Newtons First Law in 5 Minutes — Free Quick Revision
Learn Newtons First Law in about 5 minutes — a free bite-sized module with a clear explanation, a worked example and a quick quiz for Indian students.
TL;DR: Learn Newtons First Law in about 5 minutes — a free bite-sized module with a clear explanation, a worked example and a quick quiz for Indian students.
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
An object at rest stays at rest, and an object in motion stays in motion unless acted on by a net force. This is inertia.
Physics · Class 11 · about 5 minutes
The Idea
Newton's First Law of Motion states: An object at rest will remain at rest, and an object in motion will remain in motion with constant velocity, unless acted upon by a net external force.
This property of objects—to resist changes in their state of motion—is called inertia. Inertia is a measure of how difficult it is to change an object's motion. More massive objects have greater inertia.
Real-world examples: When a car suddenly brakes, your body wants to keep moving forward (inertia). When you spin a bucket of water overhead, the water stays in the bucket because it's constantly pulled toward the center by the bucket's force.
The law essentially says that motion is the natural state of objects. We don't need a force to keep something moving at constant velocity—we need a force only to *change* that motion.
Worked Example: Sudden Braking Car
Problem. A passenger is sitting in a car moving at 60 km/h. The driver suddenly brakes. Explain why the passenger lurches forward.
Solution.
The passenger and car are both moving at 60 km/h. When the car brakes, a net force (friction) acts on the car, slowing it down.
However, *no force acts directly on the passenger* at that instant. By Newton's First Law, the passenger continues moving forward at 60 km/h while the car slows down beneath them. The passenger "lurches" forward relative to the car.
A seatbelt applies a force to the passenger, bringing them to rest (or slowing them) along with the car. Without it, the passenger would continue forward until hitting the windshield.
Quick Quiz
Answer each one before reading the explanation beneath it.
1. A book rests on a table. Is there a net force on the book?
- Yes, gravity pulls it down
- No, the normal force from the table cancels gravity — correct
- Yes, friction acts on it
- No force acts on it at all
No, the normal force from the table cancels gravity
2. Which object has more inertia?
- A 2 kg bowling ball
- A 5 kg bowling ball — correct
- Both have the same inertia
- Inertia depends on speed
A 5 kg bowling ball
3. An ice skater glides across frictionless ice with no forces acting. What happens?
- They slow down and stop
- They continue moving at constant velocity indefinitely — correct
- They speed up
- They accelerate backward
They continue moving at constant velocity indefinitely
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