4Q15 5week Test Notes
Fg
Concept 1: Gravity

m1m2
2
r
How do I increase the force of gravity on an object?
Change:_____Mass of m1 ___________________________________________________
Change: ___Mass of m2______________________________________________________
Change:____The distance r between them_____________________________________
Concept 2: Weight and Mass
What is mass?__The amount of matter in an object (count of electrons, protons, neutrons)
What is weight?___The pull of gravity on a mass__________________________
Concept 3: Inverse square law
What is the inverse square law?_The rule affecting certain phenomena in which the strength
of the phenomena decreases as the square of the distance
increases.________________________________________________________________
Name some examples of things it applies to:_____Gravity, light, sound, the effect of electric
charge_____________________
________________________________________________________________________
Problems:
1) What is the mass of a 10kg object on Jupiter (where gravity is 3x stronger than the
Earth’s?)_________________10kg___________________________________________
___
2) What is the weight of a 10 pound object on the moon (where the force of gravity is 1/6
that of the Earth?)_________10/6 pounds_____________________________________
__________________________________________________________________________
3) If I double the mass of an object, how does that affect the force of gravity acting on the
object?_____________2x___________________________________________________
4) If I double the mass of the Earth, how does that affect the weight of my 10 pound cat?
_____________2x_________________________________________________________
5) If I double the distance between my cat and the Earth, how does that affect his weight?
_________Reduces by 2-squared (4 times)___________________________________
What is my cat’s new weight?___(10/4) pounds________________________________
6) If I triple the distance between my cat and the Earth, how does that affect his weight?
_____________reduces by 3-squared (9 times)_________________________________
What is my cat’s new weight?____(10/9) pounds_____________________________
7) What is my cat’s name?_________Mwane__________________________________
8) What is the mass of my 10 pound cat? (Hint: The mass of a 1 pound object is 1/(2.2kg)
on the Earth’s surface.________(10/2.2) = 4.5 kg_______________________________
9) Why do I need to tell you where my cat is before you can answer the question above?
Mass does not change, weight is totally dependent on where in the universe you are.
Concept 4: Fields around magnets
Magnetic fields are strongest at the ends of a magnet. Like poles repel, unlike poles attract.
There is no such thing as a single N or S pole – every magnet has both.
1. What can be changed to increase or decrease the current induced in a coil through which
a magnet is being moved?
a. Change velocity (increase)
b. Change the strength of the field (increase)
c. Change the number of coils (increase)
Go to: http://phet.colorado.edu/en/simulation/faraday to review this further.
Concept 5: When a generator is connected to a circuit, it is harder to turn than when it is not
connected. This is largely because when it is connected, you need energy to push the
electrons, which increases the difficulty in moving the generator. What you are feeling is
the generator “pushing back” to try to keep you from moving the electrons. So…push
harder!
Concept 6: A current through a wire produces a magnetic field. A wire moving through a
magnetic field produces a voltage.
Field around a wire: http://www.fed.cuhk.edu.hk/sci_lab/Simulations/phe/mfwire.htm
Concept 7: Magnetism starts at the level of the atoms. Magnetic material forms in domains.
In a permanent magnet, these domains stay fixed. In a “soft” magnet, the domains are
ordered only while there is magnetic field.
In a non-magnetic material, there are no domains.
A
B
C
Concept 8: Newton’s Law of cooling: The temperature of an object cools fastest the farther
it is away from the ambient (room) temperature. Or, the temperature of an object heats
up fastest the farther it is away from the ambient (room) temperature.
Below is a graph of an object cooling in a room.
Temp ( C )
90
Temperature in Celsius degrees
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
Time in Minutes
Approximately how fast is the object cooling when it starts?____2 degrees per minute_____
___________________________________________________________________________
Approximately how fast is the object cooling at the end of the period?_1/2 degree per minute
___________________________________________________________________________
If the room temperature is 25C, what is your best guess as to how long it will take the object
to get to room temperature?
a) About 20 minutes
b) About 40 minutes
c) It will never get there.
Concept 9: Convection moves heat away from a heat source by causing a fluid to expend,
become less dense, and rise.
Concept 10: Radiation uses light (usually non-visible infrared light) to carry heat in the form of
energy. Thermal radiation acts just like light – moves in a straight line and is best absorbed
by black objects. Thermal radiation diminishes as the distance increases from the source due
to the inverse-square law.
1) If I double the distance between me and a heat source (say, the Sun), how does the
amount of energy I receive change?____Reduces it by 2-squared (4)________________
2) If I triple the distance between me and a heat source (say, the Sun), how does the amount
of energy I receive change?_____ Reduces it by 3-squared (9)______________
3) If I quadruple the distance between me and a heat source (say, the Sun), how does the
amount of energy I receive change?_ Reduces it by 4-squared (16)_______________
Concept 11: Bernoulii: Bernoulli’s principle states that the faster a fluid is moving, the lower its
pressure.
For an airplane wing, that means if the air is moving faster on top of the wing, the
pressure is lower at the top than at the bottom, and the wing has “lift”.
For a house, that means a fast moving stream of air going across the roof may serve to lift
the roof off the house.
For a spoiler, that means that the fast moving stream of air below the spoiler will provide
negative “lift” and serve to push the tires more firmly to the road.
Concept 12: Temperature scales
There are three major temperature scales in use: Fahrenheit, Celsius, and Kelvin. Fahrenheith is
the common temperature scale in the USA, while Celsius is common in the rest of the world.
Kelvin is similar to Celsius but starts at a different point (absolute zero) instead of the freezing
point of water.
F = (9/5) C + 32
C = (5/9) (F - 32)
K = C + 273, or C = K -273
The following table shows some points of interest regarding these three scales:
Fahrernheit
Celsius
Kelvin
Boiling point of
212°F
100°C
373K
water
Body temperature
98.6°F
37°C
310K
Room temperature
70°F
21°C
294K
Freezing point of
32°F
0°C
273K
water
Absolute zero
-460°F
-273°C
0K