In-Class Assignment: solution
Name_____________________
Evaluate the motion of a 2.00 kg hammer that weighs 19.6 N, a 0.05 kg tennis ball that
weighs 0.49 N and a 0.01 kg feather that weighs 0.1 N dropped from a height of 4.9 m on
Earth and on the Moon.
1.
Discuss differences in gravity and air resistance associated with the Earth and
the Moon.
Gravity: The Earth’s gravitational force field is
stronger than the Moon’s because the Earth is much
larger (has greater mass). The stronger gravitational
force field on Earth causes a higher rate of
acceleration (g = 9.8 m/s2) than the weaker
gravitational force field on the Moon (gmoon = 1.6
m/s2).
Air Resistance: The Earth is surrounded by an
atmosphere of gas, which resists the motion of objects
that move through it. The Moon has a trace of gases,
but its “atmosphere” is a million times less dense and
causes no air resistance.
2.
Discuss the role of gravity on falling objects.
The gravitational force causes an object to be
accelerated towards the surface of the Earth and Moon.
Less gravity means less acceleration.
Gravity also gives an object its weight
(w = mass x gravity)-- this is Fgrav
3.
Discuss the two factors upon which air resistance depends.
Air resistance depends upon the frontal (surface) area
of an object and the speed of an object moving through
the air. An increase in either will increase air
resistance.
4.
Discuss the physical characteristics (including frontal area and weight) of each
falling object.
The hammer:
largest frontal area, greatest weight
The tennis ball: smaller frontal area than hammer,
less weight than hammer
The feather: smaller frontal area than hammer, least
amount of weight.
5.
Discuss how these three objects would fall on Earth and why. Draw three
free-body diagrams illustrating the forces acting on each object.
The hammer would fall the fastest because it has the
greatest net force and acceleration (difference
between air resistance and weight). The tennis ball
would also fall fast, but it has less weight, so its
has less net force and less acceleration.
The feather would likely reach terminal velocity
quickly (where air resistance balances weight and
net force and acceleration equal zero.
Hammer
Hammer has greatest
net force and acceleration
6.
Tennis Ball
Feather
Tennis ball is accelerating Feather is at terminal vel.
(zero net force and accel)
Discuss how these three objects would fall on the Moon and why.
Since there is no air resistance on the Moon, all
objects would be in free fall at 1.6 m/s2. This
means that they would all hit the ground at the same
time if released simultaneously from the same
height, but at a slower speed compared to objects
free falling in a vacuum on Earth.
7.
Calculate how long it would take the hammer to free-fall 4.9 m on Earth and
how long it would take the feather to fall 4.9 m on the Moon.
t =
√2d/g
(use g for earth and moon as given in question 8)
Fall Time on Earth = 1 s
Fall Time on Moon = 2.5 s
8.
Calculate the maximum (free-fall) speed for the hammer on the Earth and
Moon. (v = gt) g = 9.8m/s2
g moon = 1.6 m/s2
Maximum falling speed on Earth
after traveling 4.9 m =
9.8 m/s
Maximum falling speed on Moon
after travelin 4.9 m =
4.0 m/s
(9. 8m/s2 x 1 s)
(1.6 m/s2 x 2.5 s)
Falling Objects
Textbook
Two paper plates
Coffee Filter
Is it possible for a falling object to reach terminal velocity on the Moon? Explain.