Electromagnetism, Properties Of Waves, Motion, Gases & The Kinetic Theory Worksheet With Answers Page 21
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36Is kinetic energy conserved in collisions ?
Energy cannot be created or destroyed. However, energy can be transferred from the
kinetic energy of a colliding object (e.g. a car) into heat and sound energy which
escapes into the surroundings.
This means that it’s quite normal (even expected) that KE is ‘lost’ from the colliding
objects during a collision. Look at the situation below :
Before collision
u
=12 m/s
A
After colliding, the velocity of car A reduces to 2m/s (). If the mass of car A, m
= 1400 kg, and
A
car B, m
= 1200 kg, then by conservation of momentum,
B
momentum before
= momentum after
m
u
+
m
u
= m
v
+
m
v
A
A
B
B
A
A
B
B
16 800 +
0
= 2800 +
1200 v
B
16 800 - 2800 =
1200 v
B
14 000 = 1200 v
B
v
= 11.67 m/s (to the right)
B
Note : Since the answer is a positive number, we therefore know that it is to the right.
We can now check to see what happens to the kinetic energy of the cars :
2
2
2
KE before = KE
= 0.5 m v
= 0.5 m
u
= 0.5 x 1400 x 12
= 100 800 J
car A
A
A
KE after = KE
+
KE
= 2800 + 81 667
= 84 467 J
car A
car B
This shows that some KE is lost during the collision. Notice we do not take direction into
consideration here since kinetic energy is NOT a vector.
Elastic collision : There is no loss in kinetic energy.
Inelastic collision : There is loss in kinetic energy.
21
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