Physics 144 – Vaishnav
Spring 2010
How Things Work
Homework Set #2
Due
in class
Monday 2/8/06
Instructions
You are responsible for your own homework solutions.
Consider these problems as practice for the exam.
Consult me or other students when you have questions, but try to work the problems by yourself as much as you
can.
Collaboration is allowed, but you must
indicate your collaborators
, both in the class and outside of it.
For every problem,
show all your work
.
Simply writing down a final answer without showing your
reasoning and calculations is unacceptable.
Draw diagrams and sketches for problems, where useful.
For any
essay type questions, make sure to use complete sentences.
Problem 1:
(a)
Exercise 36 on p. 39
Look at Fig. 1.3.4, and pretend the piano is a snowflake.
If the snowflake is sitting on a sloped roof, two forces act
on it; its weight and the normal force from the ramp.
We can sum these forces like vectors to see that—as in Fig.
1.3.4—the net force acting on the snowflake is down the ramp.
On a flat roof, the weight points down and the
normal force points up.
These two forces cancel, and there is no net force on the snowflake, which therefore simply
sits there.
(b)
Exercise 38 on p. 39
As the truck goes up the ramp, its height is increasing.
The potential energy of an object at height
h
is
mgh
.
The
truck’s initial
kinetic
energy becomes
potential
energy as it moves up the ramp.
The truck’s total energy is
conserved.
(c) Problem 18, p. 40.
The potential energy of the water is mgh where m is the mass of water, and h is the height to which we raise it.
U
=
mgh
=
(1000kg)
⋅
(9.8m/s
2
)
⋅
(200m)
=
1.96
×
10
6
J
Problem 2:
A jogger stars a three part jog by running 0.24 km north, then 0.16 km east.
She then runs back to her
starting point along a straightline path.
Graphically determine the length of her path back.
We can draw the vector going north, the vector going east, and determine the length of the third vector using
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 Spring '06
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 Energy, Force, Work, net force, fan cart

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