- #1
jcruise322
- 36
- 1
Homework Statement
Two 1.0-kg carts are coupled together and placed on a very long
horizontal track that is at rest in the Earth frame of reference. The combination is launched so that at t
= 0 it is moving to the right at speed 2.0 m/s in the Earth frame, with cart #1 in front and cart #2 in
back. The coupling contains an explosive charge that is remotely ignited at time t1 > 0, releasing 18 J
of energy. Half of this released energy is dissipated into noise, thermal energy, and minor damage to
each cart. Assume zero friction between the carts and the track.
What is the final velocity (after the explosion, time t > t1) of
cart #2 in the center-of-mass (CM of the 2 cart system) reference frame?
The answer is 3m/s to the right
Homework Equations
KE=.5*m*v^2
m1*v1+m2*v2=m1*v3+m1*v4
COM velocity: V1*m1+V2+m2/(m2+m1)
The Attempt at a Solution
[/B]I already found the center of mass velocity, 2m/s.
So...we assume 9 joules of energy was transformed into kinetic energy.
So, energy of system is given by: PE+KEinitial=KE final
so: 9 joules+4 joules from both cartes=13 joules of kinetic energy final: 13=.5mv3^2+.5*mv4^2
MV is conserved: m1v1+m2v2=m1v3+m1v4
Since all the M's are the same and the velocity is 2m/s for both:
4=v3+v4
v2=(4-v1). Plug into kinetic energy equation.
26=2v^2-8v=16
v=-1, 5. Since both masses are the same, I assume 5m/s is the velocity of both carts after the collision.
Now comes the tricky part...do I assume the center of mass velocity stays the same? If so, I would subtract 2 and get the book's answer 3. How come the center of mass velocity stays the same after this collision? I thought this was only true for elastic collisions...do we treat this bomb like an elastic collision in this case, and if so, why?
Regards,
JT
Any help would be appreciated :)