🎯 By the end of this lesson, you will be able to:
- State Newton’s 1st Law
- Explain the concept of inertia and relate it to mass
- Distinguish between balanced and unbalanced forces and their effects on motion
- Apply Newton’s 1st Law to explain observations in everyday situations
D3 — Newton’s First Law (5:54)
“An object at rest remains at rest, and an object in motion continues in motion at constant velocity, unless acted upon by a net external force.”
1. Inertia
Inertia is the tendency of an object to resist change in its state of motion.
Mass determines inertia — a heavier object has more inertia and is harder to start moving, stop, or redirect. Compare pushing a football (easy) to pushing a boulder (hard). Same force applied, but the boulder’s greater mass gives it greater inertia.
Real-world examples:
- Seatbelts in cars: When a car brakes suddenly, passengers tend to keep moving forward due to inertia. The seatbelt provides an unbalanced force to change their motion.
- Tablecloth trick: When you yank a tablecloth out from under dishes, the dishes stay in place because they resist the change in motion caused by the brief friction.
- Heavy vs light objects: A light object accelerates easily when pushed; a heavy object resists acceleration and requires more force to move at the same rate.
2. Balanced and Unbalanced Forces
| Type of Forces | Effect on Motion | Example |
|---|---|---|
| Balanced Forces | No change in motion. Object remains at rest or continues at constant velocity. | Book on a desk; car driving at steady speed on highway |
| Unbalanced Forces | Change in motion (acceleration). Newton’s 2nd Law determines how much change. | Pushing a stationary object; braking or accelerating a car |
Summary Mind Map
The causal chain: unbalanced forces cause changes in motion. No unbalanced force = no change in motion.
Worked Examples
A hockey puck slides on frictionless ice. No forces act horizontally. What happens to its motion?
Answer: The puck continues moving at the same speed and direction forever.
Explanation: There are no unbalanced horizontal forces acting on the puck. By Newton’s 1st Law, it maintains its constant velocity. In reality, the puck would eventually hit the boards, but on a truly frictionless surface with no other forces, it would never slow down or change direction.
A car travels at a constant velocity of 20 m/s down a highway. The engine provides 800 N of driving force forward. What is the total friction and air resistance force?
Answer: The friction and air resistance force is 800 N backward (opposing motion).
Explanation: The car is moving at constant velocity, which means zero acceleration. By Newton’s 1st Law, balanced forces mean no change in motion. Therefore, the net force must be zero. If the driving force is 800 N forward, the total opposing force (friction + air resistance) must be exactly 800 N backward to keep the car in equilibrium.
✎ Try It Yourself
Question 1
A spacecraft in deep space (no gravity, no air resistance) has its engines turned off. Describe its motion.
Answer: The spacecraft continues at constant velocity (same speed and direction).
Explanation: With no gravity and no air resistance, there are no unbalanced external forces. By Newton’s 1st Law, the spacecraft maintains its state of motion indefinitely. Whatever velocity it had when the engines turned off, it will keep.
Question 2
Explain why passengers lurch forward when a bus brakes suddenly.
Answer: Passengers lurch forward due to inertia.
Explanation: The passengers were moving at the bus’s speed. When the bus brakes, an unbalanced force (the braking force) acts on the bus, causing it to decelerate. However, the passengers’ bodies tend to continue at their original velocity due to inertia — they resist the change in motion. Seatbelts and the friction between the seat and passenger provide the unbalanced force needed to change the passengers’ velocity along with the bus.
Further Resources
- PhET Forces and Motion Basics — Interactive simulation for exploring Newton’s laws with real-time feedback
- Physics Classroom — Newton’s 1st Law — Detailed explanation with real-world applications