5.4
La première loi de Newton : application
L'expérience suggère qu'un objet au repos reste au repos s'il est laissé seul, et qu'un objet en mouvement tend à ralentir et s'arrêter à moins qu'un…
La première loi de Newton stipule qu'une force externe est nécessaire pour tout changement dans l'état de mouvement de l'objet.
Pour mieux comprendre cela, considérons une balle au repos. Les forces qui agissent sur elle sont la force gravitationnelle et la force normale qui sont égales en intensité et opposées en direction. Par conséquent, la force nette est équilibrée.
Si, toutefois, la balle est poussée doucement d’un côté, les forces sont déséquilibrées. Il y a une force nette agissant sur l’objet et la balle commence à rouler dans le sens de la force.
Une fois en mouvement, aucune force nette n’agit sur le ballon, mais le ballon continue de rouler. Il aurait continué à rouler éternellement s’il n’y avait pas eu la résistance de l’air et les forces de frottement. Ceux-ci agissent à l’opposé de la direction du mouvement et immobilisent la balle.
La résistance de l'objet au changement de son état de repos ou de mouvement s'appelle l'inertie. Ainsi, la première loi est également connue sous le nom de loi d’inertie.
L’inertie offerte par tout objet est directement proportionnelle à sa masse.
Q1: What does Newton's first law say about objects at rest?
Newton's first law states that an external force is required for any change in an object's state of motion. An object at rest remains at rest because the forces acting on it are balanced, creating zero net force. The ball experiences equal and opposite gravitational and normal forces, so it stays stationary until an unbalanced force acts upon it.
Q2: Why does a rolling ball eventually stop if no one is pushing it?
Once a ball is in motion, it would continue rolling indefinitely if no net force acted on it. However, air resistance and frictional forces act opposite to the direction of motion, creating a net force that decelerates the ball. These forces are the cause of the ball slowing down and coming to rest, consistent with Newton's first law.
Q3: How does inertia relate to Newton's first law?
An object's resistance to change in its state of rest or motion is called inertia, making Newton's first law also known as the law of inertia. Inertia is directly proportional to an object's mass, meaning more massive objects resist changes in motion more strongly. This property applies regardless of scale, from molecules to subatomic particles.
Q4: What is the role of friction in demonstrating Newton's first law?
Friction demonstrates Newton's first law by showing that a net external force causes changes in motion. An object sliding on a rough surface slows down due to friction's force. If friction is eliminated by making the surface smoother or frictionless, the object would slide indefinitely, proving friction is the cause of deceleration.
Q5: How do balanced and unbalanced forces affect an object's motion?
When forces on an object are balanced, the net force is zero and the object maintains its state of motion or rest. When forces are unbalanced, a net force acts on the object, causing it to accelerate in the direction of that force. This distinction is central to understanding internal and external forces and how they determine whether motion changes occur.
Q6: What is the difference between cause and effect in Newton's first law?
Newton's first law emphasizes thinking in terms of cause and effect rather than simply observing behavior. The cause is a net external force, and the effect is a change in velocity or direction. This approach differs from ancient Greek philosophy and allows us to predict that without friction or air resistance, an object would continue moving indefinitely.
Q7: Why is Newton's first law called the law of inertia?
Newton's first law is called the law of inertia because it describes how objects resist changes in their motion. Inertia is the fundamental property that explains why objects at rest stay at rest and moving objects continue moving unless acted upon by an external force. This resistance to acceleration is a universal property of all matter.