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For ages people thought a force was needed just to keep something moving. Galileo and Newton showed otherwise — remove friction, and a moving object keeps going on its own. This idea became Newton's first law. Let us state it.
- In ancient times people knew a force was needed to start or stop motion.
- They wrongly thought a force was needed to keep an object moving steadily.
- In the 17th century, Galileo Galilei argued through thought experiments that a body would keep moving indefinitely if all impediments were removed.
- Isaac Newton used the word 'inertia' for an object's tendency to resist changes in its state of rest or motion.
- He presented three laws of motion in 1687 — a defining moment in science.
- The unit of force is named after him; written as newton (small case) with symbol N (capital).
- An object at rest stays at rest, and an object in motion keeps moving at constant velocity, unless a net force acts on it.
- If the net force is zero, the object cannot start moving or change its velocity.
- In that case its acceleration is zero.
- An object at rest has zero velocity.
- Constant velocity means no change in the magnitude or direction of velocity.
- If this constant velocity is non-zero, the motion is in a straight line at a steady speed.
A person pushes a moving box forward with a force equal to the friction acting on it. Will the box keep moving or stop?
Friction acts backward, equal and opposite to the push, so the two forces balance. The net force is zero, and by Newton's first law the box continues moving with constant velocity .
Draw position-time and velocity-time graphs for an object on which no net force acts.
With no net force, the object is either at rest or moving at constant velocity.
If at rest: position stays the same over time (flat position-time graph), and velocity stays zero (flat velocity-time graph at zero).
If moving at constant velocity: position increases steadily (straight sloping position-time graph), and velocity stays constant (flat velocity-time graph at a non-zero value).
- Newton's first law: no net force means no change in motion.
- Inertia is the tendency to resist changes in the state of motion.
- Zero net force gives zero acceleration.
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An object moves at constant velocity. Is there a net force on it?
No — constant velocity means zero acceleration, so the net force is zero. -
If no net force acts, which situations are possible? (i) stays at rest (ii) keeps moving at constant velocity (iii) moves with constant acceleration.
Only (i) and (ii). Constant acceleration (iii) needs a net force, so it is not possible. -
It is hard to find an object with no forces on it. How can a zero net force still be achieved?
By applying additional forces that balance the existing ones — for example, pushing a box forward exactly as hard as friction pulls back, so the net force is zero.
- With zero friction, a net force is needed to set a resting object moving.
- Once moving, no further force is needed to keep it at constant velocity.
- To change the velocity, or stop the object, a force must be applied.
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State Newton's first law in one line.
View Answer
An object stays at rest or moves at constant velocity unless a net force acts on it. -
What is inertia?
View Answer
The tendency of an object to resist changes in its state of rest or motion. -
What is the acceleration when the net force is zero?
View Answer
Zero.
- Newton's first law of motion — an object stays at rest or moves at constant velocity unless a net force acts on it.
- Inertia — the tendency of an object to resist a change in its state of rest or uniform motion.
