5.11
La troisième loi de Newton : exemples
Q1: Why don't action and reaction forces cancel each other out?
Action and reaction forces act on different objects, so they don't cancel. When a swimmer pushes the pool wall, the wall pushes back on the swimmer with equal force. These forces affect different systems: the wall's force accelerates the swimmer, while the swimmer's force acts on the wall. Since they act on separate bodies, both forces produce observable effects without canceling.
Q2: What is the difference between an action-reaction pair and forces on the same object?
An action-reaction pair involves forces between two different objects, like gravity pulling on a man and the man pulling on Earth. Forces on the same object, such as gravity and normal force acting on a seated man, are not action-reaction pairs. True action-reaction pairs obey Newton's third law and act on different bodies, while same-object forces can balance without being action-reaction pairs.
Q3: How does Newton's third law apply to non-contact forces like gravity?
Gravity is a non-contact force that obeys Newton's third law. Earth pulls on a man with gravitational force, and the man exerts an equal and opposite gravitational force on Earth. Similarly, a recliner exerts a normal force on a man, and the man exerts an equal force back on the recliner. Both contact and non-contact forces follow the action-reaction principle.
Q4: How does a hammer and nail demonstrate Newton's third law?
When you hit a nail with a hammer, the hammer exerts a force that drives the nail deeper into wood. Simultaneously, the nail exerts an equal and opposite force that pushes the hammer backward. This is a clear action-reaction pair: the hammer's action force on the nail and the nail's reaction force on the hammer act on different objects with equal magnitude and opposite direction.
Q5: Why does a professor move forward when pushing backward on the floor?
When a professor paces and pushes backward on the floor, the floor exerts an equal and opposite reaction force forward on the professor. This forward force accelerates the professor in the desired direction. The professor's backward push is the action, and the floor's forward push is the reaction. These forces act on different objects, allowing the professor to move forward.
Q6: How do helicopters use Newton's third law to create lift?
Helicopters push air downward with their rotors, creating a downward action force on the air. By Newton's third law, the air exerts an equal and opposite upward reaction force on the helicopter. This upward force lifts the helicopter against gravity. The action-reaction pair involves the helicopter pushing down and the air pushing up on different systems.
Q7: What happens when you pull down on a vertical rope?
When you pull down on a rope, you exert a downward action force on the rope. The rope exerts an equal and opposite upward reaction force on your hand. These forces act on different objects: your force acts on the rope, while the rope's force acts on you. This action-reaction pair demonstrates Newton's third law with a simple everyday object.