Laws of Motion
Laws of Motion
This domain tests how forces change motion: inertia, Newton’s three laws, momentum, impulse, friction and simple applications. Questions often ask you to identify the force pair or calculate net force rather than recite definitions.
Core rules
- Inertia is a body’s resistance to change in its state of rest or uniform motion; mass is its measure.
- Newton’s first law: if net external force is zero, velocity stays constant.
- Newton’s second law: net force F = ma; acceleration is in the direction of the net force.
- Newton’s third law: forces occur in pairs on two different bodies, equal in magnitude and opposite in direction.
- Momentum p = mv; with no external force, total momentum is conserved in collisions.
- Impulse = F t = change in momentum.
- Friction opposes relative motion; limiting friction is the maximum static friction before sliding.
Worked example
A 2 kg trolley is pushed with 10 N while friction is 4 N. Net force = 10 − 4 = 6 N. Acceleration a = F/m = 6/2 = 3 m/s². If it starts from rest, after 4 s its velocity is v = u + at = 0 + 3×4 = 12 m/s. The action-reaction pair here is the push of hand on trolley and trolley on hand, not friction and push on the same body.
Traps
Do not add action and reaction as if they cancel on one body. Use net force, not applied force alone. Keep mass in kg and acceleration in m/s². Distinguish mass from weight (W = mg). Check whether friction helps or opposes motion.