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PHS PHYSICS 1 2017 - GENERAL PHYSICS (LENGTH AND TIME (VERNIER CALLIPERS…
PHS PHYSICS 1 2017 - GENERAL PHYSICS
LENGTH AND TIME
measuring cylinders measure volume
VERNIER CALLIPERS
Vernier callipers are used to measure smaller, more precise distances (in mm)
It is an extra sliding scale fitted to some length-measuring instruments. Its divisions are set slightly closer together than normal so that one of them coincides with a division on the fixed scale.
Read the highest scale division before the 0 on the fixed scale.
Eg. 7
See were the lines coincide, read this on the sliding scale and put a decimal point in front of it.
Eg. 0.4
Add the two together.
7 + 0.4 = 7.4 mm
MICROMETERS
Micrometers are used to measure smaller, more precise distances (in mm)
This has a revolving barrel with an extra scale on it. The barrel is connected to a screw thread and, in the example shown, each turn of the barrel closes or opens the gap by half a millimetre. First, the gap is opened wide. Then it is closed up until the object being measured just fits in it (a clicking sound is heard).
Read the highest scale division that can be seen on the fixed scale.
Eg. 5.5
Read the scale on the barrel that lines up with the line, putting a decimal in front of it.
Eg. 0.32
Add the two together.
5.5 + 0.32 = 5.82 mm
clocks/stopwatches measure time
rulers are used to measure length
PENDULUM
The pendulum is a more accurate way of measuring time
The time it takes for the bob to complete one swing is called its period. You can find it accurately by measuring the time for 10 swings and divide that by 10.
ENERGY, WORK AND POWER
ENERGY
Energy is the ability to do work and brings about change.
It is measured in
Joules (J)
CONSERVATION OF ENERGY
Energy cannot be made or destroyed, but it can change from one form to another
When energy changes from one form to another, it is
transformed
. The diagram above shows a sequence of energy transformations.
During each transformation, the total amount of energy stays the same.
To do work, you have to spend energy. But, like money, energy doesn't vanish when you spend it, it goes somewhere else. People talk about 'using energy', but energy is never used up. It just changes into different forms.
TYPES OF ENERGY
Potential Energy (stored)
elastic
gravitational
chemical
nuclear
Active Energy
kinetic
heat
light
sound
electrical
In reality, energy is lost from the system at different stages. For examples, muscles convert less than 1/5 of the stored energy in food into kinetic energy. The rest is wasted as thermal energy - which is why exercises makes you sweat. And when objects move through the air, some of their kinetic energy is changed into thermal energy because of friction (air resistance). Even sound is eventually 'absorbed', which leaves the absorbing materials a little warmer than before.
WORK DONE + ENERGY TRANSFORMED
Work is done whenever a force makes something move. The greater the force and the greater the distance moved, the more work is done (Joules J).
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work done = force x distance moved in the direction of the force
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W = F x d
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Eg. if a 4 N force moves an object 3 m, the work done is 12 J.
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work done = energy transferred
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energy can be converted from one form to another (transform)
or it can be passed from one object to another (transfer)
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kinetic energy (J) = 1/2 mass (kg) x velocity ^2 (m/s)
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gravitational potential energy (J) = mass (kg) x gravitational acceleration (N/kg - 10) x height (m)
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MAJOR AND ALTERNATE SOURCES OF ENERGY
Energy
can be converted from
one form to another
including
chemical/fuel energy (regrouping of atoms)
energy from water (hydroelectric energy, waves, tides)
geothermal energy
solar energy
nuclear energy (fission of heavy atoms
wind energy (wind turbines, windmills, sailing boats and ships)
Efficiency
is a measure of how much
work
or
energy
is
conserved
in a process
Energy
can be
released
by
splitting heavy nuclei
(fission). Energy can also be released by
fusing
(joining) very light nuclei together to make heavy ones.
Fusion in the sun's core
happens when energy is released as hydrogen fuse together and convert into helium
:star: E = mc^2
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energy = mass x speed of light (3x10^8 m/s)
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SPEED, VELOCITY AND ACCELERATION
SPEED
Speed is the
rate
of how fast something is moving, or the rate of change of distance
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speed = distance
time
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(units: m/s = ms-1)
VELOCITY
Velocity is speed with a direction, it has a
vector
property
:star: velocity = distance (in a direction)
time :star:
MOTION GRAPHS
DISTANCE - TIME GRAPHS
the gradient is equal to the speed in a distance - time graph
travelling at a constant speed
accelerating at a constant rate
no distance travelled, stopped
SPEED - TIME GRAPHS
the gradient of the line is equal to the acceleration
the area under the line is equal to the distance travelled
moving with a constant speed
moving with constant acceleration
moving with changing speed,
not constant acceleration
when an object is at rest, the line is at 0
ACCELERATION
Acceleration is the rate at which speed/velocity is changing
:star: a = V - U
t :star:
(where V = final velocity
U = initial velocity
t = time
a = acceleration)
ACCELERATION DUE TO GRAVITY
All objects on Earth are attracted by its mass, so they fall to the surface at a constant acceleration of 9.8m/s^2. We round it off to 10m/s^2.
