4aNewton's First Law
Discover the law of inertia — why objects resist changes to their motion and what it means for an object to be in equilibrium.
Newton's First Law is the foundation of all classical mechanics. Understanding inertia explains why seatbelts save lives, why spacecraft can travel billions of miles without fuel, and why objects in motion behave the way they do.
Lesson Overview
Newton's First Law — the Law of Inertia — states that an object will remain at rest or continue moving in a straight line at constant speed unless acted upon by a net external force. This lesson explores what inertia means, how mass relates to inertia, and how equilibrium arises when forces balance.
Key Concepts
Law of Inertia
Objects resist changes to their state of motion
Objects at Rest
Stay at rest unless a net force acts on them
Objects in Motion
Continue at constant velocity unless a net force acts
Net Force = 0
The object is in equilibrium (no acceleration)
Inertia & Mass
Greater mass → greater inertia → harder to accelerate
Real-World Examples
Seatbelts, tablecloth trick, hockey pucks on ice
A book sits on a table and does not move. Identify all forces acting on it and explain why it remains at rest.
A hockey puck slides across frictionless ice at 3 m/s. What is its speed 10 seconds later if no force acts on it?
A 2 kg book and a 10 kg rock are both at rest on a table. Which requires a larger force to start moving, and why?
Explain why passengers lurch forward when a bus brakes suddenly.
A tablecloth is pulled quickly from under dishes. The dishes barely move. Which law explains this, and what condition makes the trick work?
A car travels at 60 km/h on a straight highway with the engine off and no friction. What happens to its speed?
Hint: Think about what net force acts on the car if there is no friction and no engine force.
Two objects, one with mass 5 kg and one with mass 50 kg, are both pushed with the same force. Which is harder to accelerate, and why?
Hint: Consider how mass relates to inertia.
A ball rolls off a table and follows a curved path to the floor. Is this consistent with Newton's First Law? Explain.
Hint: What force acts on the ball after it leaves the table?
A 500 N person stands still on a scale. What does the scale read, and what is the net force on the person?
Hint: If the person is not accelerating, what must be true about the net force?
Why do astronauts in deep space (far from any planet) float rather than fall?
Hint: Think about what forces are present — or absent — in deep space.
Key Vocabulary
Inertia
The tendency of an object to resist any change in its state of motion.
Example: A bowling ball is harder to start rolling than a tennis ball because it has more inertia.
Net Force
The vector sum of all forces acting on an object.
Example: If a 10 N force right and a 10 N force left act on a box, the net force is 0 N.
Equilibrium
The state of an object when the net force acting on it is zero, resulting in no acceleration.
Example: A lamp hanging from the ceiling is in equilibrium — tension up equals gravity down.
Newton's First Law
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force.
Example: A soccer ball stays still on the field until a player kicks it.
Interactive Practice — 5 Questions
A ball rolls along a frictionless surface at constant velocity. What is the net force on the ball?
Which property of matter describes its resistance to changes in motion?
A 1 kg ball and a 5 kg ball are both at rest. Which has greater inertia?
Why do passengers slide forward when a car stops suddenly?
An object is in equilibrium. Which statement must be true?
Independent Practice
State Newton's First Law in your own words and give two everyday examples not mentioned in this lesson.
A 3 kg object and a 15 kg object are both moving at 5 m/s. Which is harder to bring to a stop? Explain using the concept of inertia.
A spacecraft in deep space turns off its engines. Describe its motion for the next hour, assuming no gravitational forces act on it.
Draw a free-body diagram of a book at rest on a table. Label all forces and explain why the net force is zero.
★ Research the "tablecloth trick." Write a paragraph explaining the physics behind it, including the role of inertia, friction, and contact time. Predict what would happen if the tablecloth were pulled slowly.
ChallengeCommon Mistakes
Thinking that a moving object needs a continuous force to keep moving.
Newton's First Law says no force is needed to maintain constant velocity — only to change it.
Confusing inertia with weight; saying a heavier object has more inertia because gravity pulls harder.
Inertia depends on mass, not weight. Mass is the measure of inertia; weight is the gravitational force on that mass.
Assuming equilibrium means the object must be at rest.
Equilibrium means net force = 0. An object moving at constant velocity is also in equilibrium.
Math Tips
To check for equilibrium, sum all force vectors: ΣF = 0. If forces in every direction cancel, the object is in equilibrium and has zero acceleration.