F on B from A — pushes B right F on A from B — pushes A left equal magnitude · opposite direction always on different objects
Block A · moving
m_A · mass2 kg
v_A · initial speed5 m/s
Block B · at rest
m_B · mass5 kg
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After collision
v_A— m/s
v_B— m/s
F pair status—
before: no horizontal net force acting
during: F pair appears
after: zero horizontal net force
force only exists at contact
Newton's Third Law: the force pair only exists during the collision, this is a very short time span. Before and after the collision, only Newton's First Law is present — objects in motion stay in motion, objects at rest stay at rest, unless a force acts on them. NOTE: For this reason, a book on a table is not an example of Newton's Third Law.