TERMINAL VELOCITY
Gases in the atmosphere resist the downwards force of gravity. They exert an upwards force. When the upwards air resistance is equal to the downwards gravitation force, the object continues to descend at a constant velocity
MASS AND WEIGHT
MASS
Mass is a body of matter, the gravitational force does not affect somethings mass.
It is measured in kg
Mass is a property which
resists
change in motion
A gravitational field is a region in which a mass experiences a gravitational attraction. It is not uniform, it will get weaker when you move away.
An old pan balance is an advantage as it can be used in space.
The inertial balance does not need gravity.
WEIGHT
Weight is the Earth's gravitational
force
on an object
It is measured in Newtons
To calculate weight from mass:
:star: weight = m x g :star:
(m = mass
g = gravitational acceleration (10))
Weights and hence masses can be compared using a balance
FORCES
EFFECTS OF FORCES
A force may produce a change in the size and shape of a body
Elastic materials will spring back to its original shape after being stretched
The
zone of proportionality
is the zone of which a spring can be stretched
The
limit of proportionality
is the limit at which it can no longer stretch as it is carrying an object too heavy
If an object was given a single force to start it moving, it would keep on going forever at the same velocity as there is no friction and air resistance.
Two forces accelerating an object at the same point would equal both of the forces added together.
If two different forces were applied in opposite directions, the resultant unbalanced force causing the acceleration is greater force minus the smaller force.
Newtons Second Law:
An unbalanced force (f) applied to an object of mass (m) will cause it to accelerate (a)
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Force = mass x acceleration
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TURNING EFFECTS
The moments are the turning effects of a force
Eg. a wrench screwing in a screw
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Moment = force x perpendicular distance
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Equilibrium is when
moments on either
side of the pivot are equal
CENTRE OF MASS
The centre of gravity and mass is the point where the weight begins to act. Equally distributed.
The centroid is the geometric centre of a given shape
The centre of gravity is the point about which forces of gravity are balanced.
The centroid is equal to the centre of mass only when mass distribution.
Finding a centre of mass
In diagram 1, the card can be swung freely from the pin, when it is released, the forces turn the card until the centre of mass is vertically under the pin. As in diagram 2.
In diagram 3, the centre of mass lies somewhere along the plumb line, whose position is marked by the line AB. If the card is suspended at a different point a second line CD can be drawn. The centre of mass must also lie upon this line, so it is at the ponit where AB crosses CD.
Stability
If the box is pushed a little and then released, it falls back to its original position. Its position was
stable
. If the box is pushed much further, it topples. It starts to topple as soon as its centre of mass passes over the edge of its base. A box with a wider base/or a lower centre of mass is more stable. It can be tilted at a greater angle before it starts to topple.
States of equilibrium
Stable equilibrium
If you tip the cone a little, the centre of mass stays over the base. So the cone falls back to its original position.
Unstable equilibrium
The cone is balanced, but only briefly. Its pointed base is so small that the centre of mass immediately passes beyond it.
Neutral equilibrium
Left alone, the ball stays where it is. When moved, it stays in its new position. Wherever it lies, its centre of mass is always exactly over the point which is its base.
DENSITY
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density =
mass
volume
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units: g/cm^3 or g/mL
To find the density of a liquid:
Find the mass of an empty measuring cylinder. Pour liquid into the cylinder and find the volume. Weigh the cylinder with the liquid in it. Find the mass of only the liquid by subtracting the mass of the empty cylinder.
Use the mass and volume of the water to calculate density.
To find the density of a solid:
Weigh the solid to find its mass. Pour 100mL of water into a measuring cylinder. Place the solid into the water and measure the new volume. Subtract the new volume with the initial volume of the water.
Use the mass and volume of the solid to calculate density.
To find the density of an irregular solid:
Weigh the solid on a measuring balance to find the mass. Use a displacement cup and fill it with water up to the spout. Place a small measuring cylinder under the spout, so that it can catch water. Place the irregular solid into the displacement cup and wait until the water is finished displacing into the measuring cylinder.
