What force keeps the moon travelling around the Earth and the planets travelling round the Sun?
The force that keeps the moon travelling around the Earth and the planets travelling round the Sun is gravity.
What shape are the planet's orbits around the Sun?
The planet's orbits are approximately circular.
What shape is the orbit of a comet?
A comet has an orbit which is an ellipse.
What happens to a planet during its year?
During a year a planet orbits once around the Sun.
Why does Jupiter have a longer year than Earth?
Jupiter has a longer year than Earth because it is further away from the Sun and so the Sun's gravity is weaker.
Anything that travels in a circle does so because a force acts on it. What's the name of this force?
The force that pulls everything round in a circle is called centripetal force.
Centripetal force is not a type of force, but a description of a way a force is behaving. What is the centripetal force that a) keeps the planets in orbit b) keeps a bung on a string going round in circles c) a car going round a bend?
The centripetal force that a) Keeps the planets in orbit is gravity, b) keeps a bung on a string going round in circles is tension in the string and c) a car going round a bend is friction between the tyres and the road.
How do you know that something travelling in a circle is accelerating?
You know that something travelling in a circle is accelerating because it's velocity is always changing. Remember, velocity is spedd in a particular direction, so if direction changes then velocity does too.
In which direction does centripetal force act?
Centripetal force always acts towards the centre of the circle.
What are the two different types of orbit of an artificail satellite?
Geostationary and Low Polar.
What are the main characteristics of a geostationary orbit?
24 hour orbit. Remains stationary relative to the Earth's surface. High up above the equator. Used for communication (satnav, Sky etc).
What are the main characteristics of a low polar orbit?
Orbits the poles. Has a period of a few hours. Can cover the whole of the Earth's surface. Is fast because it is lower so gravity is stronger. Used for spying, monitoring (e.g. weather).
Why do low polar satellites travel faster than higher satellites?
Because they are lower so gravity is stronger and exerts more of a force on the satellite.
What's the difference between a scalar quantity and a vector quantity?
Scalars have only magnitude (size) but vectors have magnitude and direction.
Give some examples of vector quantities.
Force, velocity, acceleration, displacement, momentum are all vectors as we have to take into account direction as well as magnitude.
Give some examples of scalar quantities.
Speed, distance, mass and energy are all scalars as direction is not important. Remember, velocity is the vector version of speed.
Calculate the resultant of 2 forces at right angles, one 10 N and the other 7 N.
Use pythagoras. Draw a right angled triangle. The hypotenuse will be the resultant = 12.2 N
In the suvat equations, what do the letters stand for?
s = displacement (or distance), u = initial (starting) velocity, v = final velocity, a = acceleration, t = time.
What shape trajectory (path) do projectiles make?
Projectiles travel in a path which is a parabola.
When an object is projected horizontally as a projectile, what is its horizontal acceleration?
Zero! As no forces act on it horizontally.
When an object is projected horizontally as a projectile, what is its vertical acceleration?
Acceleration due to gravity, which on earth is around 10 m/s squared.
When an object is projected horizontally as a projectile, what is its horizontal velocity?
Nothing, it doesn't change as there is no acceleration.
When an object is projected horizontally as a projectile, what happens to its vertical velocity?
It changes due to gravity.
What happens to momentum during a collision provided no external forces act?
Momentum is conserved during a collision.
What is the formula for momentum?
Momentum = mass x velocity
What can you say about the forces that act on two objects when they collide or push apart?
When two bodies interact, they exert an equal and opposite force on each other.
What causes pressure inside a container of gas?
Collisions between the molecules of gas and the walls of the container cause a force as a result of their change in momentum.
Why does pressure go up if the temperature of a gas increases?
The hotter the gas the faster the particles move. This means their momentum chnages more when they collide with the walls of the container exerting a bigger force.
Explain using ideas about particles how the gases from a rocket can lift it off the ground.
