# Physics- making things work

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• Created by: Chesca
• Created on: 19-05-13 17:58

## Moments

Turning effect of a force (moment of the force) can be increased by:

• increasing the size of the force
• using an object with a larger handle

Levers:

• crowbar = lever = raises one edge of a heavy object
• weight of object = load
• Force applied to crowbar = effort
• Point where crowbar turns = pivot

Line that force acts along = line of action

Moment = force X perpendicular distance from the line of action of the force to the pivot

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## Centre of Mass

Race car = weight as low as possible (otherwise car overturn)

• weight of object acts at a single point = centre of mass!

Centre of mass of an object is that point at which it's mass may be thought to be concentrated.

Suspended equlibrium:

• If suspend object, then release, will come to stop with it's centre of mass directly below the point of suspension
• Object in equilibrium
• weight doesn't exert turning effect on object because centre of mass is directly below the point of suspension

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## Centre of mass of a symetrical object

Centre  of mass along the axis (line) of symmetry for symmetrical object.

If more than one axis of symmetry, centre of mass is where axis of symmetry MEET!

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## Moments in balance

'See-saw' - example where clockwise  and anticlockwise moments may balance each other out

Person 1 sits nearer pivot = heavier, meaning her anti-clockwise moment about the pivot balances the other boys clockwise moment.

Principal of moments:sum of all clockwise moments about a point = sum of the anti-clockwise moments about a point.

W1w1 =W2d2

• Work out W1 if W2 = 5.0N, d1 = 0.30m, d2= 0.15m
• W2 X d2 = 5 X 0.15 = 0.75
• W1 X d1 = 0.75 / 0.3 = 2.5
• W1 = 2.5

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## Centre of Mass Test

Attach object to a rod and stand (first point)

• 'Plumbline' (string with small weight attached) draws vertical line on card from rod downwards. (draw in line)
• Test repeated with the car suspended from second point, to give similar line. (draw in line)
• Centre of mass, where the two lines meet :)

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## Stability

Stability of object is increased by making its base as wide as possible and it's centre of mass as low as possible

• If line of action of objects weight is 'outside' base, object will topple over, as weight has a turning effect.
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• Tractors: don't topple over, as line of action of weight is within it's wheelbase.
• If tilted more, will topple, as line of action of weight is outside wheelbase.....moment about lower wheel of weight is clockwise!
• Moment of support force from ground on upper wheel is clockwise!
• Resultant force about lower wheel makes it topple over :)
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## Stability 2

• Bus tests:
• Test so see if bus is safe when going around bends and hills
• On double-decker, if all people sat upstairs bus would be unstable.
• People in concentrated area, unevenly distributed, causes all weight on top = topples over
• High Chairs:
• Needs a wide base.
• when child in....centre of mass above seat
• If narrow base and child strapped in, when leans over, chair will topple!
• Chair topples if child's weight acts outside chair base on one side.
• Topple = moment of child's weight is greater than moment of chairs weight.
• Preventing toppling:
• centre of mass of object should be as low as possible = base wide enough/clamped down
• Object topples - sum of clockwise moments is NOT equal to sum of anti-clockwise.  "Resultant moments!"

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## Hydraulics

Snow shoes used in snow....provide a larger contact area for weight = less pressure

Pressure = a force per unit area

• units = 'pescals' = N/m2

Pressure = force / area

Pressure in liquid acts equally in all directions!

• Bottle with holes, all at same depth. All hit the same distance away from bottle = all at same PRESSURE!

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## Hydraulics 2

Hydraulics - use a SMALL force to create a BIG force

Liquids are virtually incompressible (volume doesn't change when under pressure)

• Hydraulic car jack lifts car
• handle down, oil forced out of narrow cylinder and into wider one
• pressure of oil forces piston in wider cylinder outwards
• piston forces pivoted lever = raises car

Force applied to system via the piston is called the effort

• force exerted by the system, to raise car = load

Pressure in both systems, one small piston and one large, exert the same pressure, regardless of their size difference

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## Circular Motion

Satellite orbiting Earth moves in a circular motion

For an object moving in a circle at constant speed:

• objects velocity directed along tangent to the circle
• Velocity changes direction as it turns
• change of velocity, towards centre of circle

Object accelerates towards centre of the circle = called centripetal acceleration

• resultant force of object acts towards the centre of the circle

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## Centripetal force

Any object moving in circle, must have resultant force, acting towards the centre of the circle!

• resultant force = centripetal force!

Centripetal force on vehicle moving around a roundabout, due to frition between tyres and road

• centripetal force of 'gravity wheel' is due to weight of rider and downward push of the wheel.

Centripetal force of an object increases if speed of object increases.

Centripetal force increases if radius of circle decreases

Greater mass of object = greater the centripetal force!

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## Pendulums

Equlibrium position = position when swing will start/stop moving

Oscillating motion = motion of an object that moves 'to' and 'fro' on the same line.

Air resistance + friction can delay swinging

Amplitude of oscillating object - distance moved from equlibrium to highest point

Time period = time for one cycle motion

• depends on length (increases as length increases)

Frequency = number of complete cycles of oscillations per second

• unit = Hertz - 1 cycle per second

Time period = 1/ frequency of oscillations

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