# Physics Unit 3 Revision Notes

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Turning Forces

A moment, is the turning effect of a force.

Moment= Force x Perpendicular Distance

(from the line of action of the force to pivot)

M   F   x

(Nm) (N) (m)

Centre of Mass: The point at which the whole mass is concentrated

The centre of mass hangs directly below the point of suspension.

A freely suspended object will swing until it's centre of mass is vertically below the line of action of the force.

It rests in this position because there's no moment- th pivot is in line with the line of action of the force.

Balanced Moments

If the
Anticlockwise Moment= Clockwise Moment

the object will not turn.

Levers

Simple levers use balanced moments-

- The amount of work needed to do work depends on the distance the force is applied from the pivot

- Levers increase the distance from the pivot at which the force is applied- this means less force is needed to get the same amount

- Thatas why levers are known as force multipliers

Moments

If the moments acting on an object aren't equal the object will turn. (Resultant momentum)

Pendulums

The time for one pendulum swing depends on it's length.

Time Period= 1/Frequency

T=1/f

(Seconds) (Hz)

Hydraulics

Liquids are virtually incompressible.

Their volume and density stay the same.

Because liquids have this characteristic and they are also able to flow, a force applied to one point in the liquid will be passed to other points in the liquid.

Pressure in  a liquid is transmitted in all directions.

Pressure: Force per unit area

Pressure   = Force        / Cross Sectional Area

Pascals(Pa) Newtons(N) Metres2 (M2)

The pressure in liquids can be used in hydraulic systems.

- Hydraulic systems are used as force multipliers- they use a small force to produce a bigger force.

Circular Motion

Velocity is constantly changing speed and direction.

- If an object is travelling in a circle it is constantly changing direction. This means the velocity is constanlty changing- so the object is accelerating. The acceleration is towards the centre of the circle.

- There must be a resultant force acting on the object causing this acceleration.

- The force that keeps something moving in a circle is called a centripedal force.

Centripedal force depends on three things-

1) Mass: The larger the mass of an object, the bigger the centripedal force needed to keep it moving in a circle.

2) Speed: The faster the object is moving, the bigger the centripedal force has to be to keep it in a circle.

3) Radius: You need a larger force to keep it moving in a smaller circle as it has more 'turning' to do.

Magnetic Fields

A magnetic field is a region where magnetic materialsn (like iron and steel) and also wires carrying currents, experience a force acting on them.

On magnetic field diagrams the arrows always point from the…