sci_phy
Laws of Motion
Chapter summary, hard words and model exam answers for Class 9 Hindi.
Free online summary and notes (Class 9 Hindi). Read it here, no PDF download needed.
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Physics · ICSE Class 9
Summary
Mass is a measure of inertia. A heavier school bag is harder to start moving and harder to stop because it has more inertia.
This law is also called the law of inertia. It explains why passengers lurch forward when a moving bus stops suddenly.
At Class 9 level, this is used as F = ma when mass is constant. The SI unit of force, newton, comes from this relation.
Momentum is a vector quantity. Change in momentum is linked to force and time; this is why catching a cricket ball by moving hands backward reduces the force.
A swimmer pushes water backward and water pushes the swimmer forward. The pair is equal and opposite, but it does not cancel because the forces act on different bodies.
Hard words & meanings
| inertia | Resistance to change in rest or uniform motion |
| net force | Resultant of all forces acting on a body |
| momentum | Product of mass and velocity |
| newton | Force that gives 1 kg mass an acceleration of 1 m/s^2 |
| action-reaction pair | Equal and opposite forces acting on two different bodies |
| impulse | Force multiplied by time, equal to change in momentum |
Model exam answers, grammar & audio
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