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LC Physics > force and moments + density & pressure > Flashcards

force and moments + density & pressure Flashcards

1
Q

force

A

anything that changes or tends to change the motion of a body in magnitude or direction

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2
Q

what is force measured in?

A

Newtons

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3
Q

friction

A

a force that opposes motion, it can be overcome using lubricants

(we would not be able to walk or drive without friction)

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4
Q

what causes motion?

A

an unbalanced force

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5
Q

weight

A

the force with which the earth attracts a body is called the weight of the body

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6
Q

mass

A

the mass of a body is the amount of matter in it

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7
Q

momentum

A

the product of the mass and the velocity of a body

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8
Q

eg of momentum

A

a body of mass 5kg moving at 12 m/s has a momentum of 60 kgm/s

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9
Q

momentum formula

A

momentum = mass x velocity

ρ = mv

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10
Q

Newton’s Laws of motion

A

1) a body remains at rest or moving with uniform velocity in a straight line unless an unbalanced force acts on it
2) the rate of change of momentum is proportional to the force causing it and takes place in the direction of that force
3) to every action there is an equal and opposite reaction. Action and reaction never act on the same body

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11
Q

applications of newton’s laws of motion

A

1) road safety
2) sport
3) seat belt

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12
Q

road safety

A

front + rear of cars designed to crumple gradually so force is reduced while cage is rigid

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13
Q

sport

A

follow through in tennis increases contact time, increasing impulse and momentum

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14
Q

seat belt

A

designed as external forces to stretch slightly to increase contact collision time and reduce force on passenger

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15
Q

momentum unit

A

kg m/s (kilogram metre per sec)

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16
Q

principal of conservation of momentum

A

if no external force acts on a system of colliding bodies, the total momentum of the bodies remain constant
(eg. if two bodies collide)

momentum before = momentum after provided no external forces act

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17
Q

equation for initial and final momentums

A

in hback

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18
Q

what type of quanitity is momentum

A

it is a vector quantity

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19
Q

applications of momentum

A
  • jets and rockets: exhaust gas has a very high velocity and very large momentum, causing the jet/rocket to move forward
  • sport: in golf, a club called a driver is used to tee off as its head has a large mass and so the momentum is big. When it hits the small mass of the golf ball, it will travel far
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20
Q

density

A

the mass per unit volume of a substance is called the density

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21
Q

density formula

A

density = mass/volume

ρ = m/v

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22
Q

si unit of density

A

kgm⁻³ , but g cm⁻³ is more convenient

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23
Q

pressure

A

the force per unit area

P = F/a + notation

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24
Q

pressure formula

A

pressure = force/area

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25
Q

unit of pressure

A

n/m² or the Pascal (Pa)

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26
Q

earth’s atmosphere

A

we live at the bottom of a sea of air called the earth’s atmosphere, the weight of this air exerts pressure

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27
Q

standard atmospheric pressure

A

1.013 x 10⁵ Nm⁻² / pascals

or 1013 hectopascals

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28
Q

pascals and n/m²

A

1 pascal = 1 Nm⁻²

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29
Q

high atmospheric pressure

A

means dry weather

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30
Q

low atmospheric pressure

A

means wet weather

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31
Q

pressure in fluids

A

in fluids, pressure depends on depth, density, and the acceleration due to gravity

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32
Q

pressure of fluid formula

A

pressure of fluid = height x density of object x gravity

P = hρg

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33
Q

Archimedes’ Principle

A

a body wholly or partially immersed in a fluid experiences an upthrust equal in magnitude to the weight of the fluid displaced

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34
Q

archimedes principle (2)

A

(the apparent loss in weight equals the weight of the fluid dispersed)

a floating body displaces its own weight of fluid

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35
Q

law of flotation

A

states that the weight of a floating body is equal to the weight of the fluid that it displaces

36
Q

how can law of flotation be shown

A

can be shown using a mass of known weight that floats on water. If it is placed in an overflow can, the water that pours out is equal to the masses weight

37
Q

equipment to measure pressure

A

barometer

hydrometer

38
Q

hydrometer

A

the denser the liquid, the higher the hydrometer will float in it

used to find:
-the % of alcohol in beers, wines, spirits (alcohol is less dense than water)
=the % of fat in milk

39
Q

boyles law

A

states that at constant temp, the volume of a fixed mass of gas is inversely proportional to its pressure

P ∝ 1/V + notation

40
Q

for a fixed mass of gas at constant temp (formula)

A

pV = k

k - a constant

41
Q

boyle’s law apparatus

A

diagram - pg 111

  • fixed mass of gas (air)
  • air pumped into the reservoir, increasing pressure about the oil
  • pressure can be read from the Bourdon gauge
42
Q

boyles law formula

A

hardback

43
Q

centre of gravity

A

the point at which the whole weight of a body appears to act is called its centre of gravity

44
Q

equilibrium

A

objects can be considered to have a stable, unstable, or neutral equilibrium

45
Q

stable design

A

the principle of stable design is to have a low centre of gravity and a wide base

46
Q

levers

A

a lever is a rigid body that is free to rotate about a fixed point, called a fulcrum