The hot particles of gas are moving very fast. They are forced out of the end of the rocket. An equal and opposite force pushes back on the rocket lifting it up. Momentum is also conserved so the momentum of the gas down must equal the momentuim of the rocket up.
Why does a gun move backwards when it is fired?
The momentum of the bullet forwards must equal the momentum of the gun backwards so as to give a total momentum of zero. Momentum is conserved. The gun has a larger mass than the bullet so a lower velocity.
Which type of electromagnetic waves are used to send signals from Earth to satellites?
Why do microwave dishes (satellite dishes) need careful alignment?
Microwaves are short wavelength so don't diffract much. This means that the dishes must be aligned carefully to pick up the whole beam that won't have spread out much.
What happens to radio waves of frequency less than 30 MHz when they hit the ionosphere?
These relatively low frequency radio waves reflect off the ionosphere.
What happens to radio waves of frequency between 30 MHz and 30 GHz when they hit the atmosphere?
These waves can pass through the atmosphere so are useful for communication as they can reach satellites in space.
What happens to radio waves of frequency greater than 30 GHz when they hit the atmosphere?
They are absorbed and scattered by dust and rain in the atmosphere meaning they lose signal strength and aren't very good for communication.
How does the amount of diffraction depends on the size of gap and wavelength of the wave?
Diffraction is greatest when the gap or object size is the same size as the wavelength.
What is constructive interference?
When two waves meet they interfere. If the peak of one wave meets the peak of another (or a trough meets a trough) then they will interfere constructively and produce a wave of larger amplitude.
What is destructive interference?
When two waves meet they interfere. If the peak of one wave meets the trough of another (or a trough meets a peak) then they will interfere destructively and cancel out.
What must the path difference (the difference in the distance the waves travel) be for constructive interference?
A whole number of wavelengths (or an even number of half-wavelengths) so a peak always arrives with another peak.
What must the path difference (the difference in the distance the waves travel) be for destructive interference?
A whole number and-a-half wavelengths (or an odd number of half-wavelengths) so a peak always arrives with a trough.
What would you see if you passed laser light through a narrow vertical slit?
A horizontal line where the light has diffracted. This proves that light behaves like a wave.
What would you see if you passed laser light through a narrow vertical double slit?
A horizontal interference pattern of light and dark fringes where the diffracted light from the two slits has interfered constructively and destructively.
What is meant by plane polarised light?
Plane polarised light has vibrations in one plane only.
Why do polarising filters make good sunglasses?
Reflected light is partly polarised. Polarising sunglasses block out this light reducing glare.
What evidence did Newton have that light was a stream of particles?
Newton said that because light travelled in straight lines and could be reflected, it could be thought of as a stream of particles.
What evidence did Huygens have that light was a wave?
Light can be diffracted, which is something only waves do. Light will also form an interference pattern, which particles wouldn't do.
Why does light refract when it enters a different medium?
Light changes speed when it enters a different medium. The change in speed causes a change in direction.
How do you calculate the refractive index?
Refractive index = speed of light in a vacuum / speed of light in medium. It always has a value between 1 and 2.
Dispersion is the splitting up of white light into the colours of the spectrum.
Name the colours of the spectrum in order of longest wavelength to shortest wavelength.
Why does the white light split up into different colours?
The colours have the same speed in a vacuum but different speeds in a medium such as glass. This means they have different refractive indexes so refract by different amounts.
Which colour refracts the most and why?
Blue (or violet) refracts most as it has the shortest wavelength, changes speed the most, and has the highest refractive index.
What are the conditions under which total internal reflection (TIR) can occur?
TIR happens when the angle of incidence is greater than the critical angle.
How does refractive index affect critical angle?
The bigger the refractive index, the lower the critical angle.
What is the difference between a real image and a virtual image?
A real image can be projected onto a screen, a virtual image can't.
Name something that produces a virtual image.
A magnifying glass produces a virtual image.
Name something that produces a real image.
A camera or projector produces a real image.