Captain America Scene Question

[Sang Ho Lee]

I was watching Captain America: Winter Solider and was a bit confused with the scene.
I am a high school student who just learned mechanics, and I hope you guys will be able to give me an answer.

Looking at the photo, Bucky shoots a granade catrage, not moving a little bit, while Captain protects himself from the blast and flies off by like 20m until he crashes into a bus.

Wouldn't the conservation of momentum (m1v1 = m2v2) cause Captain to move as much as Bucky did (guessing less because probably m(captain)>m(Bucky))

If not, how does the explosion cause him to fly off?

Thank you,

 

[jbriggs444]

If the grenade were a simple projectile that was stopped dead by Captain America's shield then the simple $m_1v_1 = m_2v_2$ would apply. The recoil from the weapon would be equal to the momentum imparted to the target. However, the grenade does not stop dead. It explodes. Half of the blast goes back in the general direction of the firing position. Half impacts the shield and is stopped. It is the recoil from the high speed gasses expanding back toward the target that drives Captain America away.