(Further away NOT BIGGER)

There are two types of satellites you must know for your exam!

1) Geostationary

• 24 hour orbit
• Over the equator
• It has a further distance to travel than the earth so it must travel quicker
• At the same point over the earth at all times
• Communications!

2) Polar Orbit

• Over the poles
• The lower it is, the better the resolution
• Monitoring!

Test yourself

<-- geostationary orbit

Which satellite is a "Low" orbit, and which is a "Higher" orbit?

(Polar = lower, Geostationary = higher)

• Images in flat mirrors are always virtual and upright
• Angle of incidence = angle of reflection

Convex mirrors

• Always virtual

Concave mirrors

• Real image (not upright) if the object is behind the focus
• Virtual image if the object is in front of the focus

Test yourself

Is the picture showing a virtual or a real imageand why?

(if you can't see, the object is behind the focus)

(Real, behind the focus)

• Moving light waves from one medium to another, slowing down , and sometimes, changing direction
• e.g light from air --> water , slows down (bends to the normal - the middle line)
•        light from water --> air, bends away  from the normal
• This is because water is dense and air isn't!
• Refraction makes things appear closer than they actually are (like when you are having a bath and your hand looks waayy bigger lol)
• Violet lights bends further than the others (it's the first colour on the spectrum, it has to bend really tightly to keep the spectrum normal)

Test yourself

When the light moves from the air into the glass, is it moving away from, or towards the normal?

(towards)

Converging lens

• Magnified and virtual if the object is in front of the focus

Diverging - always virtual

Test yourself

Would I use a diverging lens to magnify things?

Does it create an upright, or inverted image?

(no, upright)

• Can be refracted like light
• Have longitudinal waves and therefore, cannot travel through a vacuum
• Frequency = pitch (Hz = number of waves/ time taken)
• Amplitude = Loudness (measured in m)
• Humans can hear up to 20KHz, or 20,000Hz

Test yourself

Describe each of the sounds made by the waves

(Loud and low pitched, Quiet and high pitched)

• Anything above 20KHz (20,000 Hz)
• Used for industrial cleaning
• Detecting flaws ( distance of flaw = speed of wave x time taken for reflection)
• prenatal scans

Test yourself

What does this graph represent?

(Where the flaw is, the second small jerk is it)

• A current causes a magnetic field in the wire
• The two magnetic fields (wire & magnet) interact, causing a force
• To acquire a bigger force:
  • increased currect
  • increased field
• Flemings left hand rule:
  • Thumb = force/thrust
  • First Finger = Field
  • Second finger = Current

Test yourself

What moves in the diagram?

(the wire)

• Uses force and magnestism to make a current
• The current is increased with
  • A stronger magnet
  • More coils
  • The magnet being turned faster
• A/C current created
• Must cut field lines

Test yourself

What is the opposite of the dynamo effect?

What does the dynamo effect create?

(Motor effect, A/C - alternating current)

• Uses an iron core as it easily gains and loses charge 
• Voltage of primary/Voltage of secondary = N.o coils primary/ N.o coils secondary

Step up

• More coils on the second circuit
• Increases the voltage
•  <-- step up <--- step down

Step down

• More coild on the first circuit
• Lowers the voltage

Test yourself

Which tranformer would I use to change the voltage from a power line, to a house?

Why would I use this one?

(Step-down, too high a voltage for houshold appliances)

This is the life of a star, in order of events

• Clouds of dust and Gas 
• Pulled by gravity in towards each other 
• Friction causes temperature to rise --> Protostar
• Temperatures get so high that nuclear fusion begins - hydrogen fuse to form helium nuclei --> Main Sequence Star
• Hydrogen begins to run out
• Star swells to form a Red Giant

Then three things can happen:

•  Explodes as a Supernova -> leaving a Neutron Star and a cloud of dust/gas that can form a Second Generation Star (if high mass)
•  Explodes as a Supernova -> this creates a Black Hole (if very high mass)
• Contracts to leave a White Dwarf -> Light fades to form a Black Dwarf (if low mass)

Test yourself

What happens to cause a star to collapse upon itself?

What is the process of hydrogen forming larger elements (up to iron) called?

How were elements heavier than iron formed?

(Hydrogen runs out due to fusion, fusion, when a supernova explodes and heavier elements collide to form even heavier ones)