Ch.6.3 Elastic and
Inelastic Collisions
Warm Up
How would you define “collision?”
●Is a tennis racket hitting a ball a
collision?
●Is a baseball glove catching a baseball
a collision?
●What’s different about these two
situations?
●
Collisions
[NBC Learn Video-NHL]
Collisions happen whenever two
objects impact each other
●
Sometimes the objects
bounce off of each other
Sometimes the objects
stick together
Elastic vs Inelastic Collisions
Elastic
Perfectly Inelastic
Perfectly Inelastic Collisions
In a perfectly inelastic collision, two objects collide
and stick to each other with some deformation
●
deformation
m1v1,i + m2v2,i = (m1 + m2)vf
Kinetic energy is NOT conserved because deformation
takes away energy (sound, friction, etc.): KEi ≠ KEf
●Momentum is conserved: p = p
i
f
●
Real World Examples
Textbook Example pg.213
A 1850 kg luxury sedan stopped at a traffic light is struck from the
rear by a compact car with a mass of 975 kg. The two cars become
entangled as a result of the collision. If the compact car was moving at
a velocity of 22.0 m/s to the north before the collision, what is the
velocity of the entangled mass after the collision?
D
vf = ?
F
v1,i= 0
v2,i= 22.0 m/s
A
U m1 = 1850 kg
m1v1,i + m2v2,i = (m1 +
m )v
→2vf f= m1v1,i +
m2v2,i
m1 + m 2
vf = (1850 kg)(0) + (975 kg)(22 m/s)
1850 kg + 975 kg
m2 = 975 kg
v1,i = 0
v2,i = 22.0 m/s to the north
vf = ?
S
vf = 7.59 m/s to the north
Homework
Pg.214 #1-4
Elastic Collisions
In an elastic collision, two objects collide and bounce
off of each other
●
m1v1,i + m2v2,i = m1v1,f + m2v2,f
Kinetic energy is conserved because motion
continues uninterrupted: KEi = KEf
●Momentum is conserved: p = p
i
f
●
Real World Examples
Typical Elastic Collisions
Realistic Collisions
some
deformation
Most collisions are somewhere between
elastic and perfectly inelastic
●For our class, we will be assuming collisions
are either elastic or perfectly inelastic
●
Real World Examples
Textbook Example Pg.219
A 0.015 kg marble moving to the right at 0.225 m/s makes an elastic
head-on collision with a 0.030 kg shooter marble moving to the left at 0.180
m/s. After the collision, the smaller marble moves to the left at 0.315 m/s. What
is the velocity of the 0.030 kg marble after the collision?
m2
m1
m1v1,i + m2v2,i = m1v1,f +
v1,i v2,i
D
F mv
2,f = m v + m v
→2v2,f
–
1 1,i
2 2,I
v
v1,f
2,f
mv
1 1,f
A
U
m1 = 0.015 kg m2 = 0.030 kg
v1,i = 0.225 m/s →
v2,i = 0.180 m/s ←
v1,f = 0.315 m/s ←
v2,f = ? →
S
m2
(0.15 kg)(0.225 m/s)+(.03 kg)(-.18 m/s)
– (0.15 kg)(-.315 m/s)
0.03 kg
v2,f = 0.09 m/s →
Homework
I’m not the
hero I
thought I
was…
Pg.219 #1-4a
*only do part a’s
…because I
didn’t do my
homework.