β‘Newton's First Law
The Law of Inertia
Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by a net external force.
This is the Law of Inertia. Inertia is the tendency of matter to resist changes in its state of motion.
The key word is net force. If multiple forces cancel out (net force = 0), the object behaves as if there are no forces at all β it keeps doing what it's doing.
Real-world example: When a bus suddenly stops, passengers lurch forward. Their bodies want to keep moving at the original speed β that's inertia!
What is a Force?
A force is a push or pull β an interaction between two objects. Forces are vectors with magnitude and direction, measured in Newtons (N).
Types of forces you'll encounter: - Gravity (Fg): pulls objects toward Earth's center (downward) - Normal force (N): surface pushes perpendicular to itself - Friction (f): surface force opposing relative motion - Tension (T): force in a rope or string (pulls) - Applied force (F): external push or pull
Free Body Diagrams (FBDs) show all forces on a single object as arrows. Drawing FBDs is essential for solving Newton's law problems.
Build a Free Body Diagram
Practice drawing FBDs for common scenarios. Use the toolbox to add force arrows, then rotate them to the correct direction. Click "Check My Diagram" to get feedback.
Think About It
A book sits on a table. Is the book in equilibrium? List all forces acting on it. Does the first law apply here?
Equilibrium
An object is in equilibrium when the net force on it is zero. This means either: 1. The object is at rest (static equilibrium), OR 2. The object is moving at constant velocity (dynamic equilibrium)
Both situations satisfy Newton's First Law: Ξ£F = 0 when acceleration = 0.
Equilibrium does NOT mean no forces β it means all forces balance out.
βοΈ Worked Example
Problem: A 10 kg box sits on a frictionless table. A 30 N force pushes right and a 30 N force pushes left. Describe the box's motion.
π Key Equations
Newton's First Law conditions
Ξ£ Fβ = 0 β aβ = 0 (equilibrium)aβ = 0 β constant velocity (or at rest)β οΈ Common Mistakes
Misconception: An object moving at constant velocity must have no forces acting on it.
β Correct thinking: An object can have multiple forces acting on it and still move at constant velocity, as long as all forces cancel (net force = 0). This is dynamic equilibrium.
Why: Newton's First Law is about net force, not the presence or absence of forces.
Misconception: Inertia is a force that keeps an object moving.
β Correct thinking: Inertia is a property of matter (resistance to change in motion), not a force. Forces cause changes in motion; inertia describes the resistance to those changes.
Why: Calling inertia a "force" leads to incorrect free body diagrams and faulty reasoning about why objects keep moving.
Misconception: The normal force and weight are a Newton's Third Law pair.
β Correct thinking: The normal force and weight both act on the same object (the book, for example), so they cannot be a Third Law pair. The Third Law pair of the weight is the gravitational pull the book exerts on Earth.
Why: Third Law pairs always act on two different objects. Two forces on the same object balance because of Newton's First Law, not Newton's Third.
π Practice Problems
Try these problems. Check your answer when ready.
A hockey puck slides across frictionless ice at 5 m/s. No forces act on it horizontally. What is its velocity after 10 seconds?
A 50 kg skydiver falls at constant terminal velocity. What is the magnitude of air resistance acting on her?
A box is pulled to the right by a 40 N rope at 20Β°above horizontal. A 15 N friction force acts to the left. What must the vertical normal force be for the box to stay on the surface? (box mass = 8 kg, g = 10 m/sΒ²)
Three forces act on an object: 10 N east, 6 N north, and an unknown force F. The object remains in equilibrium. Find the magnitude and direction of F.
A 1200 kg car travels at constant 25 m/s around a flat circular track. Is the car in equilibrium? Explain.
Finished reading through this lesson?