# Struggling on understanding this momentum theory based answer

I’ll attach the question and markscheme below. I understand the energy part, but there are a few parts of the momentum explanation that i don’t quite understand.

“during the explosion, there are equal and opposite forces acting between the probe and the capsule” I assume this means that because the capsule has separated from the probe, it causes a force to be exerted against the probe, which in turn causes the probe to exert an equal and opposite force

“because momentum has to be conserved, and it is a vector, the capsule must move along the original line of movement after the explosion” this part i don’t understand. What does it mean?

Any help appreciated, thank you.
Original post by LordSFelix
I’ll attach the question and markscheme below. I understand the energy part, but there are a few parts of the momentum explanation that i don’t quite understand.

“during the explosion, there are equal and opposite forces acting between the probe and the capsule” I assume this means that because the capsule has separated from the probe, it causes a force to be exerted against the probe, which in turn causes the probe to exert an equal and opposite force

“because momentum has to be conserved, and it is a vector, the capsule must move along the original line of movement after the explosion” this part i don’t understand. What does it mean?

Any help appreciated, thank you.

The first part is simply Newton 3. The forces the two bodies exert on each other during the explosion form an equal and opposite force pair.
https://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-Law
Note this is during the explosion while the capsule is still "in" the probe, not after the ejection. It is equivalent to a cannon being fired, where the capsule is the cannon ball and the probe is the cannon. The force (pair) during the explosion causes them to move in equal and opposite directions and in the diagram the probe is drawn like the barrel of a cannon.

The second part is related to the cannon - cannon ball analogy. Momentum is indeed conserved as the force pair produces equal and opposite reactions for an equal duration (same impulse magnitue is applied in equal but oppsite directions on the probe and capsule) and because the probe "barrel" is aligned with the direction of motion, the force (pair) acting on each body is along the pre explosion direction of motion. There is zero force (acceleration) perpendicular to the pre explosion direction of motion, so after the explosion the direction of motion is unchanged (possibly reversed for the capsule as its fired out the back). If the probe barrel was not aligned with the direction of motion, this would not be the case.
(edited 1 year ago)
Original post by mqb2766
The first part is simply Newton 3. The forces the two bodies exert on each other during the explosion form an equal and opposite force pair.
https://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-Law
Note this is during the explosion while the capsule is still "in" the probe, not after the ejection. It is equivalent to a cannon being fired, where the capsule is the cannon ball and the probe is the cannon. The force (pair) during the explosion causes them to move in equal and opposite directions and in the diagram the probe is drawn like the barrel of a cannon.

The second part is related to the cannon - cannon ball analogy. Momentum is indeed conserved as the force pair produces equal and opposite reactions for an equal duration (same impulse magnitue is applied in equal but oppsite directions on the probe and capsule) and because the probe "barrel" is aligned with the direction of motion, the force (pair) acting on each body is along the pre explosion direction of motion. There is zero force (acceleration) perpendicular to the pre explosion direction of motion, so after the explosion the direction of motion is unchanged (possibly reversed for the capsule as its fired out the back). If the probe barrel was not aligned with the direction of motion, this would not be the case.

Oh i see. Thank you Again!