Physics Topic 5.3 – Inelastic Collisions
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Learning Goal: You will understand how two objects behave after they collide in an inelastic collision.
Success Criteria: You will know you have met the learning goal when you can calculate the velocities of objects after they collide in an inelastic collision.
We’ve talked about how elastic collisions are ones where two objects bounce off of each other. Inelastic collisions are ones where they stick together, such
as when a person catches a ball.
The main equation involving the velocity of the objects after they collide is:
m1v1i + m2v2i = (m1 + m2)vf
Notice that only one velocity is given after they collide because the two objects become one object.
Comparing and contrasting elastic and inelastic collisions:
Perfect Elastic Collisions
Objects bounce off each other
Objects are not damaged or deformed
Momentum is conserved
Kinetic energy is conserved
m1v1i + m2v2i = m1v1f + m2v2f
Perfect Inelastic Collisions
Objects stick together
Objects are sometimes damaged or deformed
Momentum is conserved
Kinetic energy is not conserved
m1v1i + m2v2i = (m1 + m2)vf
In real life, most collisions fall somewhere between perfectly elastic and perfectly inelastic collisions, such as when two cars collide. They bounce off each
other, but they are heavily deformed in the process, thus losing a lot of their kinetic energy. Perfect inelastic collisions, however, do sometimes occur.
Task 5.3.1 (8 points): Calculate the wanted information. Also, sketch the objects before and after the collision, including their velocities.
Question
a) If a 4kg magnet moving at 10m/s and a 6kg
magnet moving at 2m/s collide and stick
together, what will their final velocity be?
b) You and your friend, whom you haven’t seen
in six months, run and jump at each other,
colliding in midair with a bear hug. If you (48kg)
are running at 7m/s, and your friend (73kg) is
running at -5m/s, how fast will you be moving
after the collision?
c) An atom of sodium-23 (mass: 22.990 amu)
flying at -106m/s and an atom of chlorine-35
(mass: 34.969 amu) flying at 156m/s collide to
form one formula unit of NaCl. What will it’s
final velocity be?
d) A 332kg satellite travelling at 95m/s tracks an
unknown object flying towards it at -162m/s. If
after the collision, the satellite and the object are
travelling at 37m/s, what was the mass of the
object?
Sketch of before and after collision, including
velocities
Work and Answer
Task 5.3.2 (4 points): Calculate the following. They may require the use of old equations.
a) 1200kg car travelling at 24m/s and a 1500kg car travelling at -28m/s
collide in an inelastic collision. How much energy goes into damaging the
cars in the collision (hint: it is the change in kinetic energy from before
the collision to after the collision).
b) A 288kg rocket, starting from rest, accelerates at 3.4m/s2 for 82
seconds before hitting a 5230kg asteroid flying at -52m/s. How fast will
the asteroid be moving after the inelastic collision?
c) If a 50kg box sliding at 8m/s collides with and sticks to a 30kg box
sliding at -9m/s, what force will each box feel if the collision takes .086s
to occur?
d) Each of the boxes from part c contain fragile electronics that can’t
withstand more than 80m/s2 of acceleration without breaking. Would
none, one, or both of the contents of the boxes break?
Task 5.3.3 (4 points): Write a two-paragraph (4 sentences per paragraph) summary of what you learned in this topic.