# Physics Unit 2B

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## Resistance

-To measure resistance an ohmmeter is used.

DC Resistance

The resistance of a given resistor or conductor grows with the length of conductor and specific resistivity of the material, and decreases for larger cross-sectional area.

AC Resistance

A wire carrying alternating current has a reduced effective cross sectional area because of the skin effect.

Thermal Resistance is a heat property - and a measure of a temperature difference, by which an object - or material resist a heat flow (heat per time unit or thermal resistance).

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## Work Done

-Work Done (j) = Force (n) x Distance (m)

-Work Done (j) = Power (w) x Time Taken (s)

Examples;

• 4 (n) x 5 (m) = 20 (j)
• 6 (w) x 120 (s) = 720 (j)

Work done is movement against the force of friction.

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## Gravitational Potential Energy

Gravitational Potential Energy (j) = Mass (kg) x Gravitational Field Strength (n/kg) x Height (m)

Example;

-30 (j) = 0.5 (kg) x 10 (n/kg) x 6 (m)

-20 (j) =1 (kg) x 10 (n/kg) x 2 (m)

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## Speed

-Speed (metres per second) = Distance (m) /Time (s)

Examples;

-450 (m) /15 (s) = 30 (meters per second)

-600 (metres per second) = 6000 (m) /10 (s)

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## Distance

-Distance (m) = Speed (meters per second) x Time (s)

Example;

-100= 25 x 4

Distance - Time Graph

When an object is stationary the line is horizontal the distance doesn't change (the object doesn't move).

When a line is diagonal going upwards then the object is going at a steady speed, the steeper the line the faster the object is moving.

If the line is going diagonally downwards the object is slowing down going back to its starting speed.

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## Velocity

-Acceleration = Final Velocity - Initial Velocity/Time

Example;

-20-0/6 = 20/6 = 3.3 (metres per second)

-50-8/3 = 42/3 = 14 (metres per second)

When two forces are balanced the object is stationary.

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## Ticker Tape Timer

Resultant force- on an object is the single force that would have the same effect as all the forces acting on the object.

Example;

-Air resistance + Friction = 400n+400n = 800n

-Forward Force - Backward Force = 1000n - 800n = 200n

Air resistance is a frictional force that opposes the motion of a solid object moving through the air.

Factors effecting air resistance:

-Gravitational pull.

-Size and shape.

-Speed.

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## Laws of Motion

1st Law;

- stationary objects stay stationary.

- moving objects stay moving at the same speed in the same motion.

2nd Law;

- If the forces on an object aren't balanced the object will accelerate in the direction of the force.

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## Weight

-The weight of an object is the force on the object due to gravity.

-Weight (n) = Mass (kg) x Gravitational Field Strength (10n).

Weight is measured in Newtons.

Mass is measured in Kilograms.

Gravity is equivalent to 10 Newtons.

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