# P3 part 2

overview of the "making things work" unit of P3

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• Created by: alex
• Created on: 04-05-13 13:38

## Centre of Mass

Centre of mass- a point where all of an objects mass appears to be concentrated.

To find the centre of mass:

Suspend the shape so it can swing freely.

Find the vertical using a plumb line and draw this vertical on.

Hang the shape from another point and find the vertical again.

Where the two lines cross is the centre of mass.

When a shape has more than one line of symmetry, the centre of mass is where they cross.

When a shape has only one line of symmetry, the centre of mass is somewhere on that line.

Centre of mass doesn't have to be inside the object. For example, in a ring it is in the centre.

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## Pendulums

Pendulum- an object attached to one end of a long thin piece of material that can swing freely under gravity.

Time period- the time a pendulum takes to move from one side of its swing to the other and all the way back again.

The number of cycles that the pendulum completes in one second is called its frequency.

So:

Time period (seconds/s)=1/frequency(hertz/Hz)

If T is the time period and F is the frequency:

T=1/F

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## Moments

Moment- using a force to turn something about a pivot.

The size of force you need to turn it depends on the distance from the pivot.

The size of a moment can be calculated with this equation:

Moment (newton-metres/Nm)= Force (newtons/N) x Perpendicular distance (metres/M)

If M is the moment in newton-metres, F is the force in newtons and d is the perpendicular distance in metres, then:

M=Fxd

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## Levers

Lever- a force converter used to turn something round a pivot

The moment about the pivot is the force multiplied by the length of the lever.

Levers can also be used as force multipliers.

Examples of levers:

Crow bar

Scissors

Bottle opener

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