EXPERIMENT 5 – Work and Energy
OBJECTIVE
The purpose of this experiment is to apply the concept of the work and energy calculate the work done
by the force of friction between a car and a wooden board as they slide relative to each other at constant
velocity. We study it for the case of the car moving up and down the incline.
MATERIALS
Wooden Board
Slotted weights
Balance
Protractor
Car
Weight hanger
Pulley
INTRODUCTION
In general, friction is the force that slows down the motion of an object. The force of friction is directed
along the surface of contact between the object and surface and directed opposite to the direction of
motion of object.
Car moving up:
For the car of mass M moving up the incline at constant
speed, we can show, from 2nd law of motion, that the force
of friction f is given by:
f = ρg – Mgsinθ
(1)
where ρ is the mass hanging from the other side of the
pulley which makes the car move up.
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Car moving down:
Now, for the car of mass M moving down the incline at constant speed, we can show,
from 2nd law of motion, that the force of friction f is given by:
f = M g sinθ – ηg
(2)
where η is the mass hanging from the other side of the
pulley which makes the car move down up.
And finally the work done by friction becomes:
Wf = f d
(3)
Where d is the distance the car travels along the board.
EXPERIMENTAL PROCEDURE
1)
Determine the mass M of the car and record it..
2)
Set up the board and the car as shown in figure 1 with an incline angle of 300 with the table.
Position the car near the bottom of the incline and record its initial height h. Add enough weights
of mass ρ on the weight hanger to make the car move up the incline at a very slow speed after
given a slight tap. Record the initial height H of this weight. Measure also the distance d that the
car travels along the board.
3)
When the car reaches near the top of the incline, record now its height h and that of the weight
hanger H. Start removing weights from the weight hanger until the car starts moving down the
incline at a slow speed after given a small tap. Record this mass η.
4)
Repeat the above procedures for an incline angle of 600.
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EXPERIMENT 5 – Work and Energy
REPORT FORM
Name ______________________
Professor _____________________
Date _____________________
Part 1
Car moving up the incline
Mass M of car ____________
Angle of
incline
300
Mass ρ
d
Initial height
of car, h
Initial height of
suspended mass, H
f
Wf
Initial height of
suspended mass, H
f
Wf
600
Part 2
Angle of
incline
300
Car moving down the incline
Mass η
d
Initial height
of car, h
600
Percent difference in Wf for θ = 300 _________
Percent difference in Wf for θ = 600 _________
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CALCULATION
1) Compute the frictional force f and the work Wf done by friction for the incline angle of 300.
2) Compute the frictional force f and the work Wf done by friction for the incline angle of 600.
3) For each angle, compute the percent difference in the two values of the work Wf .
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EXPERIMENT 5 – Work and Energy
Review Questions and Exercises
Name _____________________
Due before lab begins. Answer in the space provided.
Date ___________________
1) Write down the conservation of the mechanical energy of a system?
Is it always conserved? Explain.
2) Derive equations ( 1 ) and ( 2 ).
3) What are the main assumptions about the pulley in this experiment?
4) How does the force of friction in this experiment depend upon the angle of the incline?
5) Explain why the total energy of a system can be either positive or negative, whereas the kinetic
energy is always positive.
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EXPERIMENT 5 – Work and Energy
Post- laboratory Questions and Exercises
Name __________________
Due after completing lab. Answer in the space provided.
Date _____________________
1) Compute the percent energy lost to friction for the car moving up the incline for both angles.
2) For θ = 600, compute the work done by each force acting on the car. Should they add up to zero?
Explain.
3) For θ = 300, compute the normal force acting on each wheel of the car?
4) Is friction in this experiment only due to the car? Explain.
5) Show that, for the car moving up the incline, that the coefficient µ of friction is given by:
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