Anyone good at general physics?

[BuddyGoodness]

Two carts are put back-to-back on a track. Cart A has a spring-loaded piston; cart B, which has twice the inertial mass of cart A, is entirely passive. When the piston is released, cart A pushes against cart B, and the carts move apart. How do the magnitudes of their final momenta p and kinetic energies K compare?

1. pA > pB, KA > KB.
2. pA > pB, KA = KB.
3. pA > pB, KA < KB.
4. pA = pB, KA > KB.
5. pA = pB, KA = KB.
6. pA = pB, KA < KB.
7. pA < pB, KA > KB.
8. pA < pB, KA = KB.
9. pA < pB, KA < KB.


I am thinking momentum is conserved for 4,5,6 are candidates for the correct answer and i think due to conseervation of energy that Ka < Kb

[erikpemberton]

Re: Anyone good at general physics?

my money is
on number 9

[Gibling]

Re: Anyone good at general physics?

if i read this correctly and the two carts are initally at rest...

by cons of momentum, the total initial momentum is 0, so the final total momentum must also be zero ==> pA = -pB

so, the magnitudes of momentum must be, pA = pB

now, pA = mA*vA and kA = 1/2 mA * vA^2
since mA*vA = mB*vB, and mB = 2*mA, we know that vA = 2*vB...
so kA = 1/2*mA*vA^2 and kB = 1/2*(2*mA)*(1/2*vA)^2 = 1/2*kA so kA > kB

so I would say it is 4.

[AnewKINDofFEELING]

Re: Anyone good at general physics?

Yeah, momentum is equal. The kinetic energy of the second cart is half that of the first cart.

Everything Gibling said is correct.

[BuddyGoodness]

Re: Anyone good at general physics?

groovy, thanks