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Work – Energy
Physical Science 101
2/98
Name _____________________
Partner’s Name ___________________
Equipment
cart, track, hanger and masses, photogate, card, cushion, pulley, meter stick, lab
jack, string
Purpose
Investigate work and energy relationships by doing two different experiments.
Data
Show your work in the Data and Calculations space. Include all steps, labels
and remember your units.
Definitions
Gravitational Potential Energy = mgh
1
2
Kinetic Energy  mv2
Total Mechanical Energy = KE+PE
xy
 100
Percent Difference 
y
Part 1
m
g
h
m
v
mass
acceleration due to gravity
height above reference level
mass
speed
x
y
second value
first
value(most
value)
reliable
Gravitational Potential Energy
Purpose
In this experiment you will compare the gravitational potential energy stored in the
hanging mass to the kinetic energy that the system receives.
Procedure
1.
Set up the equipment as shown in Figure 1 on a level track.
cart
photogate
timer
M
card
20 g
track
mass - m
cushion
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h
Work – Energy
Figure 1
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Work – Energy
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2.
Place 20 g (0.02kg) on the end of a string (mass + hanger). This is mhanger.
3.
Determine cart’s mass. This is mcart.
4.
Determine the system mass. Msystem = mcart + mhanger=________________
5.
Pull the cart back so that the hanging mass is approximately 0.30 to 0.50 m above
the cushion. Measure the exact height to at least one decimal place. This is height
h.
h=________________ m
6.
7.
Compute the initial potential energy of the hanging mass relative to the cushion.
Remember : PE = mhanger g h
PE =_______________ J
Let the mass drop and measure the final velocity of the system using a photogate
timer. Compute the final kinetic energy of the system, using M system. The cart
should go through the photogate just after the mass hits the cushion.
m = Msystem=____________ kg
x = cardlength=____________ m
v
x
 _________ m/s
t
t = time from photogate =___________ s
1
KE  mv 2 =_____________ Joules
2
8.
Calculate the percent difference between the initial potential energy of the
mass (mhanger) and the final kinetic energy of the system (Msystem).
9.
Is total mechanical energy conserved?
Based on your data, would you say that energy was lost to friction? Explain
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Work – Energy
Part 2
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Potential and Kinetic Energy
Purpose
In this experiment you will compare the gravitational potential energy stored in the
cart to the kinetic energy that the cart receives by changing heights.
Procedure
1.
Set up the equipment as shown in Figure 2, on a inclined track. Measure the height
of the cart above the table at positions 1 and 2. h1 = _______ h2 = ________
Photogate
timer
1
2
Figure 2
2.
Compute the potential energy of the cart at each of the two positions.
m= mcart = ____________________ kg
PE1=mgh1=________________ Joules
PE2=mgh2=________________ Joules
3.
Compute the CHANGE in potential energy between the two positions.
PE = PE1 - PE2 = ____________Joules
4.
Place the cart at position 1 and release it from rest. Use the photogate timer to
calculate the velocity of the cart at position 2. Remember : v = x / t, where x =
length of the card and t = time from the photogate and m= mcart .
v
5.
x
1
 _________ m/s KE 2  mv2 =_____________ Joules
t
2
What is the kinetic energy of the cart at positions 1 and 2?
KE1=________________ Joules
KE2=________________ Joules
6.
Calculate the TOTAL energy, E = KE + PE of the cart at positions 1 and 2.
E1=________________ Joules
E2=________________ Joules
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Is energy conserved? Explain.
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Work – Energy
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Another important idea is the work energy theorem. It states that the work done by the
net force acting on an object equals the change in Kinetic Energy of the object.
Work = ∆ Kinetic Energy
Since Work is defined to be Force multiplied by the distance an object moves in the
force’s direction, the work energy theorem can be restated as,
Force x Distance = ∆ Kinetic Energy
Questions
1. How much work is done by an 80 N horizontal force which moves a cart 3.0 m?
2. What is the kinetic energy given to a car if it is pushed by a 20.0 N force over a
distance of 10.0 m?
3. What is the kinetic energy gained by a 20.0 kg rock just before hitting the ground if it
falls from a height of 2.0 m above the ground? Hint: what is its original potential energy
relative to the ground?
4. A rocket is projected vertically with 60 J of kinetic energy. As it goes up and comes
back down the air resistance force does -20 J of work on the rocket. What is the kinetic
energy of the rocket just before landing.
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