# AQA A level Physics - Unit 4

probably not overly useful, but i've attempted to answer the learning objective in the nelson thornes textbook

Peta 10^15 P

Tera 10^12 T

Giga 10^9 G

Mega 10^6 M

Kilo 10^3 k

Centi 10^-2 c

Milli 10^-3 m

Micro 10^-6

Nano 10^-9 n

Pico 10^-12 p

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## Momentum and Impulse

How do we calculate momentum (P)?

P = m * v

Newton's first and second laws:

An object remains at rest or in uniform motion unless acted on by a force

Rate of change of momentum is proportional to the resultant force

What is impulse? How is it measured?

an impulse is a force acting on an object multiplied by the time over which it acts

Impulse F * t = delta(mv) / t OR area under Force time graph

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## Impact Forces

What happens to the impact for if duration of force is reduced and why?

F = (mv-mu) / t

Therefore as time is reduced, the Force is increased

How do we calculate change in momentum for an object stopping or reversing?

Choose a positive and negative direction

What happens to the momentum of a ball which hits a wall?

If the ball hit the wall 'normally' the velocity after is opposite to the velocity before

If the ball hits the wall at an angle the component of the velocity which is at a normal to the wall after is the same as before

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## Conservation of Momentum

Is Momentum ever lost in a collision?

No

What do we mean by conservation of momentum?

Momentum before = Momentum after

What condition must be fulfilled if the momentun is to be conserved?

No external forces must be acting on the system

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## Elastic and Inelastic Collisions

What are elastic and inelastic collisions?

Elastic no loss of kinetic energy

Inelastic colliding objects stick together

Partially inelastic colliding object move apart with less kinetic energy

What is conserved in elastic conditions?

Momentum and kinetic energy

Are any real collisions ever perfectly elastic?

No, some of the kinetic energy is converted to other forms of energy such as sound

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## Explosions

What energy changes take place in an explosion?

Chemical energy is transformed into kinetic energy, heat and sound

What can we always say about total momentum of an explosive system?

The total momentum of the system before is equal to the momentum after

What are the consequences after an explosion when two bodies move apart?

The two bodies move apart with equal momentum, hence why a gun recoils.

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## Uniform Circular Motion

How do we recognise uniform circular motion?

For an object to move in uniform circular motion it must rotate at a steady rate

How do we measure the speed of an object moving in circular motion?

v  =  2r(pi) / T  =  2rf(pi)

What are angular displacement and angular speed?

angular displacement the angle an object moving in uniform circular motion goes through (rad)

angular speed angular displacement per second (rad per s)

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## Centripetal Acceleration

Why is the velocity not constant for a body in uniform circular motion?

Velocity is speed in a given direction, so as the direction is always changing, so is the velocity

In which direction does the acceleration take place?

Acceleration is towards the centre of the circle

How is centripetal force calculated? Where does it act?

F = (m * v^2) / r

Towards the centre of the circle

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Why are passenger thrown outwards going quickly round a bend?

The centripetal force is not big enough to hold the passenger on the circular path

Which forces are acting when going over a curved bridge?

mg - S = mv^2 / r weight and support force

What forces provide the centripetal force on a banked track?

Horizontal component of the support forces provide the centripetal force

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## At the fairground

When is the contact force on a 'big dipper' passenger greatest?

S - mg = mv^2 / r     at the bottom

What condition applies when a passenger just fails to stay in his seat?

S > mg

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## Oscillations

What is one complete oscillation?

One full cycle of motion, which ends in the same place as it started.

How are amplitude, frequency and period defined?

Amplitude maximum displacement

Frequency  number of cycles per second

Period time taken for one compete oscillation

What is the phase difference between two oscillation?

2t(pi) / T

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## SHM

What condition apply to the acceleration of SH oscillations?

always in the opposite direction to the displacement
is zero when v-t graph gradient is zero

What is the phase difference between displacement, velocity and acceleration?

D V T/4

D A T/2

V A T/4

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## Energy and SHM

How do KE and PE vary with displacement?

KE EP                                                                           maximum displacement Etotal o                                                                       equilibrium 0 Etotal

How do KE and PE vary with time if damping is negligible?

Maximum KE and Pe remain the same

What is the effect of damping on oscillations?

Light damping displacement in oscillations reduce                                               Critical damping stop oscillations just after mass passes equilibrium                     Heavy/over damping slows return to equilibrium

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## Forced oscillations and resonance

When does resonance occur?

When the applied frequency is equal to the natural frequency.

What are free and forced oscillations?

Free oscillations are not subjected to any periodic force.

Why does a resonant system reach maximum amplitude of vibration?

Because the amplitudes of both the free and forced oscillations add together.

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## Gravitational field strength

How do we represent a gravitational field?

Arrows in the direction of the field

What is gravitational field strength?

Force per unit mass on a test mass placed in a field

What is a radial field? a uniform field?

Radial field is around an entire planet

Uniform field is over a small area

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## Gravitational potential

What is gravitational potential?

Work done per unit mass to get an object from infinity to that point

What is the gravitational potential difference between two points?

Change in W = m * change in velocity

Where is the gravitational potential of an object zero?

Wherever there is no overall gravitational force acting upon it

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## Newton's law of gravitation

How does gravitational attraction vary with distance?

2d  F/4 3d   F/9 4d   F/16

What do we mean by inverse square law?

The force is inversely proportional to the distance squared

Can we treat spherical objects as point masses?

Over large distances, yes

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## Planetary fields

What is the shape of g against r for points outside a planet?

Exponential decay curve where g is proportional to r^2

What is the shape of a V - r graph?

Exponential curve where V is proportional to -1/r

What is the significance of the gradient of the V - r graph?

Gradient of a V-r graph is -g

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## Satellite Motion

When does a satellite stay in a stable orbit?

What happens to the speed of a satellite as it moves closer to earth?

The speed increases

Why must a geostationary satellite be in orbit above the equator?

So that its time period is 24h and doesn't move relative to the earth.

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## Field Patterns

How do we charge an object?

What does the direction of a field line show concerning a test charge?

the direction is that which a positive test charge would follow

How is the strength of an electric field represented?

Where the field is strongest the lines and most concentrated11

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