Physics · Class 11

Action and reaction

Each skater exerts a force on the other. The two forces are equal and opposite at the same instant, but act on different bodies. They therefore cannot cancel in the force equation for one skater. The lighter skater changes velocity more; their momentum changes remain equal and opposite.

Open the interactive experiment →

Equal pushes. Different departures.

வினை மற்றும் எதிர்வினை விசைகள் சமமாகவும் எதிர்திசையிலும் வெவ்வேறு பொருட்களில் செயல்படும். நிறை குறைந்தவரின் வேக மாற்றம் அதிகம்.

Key notes, relationships and model limits

Key relationship

FAB = −FBA; ΔvA = −Fτ/mA; ΔvB = Fτ/mB.

Remember

Common trap

Third-law forces cancel only when summing the whole two-body system, not for either body alone.

Model limits

Two initially stationary point-mass skaters on frictionless ice; constant mutual push for τ; external horizontal force zero.

What you will learn

Identify the bodies in a third-law pair and compare force, acceleration and impulse.

Try the experiment

Use equal masses, then light and heavy skaters. Observe equal momentum magnitudes and different velocities.

Think of it this way

Two people pushing apart on skateboards feel the same interaction while moving differently.

Worked example

A 40 N push for 1 s gives impulses −40 and +40 N·s. For 50 and 80 kg, velocities are −0.8 and +0.5 m/s.

Why this matters

Labeling the receiving body prevents common free-body diagram errors.

Before you start

Vector components · velocity · SI units

Check your understanding

Why do the mutual pushes not cancel for skater A?

Reaction occurs later
The other force acts on skater B
The forces have unequal magnitudes

Read the explanation

A’s force equation includes forces acting on A. The paired reaction acts on B.

Course connection: CBSE XI Chapter 4; NEET/Main Unit 3; JEE Advanced Mechanics. Apply each exam’s stated assumptions.