PH1004
Exp 2: Forces
1
PH 1004 Laboratory Instructions
Experiment 2
Forces
Background
Newton’s first law states that an object at rest remains at rest unless acted upon by
an outside force, and an object in motion continues to travel at constant velocity unless
acted upon by an outside force.
This law is also referred to as the law of inertia.
Newton’s second law deals with force.
Force can be defined as an external influence
(sometimes described as a push or a pull) on an object that causes it to accelerate relative
to an inertial frame of reference.
The direction of the force is the same as the direction of
the acceleration, assuming that there are no other forces acting on the object.
The
magnitude of the force is equal to the acceleration of the object times its mass.
Thus,
mass is a measure of an object’s inertia, or resistance to acceleration.
Experimentally it is
observed that forces add as vectors, and therefore, all the forces on an object act together
like one net force.
Newton’s Second Law can then be written as
∑
=
=
a
m
F
F
net
r
r
r
.
(21)
In this experiment, we use measurements for the acceleration of the glider to
calculate the acceleration due to gravity,
g
.
As in Experiment 1, an air track will be used.
In Part A of this experiment, the
force on the glider is again exerted by a string, which is connected to the glider, draped
over a pulley, and attached to the hanging mass.
In Part B, one end of the air track will
be raised so that the glider will accelerate toward the other end.
In this way, the force
that causes acceleration is a component of the gravity force.
Apparatus
Required equipment:
1.
An air track assembly including air supply pump, and other accessories.
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PH1004
Exp 2: Forces
2
2.
A glider with a flag.
3.
Two photogates, labeled G1 and G2.
4.
Hanging weights.
5.
A string (to attach the glider to a weight via a pulley).
6.
Labjack (for raising one end of the air track).
7.
A data acquisition box and a computer.
Part A. Pulling Force
A pulling force will be applied to the glider on the air track using a string passed
over a pulley and a hanging mass. The relationship between the glider’s acceleration
a
and the gravitational acceleration
g
is given by
g
m
m
m
a
gl
+
=
(22)
where
m
is the mass of the hanging weight, and
m
gl
is the mass of the glider.
Equations
23 that represent Newton’s Second Law for the horizontal and vertical components of
the net forces acting on the glider and hanging mass respectively, explain how equation
22 is derived:
a
m
m
mg
ma
T
mg
a
m
T
gl
gl
)
(
+
=
∴
=
−
=
(23)
Note from equation 22 that glider acceleration,
a
, is expected to be a linear function of
the ratio
gl
m
m
m
+
, with zero intercept:
Based on equation 22 we develop the following method for determining the
acceleration due to gravity
g
:
•
Accelerations of the glider,
a
, are found for several values of the hanging mass,
m
.
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 Spring '08
 WORMER
 Physics, Force, Velocity

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