P2 basic knowledge

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P2.1.1 Resultant forces

Whenever two objects interact, the forces they exert on each other are equal and opposite

A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the resultant force.

A resultant force acting on an object may cause a change in its state of rest or motion.

If the resultant force acting on a stationary object is:

  • zero, the object will remain stationary
  • not zero, the object will accelerate in the direction of the resultant force.

If the resultant force acting on a moving object is:

  • zero, the object will continue to move at the same speed and in the same direction
  • not zero, the object will accelerate in the direction of the resultant force.
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P2.1.2 Forces and motion

The acceleration of an object is determined by the resultant force acting on the object and the mass of the object.

  • F is the resultant force in newtons, N
  • m is the mass in kilograms, kg
  • a is the acceleration in metres per second squared, m/s2

The gradient of a distance–time graph represents speed.

The velocity of an object is its speed in a given direction.

The acceleration of an object is given by the equation: The gradient of a velocity–time graph represents acceleration.

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P2.1.3 Forces and braking

When a vehicle travels at a steady speed the resistive forces balance the driving force.

(Most of the resistive forces are caused by air resistance)

The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.

The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver's reaction time (thinking distance) and the distance it travels under the braking force (braking distance)

A driver's reaction time can be affected by tiredness, drugs and alcohol.

When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase.

A vehicle's braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

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P2.1.4 Forces and terminal velocity

The faster an object moves through a fluid the greater the frictional force that acts on it.

An object falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity

When a parachute is opened the force of air resistance becomes greater than gravity.

The net force on the skydiver now has negative acceleration or deceleration.

This causes a rapid decrease in the skydiver's velocity.

The gradient of a velocity time becomes stronly negative when the parachute is opened.

As the speed of descent decreases, the amount of air resistance also falls, until once again a terminal velocity is reached because Fw = FAR.

This second terminal velocity is now slow enough to allow the skydiver can land safely.

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P2.1.5 Forces and elasticity

A force acting on an object may cause a change in shape of the object.

A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy. 

For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded

F = k × e

  • F is the force in newtons, N
  • k is the 'spring constant' in newtons per metre, N/m
  • e is the extension in metres, m

Work is done on an elastic object when its shape changes and it stores elastic potential energy

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P2.2.1 Forces and energy

When a force causes an object to move through a distance work is done.

Energy is transferred when work is done.

Work done = force x distance

Power is the work done or energy transferred in a given time.

Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field.

Earth's gravitational field is approximately 10 N/Kg

GPE (J) = mass (kg) x gravitational field strength (N/kg) x height (m)

The kinetic energy of an object depends on its mass and its speed.

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P2.2.2 Momentum

Momentum is a property of moving objects.

p = m × v

  • p is momentum in kilograms metres per second, kg m/s
  • m is the mass in kilograms, kg
  • v is the velocity in metres per second, m/s

In a closed system the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.

Velocity is speed in a particular direction, so the momentum of an object also depends on the direction of travel. This means that the momentum of an object can change if:

  • the object speeds up or slows down
  • the object changes direction
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P2.3.1 Static electricity

When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and onto the other.

The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

If you rub an acetate plastic rod with a duster

  • electrons move from the rod to the duster
  • the duster becomes negatively charged and the rod becomes positively charged

The opposite thing happens with a polythene rod:

  • electrons move from the duster to the rod
  • the rod becomes negatively charged and the duster becomes positively charged

When two electrically charged objects are brought together they exert a force on each other.

Two objects that carry the same charge repel. Two objects that carry different charges attract.

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P2.3.1 Static electricity 2

  • V is the potential difference in volts, V
  • I is the current in amperes (amps), A
  • R is the resistance in ohms, Ω

The current through a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component.

The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected).

For components connected in series:

  • the total resistance is the sum of the resistance of each component
  • there is the same current through each component
  • the total potential difference of the supply is shared between the components.

For components connected in parallel:

  • the potential difference across each component is the same
  • the total current through the whole circuit is the sum of the currents through the separate components.
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P2.4.1 Household electricity

Cells and batteries supply current that always passes in the same direction- direct current (d.c.).

An alternating current (a.c.) is one that is constantly changing direction.

Mains electricity is an a.c. supply. In the UK it is about 230 V.

If an electrical fault causes too great a current, the circuit is disconnected by a fuse or a circuit breaker in the live wire as if current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit.

Some circuits are protected by Residual Current Circuit Breakers (RCCBs).

RCCBs operate by detecting a difference in the current between the live and neutral wires

Appliances with metal cases are usually earthed.

Some appliances are double insulated, and therefore have no earth wire connection.

The earth wire and fuse together protect the wiring of the circuit

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P2.4.2 Current, charge and power

When an electrical charge flows through a resistor, the resistor gets hot.

Candidates should understand that there is a choice when buying new appliances in how

The rate at which energy is transferred by an appliance is called the power- P = E/t

  • P is power in watts, W
  • E is energy in joules, J
  • t is time in seconds, s

P = I x V

  • P is power in watts, W
  • I is current in amperes (amps), A
  • V is potential difference in volts, V
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P2.5.2 Atoms and radiation

Some substances give out radiation from the nuclei of their atoms all the time, whatever is done to them. These substances are said to be radioactive.

Radioactive Decay is random

Nuclear Background radiation includes both natural sources, such as rocks and cosmic rays from space, and man-made sources such as the fallout from nuclear weapons tests and nuclear accidents.

Alpha particles are two neutrons and two protons, the same as a helium nucleus, a beta particle as an electron from the nucleus and gamma radiation as electromagnetic radiation

Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not. Alpha particles are deflected less than beta particles and in an opposite direction.

The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve

Nuclear radiation ionises materials. This changes atoms or molecules into charged particles

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P2.6.1 Nuclear fission

There are two fissionable substances in common use in nuclear reactors: uranium-235 and plutonium-239.

The majority of nuclear reactors use uranium-235.

Nuclear fission is the splitting of an atomic nucleus.

For fission to occur the uranium-235 or plutonium-239 nucleus must first absorb a neutron.

The nucleus undergoing fission splits into two smaller nuclei and two or three neutrons and energy is released.

The neutrons may go on to start a chain reaction.

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P2.6.2 Nuclear fusion

Nuclear fusion is joining 2 atomic nuclei to form a larger one & how energy is released in stars.

Stars form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets.

The early Universe contained only hydrogen but now contains a large variety of different elements.

In the main sequence period of its life, a star is stable because the forces within it are balanced.

A star goes through a life cycle. This life cycle is determined by the size of the star.

Fusion processes in stars produce the naturally occurring elements. These may be distributed throughout the Universe by an explosion of a big star (supernova) at the end of its life

Stars have enough Hydrogen to last millions of years. As the star runs out of hydrogen, other fusion reactions take place forming the nuclei of other elements

Candidates should know that elements up to iron are formed during the stable period of a star. Elements heavier than iron are formed in a supernova.

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