10.3 Content
10.3.1 Newton’s First Law (Law of Inertia)
If the net force on an object is zero, it remains at rest or moves at constant velocity.
This law describes inertia—the tendency of objects to resist changes in their motion.
Key implications:
- An object at rest stays at rest unless a net force acts
- An object in motion continues at constant velocity unless a net force acts
- “Constant velocity” means constant speed AND direction
When a car brakes suddenly, passengers continue forward—they have inertia. Seatbelts provide the force needed to decelerate you with the car.
10.3.2 Interactive: First Law Demonstration
A puck on a frictionless surface maintains constant velocity:
Observation: Equal spacing between positions means constant velocity. Zero net force → zero acceleration.
10.3.3 Newton’s Second Law
The acceleration of an object is proportional to the net force and inversely proportional to its mass.
\[\vec{F}_{net} = m\vec{a}\]
Or equivalently: \(a = \frac{F_{net}}{m}\)
This is the most-used equation in mechanics. It connects:
- Force (N) - the cause
- Mass (kg) - the resistance to acceleration
- Acceleration (m/s²) - the effect
10.3.4 Interactive: Force, Mass, and Acceleration
See how doubling force or doubling mass affects acceleration:
Observation: Increasing spacing between positions indicates acceleration. The velocity arrows grow because \(v = v_0 + at\).
10.3.5 Interactive: Free-Body Diagram with Net Force
A 12 kg crate with 30 N applied force:
With \(F_{net} = 30\) N and \(m = 12\) kg: \[a = \frac{F_{net}}{m} = \frac{30}{12} = 2.5 \text{ m/s}^2\]
10.3.6 Newton’s Third Law
For every action force, there is an equal and opposite reaction force acting on a different object.
Key features of action-reaction pairs:
- Equal magnitude and opposite direction
- Act on different objects
- Same type of force (both gravitational, both contact, etc.)
- Exist simultaneously
Action-reaction pairs never cancel because they act on different objects. Forces only cancel when they act on the same object.
10.3.7 Interactive: Action-Reaction Pairs
When you push on a wall, the wall pushes back on you:
Important: These forces act on different objects—one on the wall, one on you.
10.3.8 Connecting the Laws
| Law | Statement | Key Equation |
|---|---|---|
| First | No net force → no acceleration | \(\vec{F}_{net} = 0 \Rightarrow \vec{a} = 0\) |
| Second | Net force causes acceleration | \(\vec{F}_{net} = m\vec{a}\) |
| Third | Forces come in pairs | \(\vec{F}_{AB} = -\vec{F}_{BA}\) |