Experiment
18
Work and Energy
Work
is a measure of energy transfer. In the absence of friction, when positive work is done on an
object, there will be an increase in its kinetic or potential energy. In order to do work on an object, it
is necessary to apply a force along or against the direction of the object’s motion. If the force is
constant, work can be calculated using the vector dot product
where
F
is the constant force and
s
the displacement of the object. If the force is not constant, we
divide the overall displacement into short segments,
s
, the force is nearly constant during each
segment. The work done during that segment can be calculated using the previous expression. The
total work for the overall displacement is the sum of the work done over each individual segment:
This sum can be determined graphically as the area under the plot of force
vs.
distance.
1
These equations for work can be easily evaluated using a force sensor and a Motion Detector. In
either case, the workenergy theorem relates the work done to the change in energy as
W
NC
= U + K
where
W
NC
is the work done (by nononservative forces),
∆
U
is the change in potential energy, and
∆
K
the change in kinetic energy.
In this experiment you will investigate the relationship between work, potential energy, and kinetic
energy.
OBJECTIVES
•
Use a Motion Detector and a force sensor to measure the position and force on a hanging
mass, a spring, and a dynamics cart.
•
Determine the work done on an object using a force
vs.
distance graph.
•
Use the Motion Detector to measure velocity and calculate kinetic energy.
•
Compare the work done on a cart to its change of mechanical energy.
•
Test the Conservation of Mechanical Energy Principle.
MATERIALS
Power Macintosh or Windows PC
dynamics cart
Vernier Motion Detector
masses (200 g and 500 g)
Vernier Force Sensor
spring with a low spring constant (10 N/m)
Universal Lab Interface
Tape
and rubber band
1
If you know calculus, you may recognize this sum as leading to the integral .
Physics with Computers
18  1
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View Full DocumentExperiment 18
Logger
Pro
wire basket (to protect Motion Detector)
PROCEDURE
Part I
Work When The Force Is Constant
In this part you will measure the work needed to lift an object straight upward at constant
speed. The force you apply will balance the weight of the object, and so is constant. The work
can be calculated using the displacement and the average force, and also by finding the area
under the force vs. distance graph.
1.
Connect the Vernier Motion Detector to PORT 2 and the Vernier Force Sensor to DIN 1 of the Universal
Lab Interface.
2.
Open “Exp18DRA” from the
Physics with Computers
experiment files of Logger
Pro.
Set the range
switch on the sensor to 10 N. Three graphs will appear on the screen: distance
vs.
time, force
vs.
time,
and force
vs.
distance. Data will be collected for 5 s.
3.
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 Spring '08
 Roberts
 English, Energy, Force, Kinetic Energy, Potential Energy, Velocity, Motion detector

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