# IGCSE Physics Section A (Forces and Motion)

IGCSE Section A Physics Notes :)

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## Speed

Speed = Distance / Time

Distance = Speed X Time

Time = Distance / Speed

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## Acceleration

Acceleration = Change in velocity / Time measured m/s

In a velocity time graph we look for the area under the graph to show us distance travelled. This helps us find speed and time.

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## Forces

• Weight- force acting on a body because of gravity.
• Friction- force that opposes motion.
• Air resistance- friction between an object and the air
• Viscous drag- similar to air resistance but when an object is moving through a liquid.
• Upthrust- the upward force liquids and gases exert on objects
• Magnetic- force that magents exert on other magnets
• Electrostatic- force between electrically charged objects

Forces are measured in Newtons.

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## Hooke's Law

When an elastic object - such as a spring - is stretched, the increased length is called its extension. The extension of an elastic object is directly proportional to the force applied to it:

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

This equation works as long as the elastic limit (the limit of proportionality) is not exceeded. If a spring is stretched too much, for example, it will not return to its original length when the load is removed.

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## Movement

Acceleration = Force / Mass

Mass = Force / Acceleration

Force = Mass X Acceleration

This is important when it comes to the science about driving.

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## Acceleration due to gravity and weight

On Earth, 10N = 1 kg

On the Moon 1kg = 1.67N this is because 1.67 N/kg is the gravitational field strength on the Moon.

Weight = Mass X Gravitational field strength.

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## Momentum

Momentum = Mass X Velocity

Momentum is a measure of how easily an object maybe brought ot rest. An object with a large momentum will need a greater force to bring it too rest.

Conservation of Momentum

The momentum of two bodies colliding is always conserved, provided that the force acting on the bodies are the force of one body on another.

Total momentum before = Total momentum after.

As momentum is a vector quantity, you must take the direction of movement into account.

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## The turning effects

Moment = Force X Perpendicular distance from the pivot

The moment of a force is the turning effect. In real life, levers are used to increase the turning effects by increasing the distance between the forces applied and the point around which the object will turn.

Sum of clockwise moments = Sum of anti-clockwise moments.

Example:

Child A is 1.5m away from pivot at 400N

Child B is d m away from pivot at 300N

( 400 X 1.5 ) / 300 = d                           d = 2m

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## Centre of gravity

The whole of the weight of an object acts through one point is called the centre of gravity. If a shape is suspended from a point so that it can turn, it will rest with the centre of gravity immediately below the point of which it has been suspended from.

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## The solar system

The Solar System consists of the Sun and its planetary system of eight planet and moons. It formed 4.6 billion years ago from the collapse of a giant molecular cloud.. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mecury, Venus, Earth and Mars are primarily composed of rock and metal. The four outer planets, called the gas giants are substantially more massive. The two largest, Jupiter and Saturn are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic plane.

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## The strength of gravity

Gravity is the force that attracts objects with mass to each other. The size of gravitational force depends on:

• The size of the masses involved. Large masses like starts and plants produce high gravitation forces.
• The distance between the masses. The greater the distance between two masses the smaller the gravitational force between them is.

The effect of gravity between two small masses is extremely small.

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## Orbital speeds of satellites

Orbital speed = 2 X Pi X Radius / Time

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