PASCO ME-6964 PAStrack Wheel Set - Manual (Page 5)

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M o d e l N o . M E - 6 9 6 4 S u g g e s t e d A c t i v i t i e s
5
4. Raise the right end of the stationary track slightly and repeat step 3. The
cart-track system should now move to the right slower after the cart stops run-
ning. Adjust the incline of the track and repeat step 3 until the cart-track system
stops when the cart stops running.
You should find that cart-track system remains stationary if you hold it and let go
very carefully (because static friction is higher than kinetic friction), and it will
travel at a constant velocity if you nudge it to left (because the component of
gravity parallel to the track cancels the friction).
1
Procedure
1. Place the PAStrack with wheels on the stationary track and place the cart at the
left end of the PAStrack with wheels. Using tape, mark:
• the position of the cart on the PAStrack with wheels,
• the position of the cart on the stationary track, and
• the position of the PAStrack with wheels on the stationary track.
2. Press START on the Time Pulse Accessory and allow the cart to drive across the
track from left to right as the track moves in the opposite direction.
3. After the cart stops, measure:
•
∆x
cart-track
, the displacement of the cart on the PAStrack with wheels;
•
∆x
cart
, the displacement of the cart on the stationary track;
•
∆x
track
, the displacement of the PAStrack with wheels on the stationary
track.
Analysis
1. What is the relationship between
∆x
cart-track
,
∆x
cart
, and
∆x
track
?
(Hint: do
∆x
cart
and
∆x
track
have the same sign or opposite signs? How should
they be combined to equal
∆x
cart-track
?)
2. The change in position of the center of mass of the cart-track system is
where M is the mass of the system, m
cart
is the mass of the cart, and m
track
is the
mass of the PAStrack with wheels. Measure the masses and calculate . Did
the system’s center of mass change significantly?
3. Add a 250 g mass to the cart and repeat the experiment. Move the mass from the
cart to the track (as pictured) and repeat the experiment again.
In the three cases, what variables change? What variables do not change
significantly?
1 Another way to counter-
act friction is to attach a
small mass hanging on a
string over a pulley.
Adjust the mass so that
the system stops when
the cart stops running, or
use a photogate to moni-
tor the speed of the pul-
ley, and adjust the mass
so that it moves at a con-
stant velocity when you
nudge the track.
∆x
cm
1
M
----- m
cart
∆x
cart
m
track
∆x
track
+
( )
=
∆x
cm