P3 Revision - Moments, Centre of Mass + Circular Motion

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  • Created by: Théa
  • Created on: 21-05-13 14:47

Moments

Levers:

-For example: see-saw, door, crowbar, wheelbarrow, etc

-Doing all of these things is called a "turning force"

-What defines this is the amount of force applied and how far it is from the pivot

-Mechanical advantage / multiplying effect = distance from pivot to effort force / distance from pivot to load force

Moment:

-Turning effect of a force

- Moment (Nm) = force (N) x perpendicular distance from pivot to line of action

  M = F x d

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Circular Motion

Examples:

Electrons orbiting the nucleus of an atom - maintained by electrostatic attraction between positive protons and negative electrons 

Planets orbiting the sun - Gravitational attraction and pull of the sun

A conker spun on a string - Tension in the string or CENTRIPETAL FORCE

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Centripetal Force

-Think of the conker on a string. If the tension is removed, then the circular path will not be maintained

-This force acts towards the centre of the circle and is refferred to as the CENTRIPETAL FORCE

-This is an unbalanced force and is responsible for the conker changing direction constantly

-It thus causes the velocity to change 

-The centripetal force, therefore causes an acceleration that acts towards the centre of the circular path

-The force and acceleration MUST be at right angles, TOWARDS the centre of the circle

To INCREASE the Centripetal Force:

-INCREASE the object's mass

-INCREASE the object's speed

-DECREASE the radius of the circle

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Centre of Mass

The centre of mass hangs directly below the point of suspension

-A freely suspended object will swing until its centre of mass is vertically below the point of suspension, where it will rest

-This is because: the object's weight acts at a distance from the pivot, which creates a moment about the pivot, causing it to swing., However, when the object is resting, the line of action of the weight is in line with the pivot and the centre of mass, therefore there is no moment and the basket doesn't swing.

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Pendulums

The time for one pendulum swing depends on its length

Time period = 1 / Frequency

T = 1 / F

The longer the pendulum, the greater the time period, the shorter the pendulum, the shorter the time period

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