How this calculator works
The two forces in an interaction act on different bodies, have equal magnitudes and point in opposite directions. They therefore do not cancel in either individual body's free-body diagram. Different masses can accelerate by different amounts under the same interaction force. If other forces are present, sum them for each body before using its acceleration to infer the interaction.
Formula
F2 = -F1; a1 = F1/m1; a2 = -F1/m2.
Worked example
If 2 kg accelerates at 3 m/s² solely from an interaction with 4 kg, the forces are +6 N and -6 N. The second acceleration is -1.5 m/s².
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Frequently asked questions
Do action and reaction happen at different times?
No. The pair describes the same interaction acting on the two bodies simultaneously in this Newtonian model.
Why do the forces not cancel?
They act on different objects. Only forces on the same object are summed to determine its acceleration.
Does the heavier object exert a larger force?
No. The force magnitudes are equal, while the heavier object has a smaller acceleration if no other forces act.