9th Class Physics Chapter 3 Notes: Dynamics

Identify the object and its forces

A force is an interaction with direction as well as magnitude. For a free-body diagram, show only forces acting on the selected object. Along one line, add forces with a consistent sign convention. Balanced forces have zero resultant, while an unbalanced resultant changes the object’s velocity.

• Weight acts downward near Earth.
• A normal contact force acts perpendicular to the contact surface.
• Friction opposes relative sliding, or the tendency for sliding, at a contact.

Newton’s three laws

First law: in an inertial reference frame, zero resultant force leaves an object at rest or moving with constant velocity. Mass measures inertia. Second law: for constant mass, the resultant force equals mass multiplied by acceleration. Third law: interacting objects exert equal and opposite forces on one another at the same time.

• ΣF = ma; 1 N = 1 kg·m/s².
• A zero resultant force means zero acceleration, not necessarily zero velocity.
• Third-law partners act on different objects, so they do not cancel in a free-body diagram for one object.

Mass and weight are different quantities

Mass is measured in kilograms and does not change merely because an object moves to a location with different gravity. Weight is a gravitational force. Near Earth, W = mg; use the value of g given in the question, or state the approximation you choose. A balance and a force meter serve different measurement purposes.

• For these notes, g ≈ 9.8 N/kg unless the problem states otherwise.
• A book at rest on a level table has balanced vertical forces if no other vertical force acts.
• The book’s weight and the table’s upward force are not a Newton’s-third-law pair; both act on the book.

Momentum and impulse

Momentum combines mass and velocity: p = mv. Impulse is the change in momentum. For a constant resultant force, impulse equals force multiplied by the time interval. Total momentum is conserved for a system when its net external impulse is negligible over the interaction.

• Momentum unit: kg·m/s.
• Impulse unit: N·s, equivalent to kg·m/s.
• Choose the system and direction before adding momenta. Do not assume kinetic energy is conserved in every collision.

Use an appropriate model

Elementary Newtonian calculations work well for the everyday speeds and sizes considered here. They are not a complete description of motion at speeds close to light or of atomic-scale systems. State simplifying assumptions such as negligible air resistance rather than treating them as universal facts.

Worked examples

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Common mistakes to avoid

• Using the applied force instead of the resultant force in F = ma.
• Giving weight in kilograms or leaving out its direction.
• Pairing two forces on the same object as a third-law pair.

Sources and scope

• PECTAA curriculum and e-book directory
• OpenStax Physics: inertia
• OpenStax Physics: Newton’s second law
• OpenStax Physics: Newton’s third law
• OpenStax Physics: momentum and impulse

The chapter sequence is checked against the official directory and source book. The explanations, numerical values and practice are independently written for SchoolOrbit. No PECTAA endorsement, complete exercise key or examination-session coverage is claimed.

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