2024 WA3 Physics

• What is a moment?

  Turning effect of force
• Which is the perpendicular distance?

  a)6cm is the perpendicular distance

  b)3m is the perpendicular distance

  c)5m is the perpendicular distance

  
• When mass of beam is 1.0kg, what is the net moment of force and its effect in SI unit?

  g: 10N/kg

  Total anticlockwise moment = (20*10)(20/100)

  Total clockwise moment =(1.0*10)(10/100) + (50*10)(40/100)

  Net moment = clockwise - anticlockwise moment

  =201-40

  Effect is clockwise

  
• How can the body be in balance?

  sum of clockwise moments due to individual particles = sum of anticlockwise moments

  -equivalent to having a force acting through a point below pivot

  -turning effect = 0
• What does the resultant force acting through the point below a pivot represent?

  -sum of all individual particles' weight

  -resultant force is weight of body
• Where is the centre of gravity for an object that has a regular shape and uniform density?

  centre of object
• centre of gravity can be outside the object => e.g ring
• What happens when the block is tilted slightly? (before reaching pivot)

  -can return to original position

  -vertical line of action of weight still falls within base

  -weight creates (effect) moment about pivot

  -> helps block return to original position -> stable
• What happens when the block is tilted more? (When center of gravity is aligned with pivot)

  -reaches a point where vertical line of action of weight is in line with the pivot

  -block balances on its edge

  -no moment is created
• What happens when block is tilted further? (pivot is past center of gravity)

  -block will topple

  -vertical line of action of weight falls outside base

  -weight of the block creates a (effect) moment about pivot

  -causes block to topple -> unstable
• What is the relationship between the position of center of gravity and stability?

  -the lower the center of gravity, the more stable it is

  ->larger tilt angle needed for center of gravity to fall outside the base
• Why does block A (a vertical block) fall before block B? (a horizontal block)

  -block A has a higher centre of gravity

  -vertical line through the centre of gravity falls outside the base at a smaller tilt angle
• Calculate reaction force Fp

  Total anticlockwise moments about Q = 1200(0.6) + 700(0.4) = 1000 Nm

  By principle of moments,

  Fp (1.2) = 1000

  Fp = 830 N

  
• Calculate reaction force Fq when Fp=830N

  Total upward forces = total downward forces (resultant force=0)

  Fq + 830 = 1200 + 700

  Fq = 1070 N

  
• What does it mean when an object is neutral?


  stays in original position even after it's pushed
• How can acceleration be found?

  FORCE divided by MASS
• Surface area vs contact area

  Contact area: area touching

  Surface area: areas that can be seen
• What is the relationship between force and pressure?

  -the greater the magnitude of force, the greater the pressure
• If the weight of a man is 700N, each foot area is 0.50m^2, he is standing on 2 feet. Calculate pressure exerted by 1 foot.

  M1: 700 N / 2 = 350 N (Weight of each foot)

  350 N / 0.50 m^2 = 700 N/m^2

  M2: Pressure = total weight / total contact area

  = 700 N / 0.50 m^2 * 2

  = 700 N/m^2
• How does hydraulic system work?

  -Force 1 applied on piston 1, enclosed liquid experiences a pressure 1 of applied force 1 / contact area 1 (of piston 1)

  -Pressure 1 is transmitted equally to other parts of the enclosed liquid and to piston 2

  -Pressure 2 exerted on piston 2 is the same as pressure 1

  -Force exerted by piston 2 is pressure 2 * contact area 2
• How do you find force exerted on the second piston?

  Force 2 = Force 1 * Area 2 / Area 1
• What is the force needed to lift the car of mass 800kg?

  Radius of piston 1 = 10cm

  Radius of piston 2 = 100cm

  800kg = 8000 N

  Contact area of piston 1 = pi * (10)^2 = 100 pi cm^2

  Contact area of piston 2 = pi * (100)^2 = 10 000 pi cm^2

  Pressure at 1 = Pressure at 2

  Force 1 / 100 pi = 8000 / 10 000 pi

  Force 1 / 100 = 8 / 10

  Force 1 = 80 N
• How do you determine the density of a liquid?

  1.Weigh empty measuring cylinder with calibrated electronic balance

  2.Pour liquid into measuring cylinder and weigh both measuring cylinder and liquid

  3.Find mass of liquid by subtraction of readings

  4.Measure volume of liquid using measuring cylinder

  5.Calculate density
• When is a displacement can used?

  When mouth of measuring cylinder is too small
• How do you level a liquid in a displacement can accurately?

  -Pour liquid into can until it overflows

  -Wait until liquid drains from can and levels
• What is the application of density?

  -Identify substances

  -Identify purity of substances
• Water pressure is exerted in all directions
• What is the relationship between depth and pressure? Why?

  -The lower the depth, the greater the pressure

  -pressure underwater comes from weight of water above

  -Deeper-> weight of water is greater, thus an increase in water pressure
• Where do you find height when calculating pressure

  from surface of liquid to point given in the question/bottom of liquid

  => depth
• What is the water pressure at air-liquid surface?

  -Zero -> height = 0

  -Only atmospheric pressure is exerted

  -air-liquid surface does not experience water pressure
• When is there no atmospheric pressure?

  -in space

  -in enclosed spaces (vacuum)
• What is the purpose of a mercury barometer?


  -measure any gas pressure
• What is in the space above mercury in a barometer?

  a vacuum
• What is the density of mercury?

  13600 kg/m^3
• What happens when the pressure inside mercury is equal to atmospheric pressure?

  mercury does not move upwards anymore
• What happens to the length of mercury when the tube is lowered?

  unchanged
• What happens to the length of mercury when the tube is tilted?

  unchanged
• What happens to the length of mercury when the tube is narrower?

  unchanged
• What happens to the length of mercury when the tube is lifted?

  unchanged
• What is the total pressure made up of at this point?

  Water pressure + atmospheric pressure