47
Q

moment of the force

A

the turning effect of a force is called the moment of the force (Nm) (Newton metre)

M = Fd
(notation: M = moment, F = force, d = perp distance)

48
Q

moment of a force formula

A

M = Fd

force x perpendicular distance from the point to line of action of the force

49
Q

principle of moments

A

when a body is in equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anti-clockwise moments about that point

50
Q

vector sum

A

the vector sum of the moments about any point is zero

51
Q

2 conditions for equilibrium

/ laws of equilibrium

A

1- the principle of moments must apply
(when a body is in equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anti-clockwise moments about that point)

2- the sum of the forces in any direction must equal the sum of the forces in the opposite direction

52
Q

couples (torque)

A

two parallel forces with the same magnitude acting in opposite directions are called a couple

53
Q

resultant of couples

A

he resultant of these two forces is zero

54
Q

moment of a couple

A

the moment of a couple is called the torque of the couple

55
Q

symbol + unit for moment of a couple

A

symbol: T
unit: Newton Metre (Nm)

56
Q

formula for moment of a couple

A

T = Fd

57
Q

example of couples

A

taps and doors
handlebars on a bike
moving coil meters and simple motor

58
Q

gravitation

A

the attraction that all masses in the universe have for each other

59
Q

What did Newton publish and when?

A

1680 - published his law of gravitation

60
Q

newton’s law of universal gravitation

A

states that any two point masses in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres

F = Gm₁m₂/d²
(notation)

61
Q

newtons law of universal gravitation formula

A

hardback

62
Q

gravitational force

A
  • always attractive
  • size of force on each mass is the same
  • distance between two bodies is the same between their centres
63
Q

gravitational force number

A

G = 6.7 x10⁻¹¹ Nm²kg⁻²

64
Q

gravitational force between two bodies

A

gravitational force between two bodies is negligible unless at least one of them has a very large mass

65
Q

gravity and weight equations

A

hardback

66
Q

objects above the surface of the earth

A

If d is the distance from the centre of the earth to an object in orbit around the earth, then the acceleration due to gravity (gd) is given by:

gd = GM/d²

67
Q

how to demonstrate law of Flotation

A
  • weigh object that floats on spring balance
  • fill overflow can w/ water + allow water to settle
  • lower object slowly into can until it floats
  • collect water displaced in a previously weighed beaker
  • weigh beaker w displaced water in it
  • subtract to find weight of displaced water

result: weight of object will be equal to weight of water displaced, thus verifying the law

68
Q

Diving & pressure - The Bends

A
  • human body: designed to operate at normal atmospheric pressure
  • if divers breathe normal air (79% nitrogen) while diving deeply, pressure causes too much Nitrogen to become dissolved in thieir blood
  • Form bubbles in blood + would experience ‘the bends’
  • Painful, can kill
69
Q

The Bends cure

A

place deep sea divers in decompression chamber

70
Q

demonstrating atmospheric pressure

A

The Collapsing Can:

  • place small amount of water in empty can
  • Place over Bunsen, bring water to boil
  • Can becomes full of steam which pushes air out of it
  • Turn off heat source quickly + place cap on can
  • As it cools down, steam inside turns back to water, creating partial vacuum (ie. very low pressure) in can
  • Atmospheric pressure acts on can from outside + is bigger than pressure inside
  • Thus, can collapses dramatically
71
Q

Boyles Law proportions

A
  • If pressure doubled, volume halved
  • Pressure trebled, volume decreased to a third
  • Pressure quartered, volume increased four times
72
Q

what keeps the earths atmosphere attached to the earth?

A

the force of gravity.

Also occurs on our moon and explains why moon has no atmosphere

73
Q

show that f=ma is a special case of Newton’s second law

A

write derivation of f=ma

74
Q

when figuring out centripetal force, etc

A

use the centripetal acceleration for a in f=ma

75
Q

A toy is knocked over, it always returns to the upright position. Explain why

A

has a turning moment about fulcrum / has zero turning moment when toy is in vertical position

76
Q

conditions necessary for the equilibrium of a body under a set of co-planar forces

A
  • sum of forces = zero / forces up = forces down

- sum of the turning moments about any point = zero / clockwise turning moments = anticlockwise turning moments

77
Q

centripetal force on a merry go round

A

friction

78
Q

if there was no force of friction on someone on a merrygoround, what direction would they move as the merry go round rotates?

A

they would remain stationary

79
Q

relationship between period and radius

A

T² ∝ R³

80
Q

a rotating object can be in equilibrium. Explain how this can happen

A

constant angular velocity

81
Q

what a zero net force tells you about someone’s motion

A

moving at constant speed (or at rest)

82
Q

is pressure a vector or scalar quantity? why?

A

Scalar

It acts in all directions / it has no direction

83
Q

why is it easier to turn a nut using a longer spanner than a shorter one

A

greater torque

84
Q

A hammer is spun in a circle at constant speed, why does it accelerate even though it moves at constant speed?

A

direction changes

85
Q

if you exert a force on something, what is the force it exerts on you?

A

the same force, in the opposite direction (make sure to write “in the opposite direction”)

LC Physics (25 decks)

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