# Using Physics to Make Things Work

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MOMENTS

A moment - the turning effect of a force

The size of the moment is given by the equation:

M = f x d, where M=moment of the force in newton-metres, Nm, f=force in N and d=perpendicular distance from the line of action of the force to the pivot in m

A door opens when you apply a force of 20N at right angles to it, 0.6m from the hinge.  what is the moment of the foce about the hinge?

20 x 0.6 = 12Nm

To increase the moment, you could increase the force or the distance from the pivot.

We use a lever to make a job easier.  When using a lever, the force we are trying to move is called the load and the force applied to the lever is the effort.  A lever acts as a force multiplier, so the effort we apply can be much less than the load.

CENTRE OF MASS

The mass of any object is concentrated at one single point.  This is the centre of mass.  Any object that is freely suspended will come to rest with its centre of mass directly below the point of suspension.  The object is then in equilibrium.

You can find the centre of mass of a sheet of an irregularly-shaped material by suspending the sheet from a pin held in a clamp stand.  When it comes to rest, hang a plumbline from the same pin. Mark the position of the plumbline against the sheet.  Hang the sheet with the pin at another point and repeat the procedure.  The centre of mass is where the lines that marked the position of the plumbline crosss.  The position of the centre of mass depends on the shape of the object.  Sometimes it lies outside the object.  For a symmetrical object, the centre of mass is along the axis of symmetry.  If the object has more than one line of symmetry, the centre of mass is where the lines of symmetry meet.

MOMENTS IN BALANCE

If an object is in equilibrium it is balanced and not turning.  We can take the moments about any point and will find that the total clockwise moment and the total anti clockwise moment are equal.  This is the principle of moments.

STABILITY

The line of action of the weight of an object acts through the centre of mass.  If the line of action of the weight lies outside