5.4
A primeira lei de Newton: aplicações
A experiência sugere que um objeto em repouso permanece em repouso se deixado sozinho, e que um objeto em movimento tende a desacelerar e parar a meno…
A primeira lei de Newton afirma que uma força externa é necessária para qualquer mudança no estado de movimento do objeto.
Para entender isso melhor, considere uma bola em repouso. As forças que atuam sobre ele são a força gravitacional e a força normal que são iguais em magnitude e opostas em direção. Portanto, a força resultante é equilibrada.
Se, no entanto, a bola for empurrada suavemente de um lado, as forças serão desequilibradas. Há uma força resultante agindo sobre o objeto e a bola começa a rolar na direção da força.
Uma vez em movimento, nenhuma força resultante atua sobre a bola, mas a bola continua rolando. Teria continuado a rolar para sempre se não fosse pela resistência do ar e pelas forças de atrito. Estes agem de forma oposta à direção do movimento e fazem a bola parar.
A resistência do objeto à mudança em seu estado de repouso ou movimento é chamada de inércia. Assim, a primeira lei também é conhecida como lei da inércia.
A inércia oferecida por qualquer objeto é diretamente proporcional à sua massa.
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.