Newton’s Third Law and Contact Pairs
For interacting A and B, F_AB = -F_BA. Partners have the same interaction type and act on different objects.
Why this shows up in the exam
Support reactions · Propulsion statements · Contact-force pairs
Learn the idea
Interaction forces are equal and opposite but act on different bodies. A push creates a simultaneous partner force; the pair cannot cancel on one body’s FBD.
🧠 Memory hook: Equal and opposite, on different objects.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- F_AB = -F_BA — third-law pair
- sum F_ext = dP_system/dt — internal pairs cancel in a system balance
How to approach it
- 1Name both bodies
- 2Label who acts on whom
- 3Draw only forces on the chosen body
Common slip-ups that cost marks
- •Cancelling on one FBD
- •Pairing weight with normal
- •Saying rockets require air
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A 2 kg block moves on a horizontal rough surface with coefficient of kinetic friction 0.2. A horizontal force of 10 N acts on it. Take g = 10 m/s^2. What is its acceleration?
More from Laws of Motion
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Understand and apply Newton's first, second, and third laws to analyze forces, motion, and interactions in various physical situations.
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Explore the principles of linear momentum, impulse, and their conservation in collisions and explosions.
Inertia and Inertial Frames
In an inertial frame, sum F_ext = 0 implies a = 0. Earth-fixed frames are approximate when rotational effects are negligible.
Newton’s Second Law in Vector Form
In an inertial frame sum F_ext = dp/dt; for constant mass this becomes m dv/dt = ma.