5.6
A segunda lei de Newton
A segunda lei de Newton está intimamente relacionada com a sua primeira lei do movimento. Matematicamente, ela estabelece a relação de causa e efeito…
Considere uma senhora empurrando um carrinho de bagagem enquanto se move. A força externa exercida por ela acelera o carrinho na direção da força aplicada.
Se ela empurrar com uma força maior, o carrinho se moverá mais rápido. Para parar o carrinho, ela precisa aplicar uma força externa na direção oposta ao seu movimento.
Se ela colocar mais bagagem no carrinho, a mesma força produz uma aceleração menor na direção da força externa líquida.
Compilando-os, obtemos a segunda lei do movimento de Newton, que afirma que um objeto acelera proporcionalmente e na direção da força externa líquida e inversamente à sua massa.
Ou, a força externa resultante que atua sobre um objeto é o produto de sua massa e sua aceleração. Todos os objetos em aceleração seguem a segunda lei de Newton.
Q1: How does Newton's second law relate force and acceleration?
Newton's second law states that acceleration is directly proportional to the net external force acting on an object and occurs in the same direction as that force. A larger net force produces greater acceleration. The mathematical relationship is expressed as F = ma, where force equals mass times acceleration, quantifying the cause-and-effect relationship between force and motion changes.
Q2: Why does mass affect how an object accelerates under force?
Acceleration is inversely proportional to mass, meaning larger mass produces smaller acceleration from the same force. This inverse relationship reflects inertia—the object's resistance to motion changes. Experiments confirm that acceleration depends only on net external force and mass. Adding luggage to a cart demonstrates this: the same pushing force produces less acceleration with increased mass.
Q3: What is the mathematical expression of Newton's second law?
Newton's second law is expressed as Fnet = ma, where Fnet is the net external force, m is mass, and a is acceleration. This equation shows that net force equals mass multiplied by acceleration. All accelerating objects follow this relationship. It provides a quantitative method to calculate what happens in situations involving forces acting on systems.
Q4: How does Newton's second law connect to his first law of motion?
Newton's second law is closely related to his first law, which states objects maintain constant motion unless acted upon by force. The second law mathematically describes what happens when force is applied, showing how objects respond to external influences. Together, they explain both why objects resist motion changes and how forces cause those changes to occur.
Q5: Can Newton's second law be expressed using momentum?
Yes, Newton also outlined his second law in terms of momentum: the instantaneous rate at which a body's momentum changes equals the net force acting on it. This formulation emphasizes how force drives changes in an object's motion state. Both the force-acceleration and momentum-change expressions describe the same physical principle governing mechanical response.
Q6: What does it mean that acceleration is proportional to net force?
When net external force increases, acceleration increases proportionally in the same direction. If a person pushes a luggage cart harder, it accelerates faster. Conversely, applying force opposite to motion slows the cart. This direct proportionality means doubling the net force doubles the acceleration, demonstrating the quantitative relationship central to Newton's second law.
Q7: How do you identify all forces acting on an object to find net force?
All external forces acting on a system add together to produce net force. To find net force, you must identify every force acting on the object, including applied forces, friction, and gravity. Drawing free body diagrams helps visualize these forces systematically. The net force is the vector sum of all individual forces, which then determines the object's acceleration using F = ma.