# Resultant Forces

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## Resultant Forces

Resultant Forces

Whenever two objsscts 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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## 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 = ma (a = m/s squared

The gradient of a distance–time graph represents speed.

Calculation of the speed of an object from the gradient of a distance–time graph.

The velocity of an object is its speed in agiven direction.

The acceleration of an object is given by the equation: acceleration = change in velocity / time

The gradient of a velocity–time graph represents

acceleration.

Area underneath a velocity-time graph is distance

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## Stopping Distance

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

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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## 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 (steady speed). When falling objects first set off, the force of gravity is much stronger than the frictional force slowing them down, so they accelerate. As the speed increase, so does the frictional force. When they are both ewaul, the object falls at a steady speed and has reached it's terminal velocity.

Calculate the weight of an object using the force exerted on it by a gravitational force:

W = m x g

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## 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 x e

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## Forces and Energy

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

Work done, force and distance are related by the equation:

W = F x d

Energy is transferred when work is done.

Work done against frictional forces.

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

P = E/t

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

Ep = m x g x h

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

Ek = 1/2 mv squared

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## Momentum

Momentum is a property of moving objects.

Momentum = mass x velocity

b) 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.

When working out momentum after, have one as positive and one as negative.

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## 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.

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

Two objects that carry the same type of charge repel.

Two objects that carry different types of charge attract.

Electrical charges can move easily through some substances, eg metals.

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## Electrical Circuits

Electric current is a flow of electric charge. / number of coulombs per second

The size of the electric current is the rate of flow of electric charge. The size of the current is given by the equation:

Current (A) = charge (C) / time (s)

The voltage between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points so amount of energy

Voltage (V) = Work done (J) / Charge (C)

Current–potential difference graphs are used to show how the current through a component varies with the potential difference across it.

The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor.

Calculate current, potential difference or resistance using the equation:

Voltage (V) = current (A) x resistance (Ohms)

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## Electrical Circuits 2

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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## Household Electricity

Cells and batteries supply current that always passes in the same direction. This is called 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 has  a frequency of 50 cycles per second (50 hertz) and is about 230 V.

Most electrical appliances are connected to the  mains using cable and a three-pin plug.

RCCBs operate by detecting a difference in the current between the live and neutral wires. and operate much faster than a fuse.

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

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

j) Appliances with metal cases are usually earthed.The earth wire and fuse together protect the wiring of the circuit.

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## Current, Charge and power

When an electrical charge flows through a resistor, the resistor gets hot. The ions vibrate, which make it harder for charge-carrying coloumbs to get through

b) The rate at which energy is transferred by an appliance is called the power.

Power (W) = Energy / time

Power, potential difference and current are related by the equation:

Power(W) = Current x Voltage

Energy transferred, potential difference and

charge are related by the equation:

Energy = Voltage (V) x Charge (C)

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## Atomic Structure

The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons.

The relative masses and relative electric charges of protons, neutrons and electrons.

In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no overall electrical charge. Atoms may lose or gain electrons to form charged particles called ions.

The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope. The total number of protons in an atom is called its atomic number. The total number of protons and neutrons in an atom is called its mass number.

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## 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.

Identification of an alpha particle as 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 -He Nuclei (2 protons, 2 neutrons) Big, heavy, slow. Strongly ionising
Beta particles - electrons. For every one emitted a neutron converts to a proton. Moderatly ionising
Gamma - waves. Weakly ionising. pass through materials. No mass or charge.

Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not.

The uses of and the dangers associated with each type of nuclear radiation.

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, or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.

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## Nuclear Fission

During the process of nuclear fission atomic nuclei split. This process releases energy, which can be used to heat water and turn it into steam. The steam drives a turbine, which is connected to a generator and generates electricity.

There are two fissionable substances in commonuse in nuclear reactors: uranium-235 and

plutonium-239.

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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## Nuclear Fusion

Nuclear fusion is the joining of two atomic nuclei to form a larger one.

Nuclear fusion is the process by which 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.

During the ‘main sequence’ period of its life cycle 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.

Protostar(gas and dust) - main sequence star - red giant - black dwarf - white dwarf

Protostar(gas and dust) - main sequence star - red super giant - supernova - neutron star (very dense) / black hole

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

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