force and moments + density & pressure Flashcards
(85 cards)
1
Q
force
A
anything that changes or tends to change the motion of a body in magnitude or direction
2
Q
what is force measured in?
A
Newtons
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)
4
Q
what causes motion?
A
an unbalanced force
5
Q
weight
A
the force with which the earth attracts a body is called the weight of the body
6
Q
mass
A
the mass of a body is the amount of matter in it
7
Q
momentum
A
the product of the mass and the velocity of a body
8
Q
eg of momentum
A
a body of mass 5kg moving at 12 m/s has a momentum of 60 kgm/s
9
Q
momentum formula
A
momentum = mass x velocity
ρ = mv
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
11
Q
applications of newton’s laws of motion
A
1) road safety
2) sport
3) seat belt
12
Q
road safety
A
front + rear of cars designed to crumple gradually so force is reduced while cage is rigid
13
Q
sport
A
follow through in tennis increases contact time, increasing impulse and momentum
14
Q
seat belt
A
designed as external forces to stretch slightly to increase contact collision time and reduce force on passenger
15
Q
momentum unit
A
kg m/s (kilogram metre per sec)
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
17
Q
equation for initial and final momentums
A
in hback
18
Q
what type of quanitity is momentum
A
it is a vector quantity
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
20
Q
density
A
the mass per unit volume of a substance is called the density
21
Q
density formula
A
density = mass/volume
ρ = m/v
22
Q
si unit of density
A
kgm⁻³ , but g cm⁻³ is more convenient
23
Q
pressure
A
the force per unit area
P = F/a + notation
24
Q
pressure formula
A
pressure = force/area
25
unit of pressure
n/m² or the Pascal (Pa)
26
earth's atmosphere
we live at the bottom of a sea of air called the earth's atmosphere, the weight of this air exerts pressure
27
standard atmospheric pressure
1.013 x 10⁵ Nm⁻² / pascals
or 1013 hectopascals
28
pascals and n/m²
1 pascal = 1 Nm⁻²
29
high atmospheric pressure
means dry weather
30
low atmospheric pressure
means wet weather
31
pressure in fluids
in fluids, pressure depends on depth, density, and the acceleration due to gravity
32
pressure of fluid formula
pressure of fluid = height x density of object x gravity
P = hρg
33
Archimedes' Principle
a body wholly or partially immersed in a fluid experiences an upthrust equal in magnitude to the weight of the fluid displaced
34
archimedes principle (2)
(the apparent loss in weight equals the weight of the fluid dispersed)
a floating body displaces its own weight of fluid
35
law of flotation
states that the weight of a floating body is equal to the weight of the fluid that it displaces
36
how can law of flotation be shown
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
equipment to measure pressure
barometer
| hydrometer
38
hydrometer
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
boyles law
states that at constant temp, the volume of a fixed mass of gas is inversely proportional to its pressure
P ∝ 1/V + notation
40
for a fixed mass of gas at constant temp (formula)
pV = k
k - a constant
41
boyle's law apparatus
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
boyles law formula
hardback
43
centre of gravity
the point at which the whole weight of a body appears to act is called its centre of gravity
44
equilibrium
objects can be considered to have a stable, unstable, or neutral equilibrium
45
stable design
the principle of stable design is to have a low centre of gravity and a wide base
46
levers
a lever is a rigid body that is free to rotate about a fixed point, called a fulcrum
47
moment of the force
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
moment of a force formula
M = Fd
| force x perpendicular distance from the point to line of action of the force
49
principle of moments
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
vector sum
the vector sum of the moments about any point is zero
51
2 conditions for equilibrium
/ laws of equilibrium
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
couples (torque)
two parallel forces with the same magnitude acting in opposite directions are called a couple
53
resultant of couples
he resultant of these two forces is zero
54
moment of a couple
the moment of a couple is called the torque of the couple
55
symbol + unit for moment of a couple
symbol: T
unit: Newton Metre (Nm)
56
formula for moment of a couple
T = Fd
57
example of couples
taps and doors
handlebars on a bike
moving coil meters and simple motor
58
gravitation
the attraction that all masses in the universe have for each other
59
What did Newton publish and when?
1680 - published his law of gravitation
60
newton's law of universal gravitation
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
newtons law of universal gravitation formula
hardback
62
gravitational force
- always attractive
- size of force on each mass is the same
- distance between two bodies is the same between their centres
63
gravitational force number
G = 6.7 x10⁻¹¹ Nm²kg⁻²
64
gravitational force between two bodies
gravitational force between two bodies is negligible unless at least one of them has a very large mass
65
gravity and weight equations
hardback
66
objects above the surface of the earth
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
how to demonstrate law of Flotation
- 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
Diving & pressure - The Bends
- 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
The Bends cure
place deep sea divers in decompression chamber
70
demonstrating atmospheric pressure
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
Boyles Law proportions
- If pressure doubled, volume halved
- Pressure trebled, volume decreased to a third
- Pressure quartered, volume increased four times
72
what keeps the earths atmosphere attached to the earth?
the force of gravity.
Also occurs on our moon and explains why moon has no atmosphere
73
show that f=ma is a special case of Newton's second law
write derivation of f=ma
74
when figuring out centripetal force, etc
use the centripetal acceleration for a in f=ma
75
A toy is knocked over, it always returns to the upright position. Explain why
has a turning moment about fulcrum / has zero turning moment when toy is in vertical position
76
conditions necessary for the equilibrium of a body under a set of co-planar forces
- sum of forces = zero / forces up = forces down
| - sum of the turning moments about any point = zero / clockwise turning moments = anticlockwise turning moments
77
centripetal force on a merry go round
friction
78
if there was no force of friction on someone on a merrygoround, what direction would they move as the merry go round rotates?
they would remain stationary
79
relationship between period and radius
T² ∝ R³
80
a rotating object can be in equilibrium. Explain how this can happen
constant angular velocity
81
what a zero net force tells you about someone's motion
moving at constant speed (or at rest)
82
is pressure a vector or scalar quantity? why?
Scalar
It acts in all directions / it has no direction
83
why is it easier to turn a nut using a longer spanner than a shorter one
greater torque
84
A hammer is spun in a circle at constant speed, why does it accelerate even though it moves at constant speed?
direction changes
85
if you exert a force on something, what is the force it exerts on you?
the same force, in the opposite direction (make sure to write "in the opposite direction")
