Download Inelastic Collisions

Inelastic Collisions
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Equipment:
Computer running LoggerPro3: Two-Gate Timer Program Running in Gate Mode.
(2) Photogates linked to computer for data acquisition.
Dynamics Track with (2) Dynamics Cars, (1) Flag and (1) Gram Mass Set
Introduction:
When two objects collide they share a single interaction which creates equal sized forces
directed in opposite directions. This interaction may speed-up one object and slow-down the
other object, or, the interaction may slow-down both objects such as in a head-on collision of
two cars. The force-time product of this interaction is known as “impulse” is defined as the size
of the average force multiplied by the time-length of the interaction. The impulse size is equal
to the size of the change in momentum. By Newton’s 3rd Law, each object receives equal sized
but oppositely directed changes in momentum.
A. Pre-Lab Questions.
The following data was taken with two frictionless cars moving on a level surface. Car1, mass
4kg moves in +x direction with +3.0m/s velocity. Car1 collides with Car2, mass 2kg, which is
initially at rest. Car1 and Car2 each move with a velocity of +2.0m/s after the collision.
A1-A3) Calculate the SI value of each cell in the table below. Show the calculation and the
value in the cell.
Table1
Object
Initial (before collision)
Final (after collision)
Final – Initial
p1 Car1
p2 Car2
Totals 
A4) Explain why the overall momentum of both masses considered together was the same
before and after the collision.
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A5) Calculate the change in velocity of Car1 by filling in the blanks
v1 = v1f – v1i = ____________ – ______________ = __________________
A6) Calculate the change in velocity of Car2 by filling in the blanks
v2 = v2f – v2i = ____________ – ______________ = __________________
You should have found that the sizes of the changes in velocity were not equal. On the other
hand, the momentum changes were equal in size but opposite in direction. The momentum
concept was introduced due to this observation.
A7-A8) Calculate the kinetic energy of each car before and after the collision. Show
calculation and SI answer in each cell.
Type
Table 2. Write Energies in Joule Units
Initial (before collision)
Final (after collision)
KE Car1
KE Car2
Thermal
0
Utherm =

Column
Totals 
A9) Calculate the total energy before the collision and enter it into Table 2. Use energy
conservation to calculate Utherm. Show your calculation here.
A10) Was the increase in kinetic energy of Car2 equal in size to the decrease in KE of Car1?
Why or why not?
A11) Was kinetic energy conserved in the collision? ____ Explain.
A12) Was energy conserved in the collision? ______ Explain.
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B. Experiment: Complete Inelastic Collision.
Load Car1 with 800grams added mass. Car2 will be empty of extra mass and at rest before the
collision. We will assume that the Car1 moves in the +x-direction.
Car1 mass m1 = _____________grams = ______________kg
Car2 mass m2 = _____________grams = ______________kg
v1i = ______________m/s
v2i = 0m/s
v1f = v2f = ________________m/s
Calculate the following, showing calculation and units:
B1) p1 = m1(v1f – v1i) = (________)(_______– _________) = __________kgm/s
B2) p2 = m2(v2f – v2i) = (________)(_______– _________) = __________kgm/s
B3) Is it true that p1 and p2 are nearly equal in size but opposite in sign?_____
B4) What is the direction of the collision force on Car1 with respect to its initial (+x) velocity ?
Is the collision force parallel or anti-parallel (opposite) to v1i?
B5) Does the collision force speed-up or slow-down Car1?
B6) Assuming the collision lasts 0.0055 seconds, calculate the size of the average collision
force that acts on Car1 using Favg = p1/t. Show formula, calculation, and units.
B7) Assuming the collision lasts 0.0055s calculate the size of the average collision force that
acts on Car2 using Favg = p2/t. Show formula, calculation, and units. What direction does
this force act on Car2?
B8) If the collision-time were 10 times greater, all other things remaining the same, how would
this affect the values of the collision force calculated in the last two questions?
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C. Momentum and Velocity
Transfer your data from the previous page to the spaces below.
Car1 mass m1 = _____________grams = ______________kg
Car2 mass m2 = _____________grams = ______________kg
v1i = ______________m/s
v2i = 0m/s
v1f = v2f = ________________m/s
Make the following calculations of the system-momentum P:
C1) Pi = m1v1i + m2v2i = (______)(_______) + (______)(_______) =___________kg·m/s
C2) Pf = m1v1f + m2v2f = (______)(_______) + (______)(_______) =___________kg·m/s
C3) Estimate the percent change in system momentum: ______. Was this an increase or a
decrease in system momentum?___________What experimental factors might have caused this
change?
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C4) Calculate the following energies:
Ki = ½m1(v1i)2 + ½m2(v2i)2 = ½(______)(______)2 + ½(______)(_______)2 =___________J
Kf = ½m1(v1f)2 + ½m2(v2f)2 = ½(______)(______)2 + ½(______)(_______)2 =___________J
C5) Estimate the percent change in system kinetic energy: ______. Was this an increase or a
decrease in system kinetic energy? ___________ Where did this energy go to or come from?
_____________________________________________________________________
C6) How much thermal energy was created in the collision? ________________. Where did
this energy come from?
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