# Physics 3

revision cards for AQAs Physics unit 3 for additional science.

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## Moments and Levers

Moment: One example of a moment is the turning of a spanner. The longer the spanner the handle, the less force you need to exert on it to undo a nut.

To increase moment of the force:

increase size of force OR   using a spanner with a longer handle

Levers: The weight is called LOAD. The force applied by the person is called EFFORT. The point of turn is called PIVOT.

Moment (newton metres Nm) = Force (newtons N) X Perpendicular distance from pivot to line of action (of force) (m)

Centre of mass (gravity) is the point where an objects mass may be thought to be concentrated.

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## Suspended Equilibrium and Moments in Balance

A suspended object will always rest below its centre of mass/gravity or point of suspension.

For a symmetrical object its centre of mass along its axis of symmetry.

Moment of force about the pivot = Force X Distance

See diagrams for Moments in Balance

sum of all Clockwise       =      sum of all Anticlockwise Moments about any point  Moments about any point

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## Circular Motion and Stability

For an object moving in a circle at a constant speed, at any instant:

• the objects velocity is directed along a tangent to the circle.
• its velocity changes direction as it moves round.
• the change of velocity is towards the centre of the circle.

Accelerates continuously towards centre of circle.

Force on the object acts towards centre of circle.

Stability: If you tip an object over its centre of mass and pivot the weight knocks it over.

For example Tractors: line of action of its weight acts within its wheel base. If it tiles more, it will topple over when the line of action of its weight acts outside the wheelbase. (Clockwise turning effect about the lower wheel)

LARGE WHEEL BASE over centre of mass

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## Centripetal Force and Gravitational Attraction

CF: Object moving in a circle must be acted on by a resultant force that acts towards circle centre. The greater the mass of object, the greater the CF is.

Friction on uplifting force and weight (gravity?)  Resultant of these 2 forces = centripetal force (CF)

If too fast: CF needs increase if speed increased                                                        If too slow: CF needs increase if radius of circle decreased

Gravity: 10N per kilogram on Earth AND 1.6N per kilogram on the Moon

Any 2 objects exert a force of gravitational attraction on each other. Force depends on mass of each object

If its bigger, the greater the mass of each object is                                                      If its smaller, the greater the distance between the two objects.

Space probe: decreases as it moves away from Earth but increases towards the Moon. Earth mass is greater so it exerts a greater force on an object on its surface than the Moon does.

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## Planetary Orbits

Most orbits are a slightly squashed circle (elliptical). An object orbits a much bigger objects. Centripetal force on the orbiting object is due to force of gravitational attraction between it and the larger object.

Force of gravity on planet due to

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