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Density
Density is the ratio of the mass of an object to its volume
Density = Mass
Volume
This equation can be written in different ways to solve for density, mass or volume.
D = m/V
m=DxV
V = m/D
Common units that we use when measuring density:
g/cm3, g/mL, kg/m3, kg/L
If something has a large mass compared to its volume, it has a high density.
This is like a set of weights which can be small but heavy.
But if an object has a small mass compared to its volume it has a lower density.
This is like a piece of wood which can seem light for its size.
Different types of plastic, metal, wood, and other materials have different densities. The density of a material is based
on the atoms or molecules the substance is made from. For example, a copper and an aluminum cube of the same
volume feel very different when you hold them. The copper cube feels much heavier than the aluminum cube. If you put
them on a balance, you see that the copper cube has more mass than the aluminum cube. Since the cubes have the
same volume and the copper has a greater mass, the copper cube is more dense than the aluminum cube. This is true
since D = m/v. If the cubes have the same volume, the one with the greater mass must be more dense.
If you compared the density of a copper cube and a plastic
cube of the same volume, the copper is more dense. This is
because copper is made from small massive atoms that are
packed closely together. This gives copper a fairly high density.
Plastics are made mostly of carbon and hydrogen which are
not as massive for their size. They are connected together in
long chains and not packed as tightly as the atoms in copper.
This makes plastic less dense than copper.
Density in sinking and floating
The density of an object and the density of the liquid it is placed in determine whether an object will sink or float. Even
though one object might be heavier than another, the heavy one might float and the lighter one might sink. An example
is a small piece of clay and a heavier block of wood placed in water. Even though the wood is heavier than the clay, the
wood floats and the clay sinks. This is possible because its not the mass of the object that matters in sinking and floating
but its density compared to the density of water.
An object that is less dense than water will float. An object that is more dense than water will
sink. This is why the clay sinks in water and the wood floats. The clay is more dense than water and
the wood is less dense than water.
Measuring Time
The SI base unit for time is seconds (s)
Units used to measure time:
Seconds (s), minutes (min), hours (hr), days, weeks, months, years
Tools or instruments used to measure time:
Clock, watch, stopwatch, hour glass, iphone
How many hours are there in 3 months?
3 week x 7 day/week x 24 hr/day = 504 hr
Measuring Temperature
Metric units for measuring temperature:
Celsius (°C) and Kelvin (K)
Instrument used to measure temperature:
Thermometer
In the laboratory we measure temperature using the Celsius Scale, a
scale made by defining the normal freezing point of water as 0°C and
the normal boiling point of water as 100°C. This gives us 100 divisions
between the freezing point and the boiling point. Each division is 1°C.
The Kelvin scale is very similar to the Celsius scale in that the scale is
set such that normal freezing point of water is 273K ( 0 + 273 = 273)
and the normal boiling point as 373K (100 + 273 = 373). Thus both
scales are very similar.
The third scale, the Fahrenheit scale is different but the basic idea is
the same. With this scale the normal freezing point of water is set at
32°F and the normal boiling point at 212°F. Thus this sale has 180
divisions between the freezing point of water and its boiling point.
Thus 1°F is a smaller division that 1°C. (In fact it is 100/180 or 5/9
times smaller).
K = °C + 273.5
°C = K – 273.5
°C = 5/9 (°F – 32)
°F = (1.8 x °C) + 32