5.5
Interne und externe Kräfte
Newtons erstes Gesetz besagt, dass eine resultierende äußere Kraft eine Veränderung der Bewegung verursacht. Externe Kräfte wirken auf einen Gegenstan…
Was lässt ein Auto aus dem Stand beschleunigen?
Ist es die Kraft des Motors des Autos auf die Antriebswelle oder die Kraft seiner Achsen auf die Räder? Oder ist es die Reibungskraft der Straße auf die Räder des Autos?
Die Antriebswelle überträgt die innere Kraft des Motors auf die Achsen. Die Achsen drehen dann die Räder. Die rotierenden Räder erfahren eine äußere Reibungskraft durch die Straße und das Auto bewegt sich vorwärts.
Wenn keine Reibungskraft vorhanden ist, z. B. wenn ein Auto auf einer vereisten Straße feststeckt, drehen sich die Räder, aber das Auto bleibt an der gleichen Stelle stecken.
Folglich ist es die Reibung, die von der Straße ausgeübt wird, die das Auto vorwärts treibt.
Solche Kräfte, die außerhalb des Zinssystems entstehen, sind äußere Kräfte. Nur äußere Nettokräfte können den Bewegungszustand eines Objekts verändern.
Die Schnittgrößen hingegen entstehen innerhalb eines Systems und werden innerhalb der Teile des Systems ausgetauscht. Sie verursachen keine Beschleunigung.
Q1: Why does friction from the road propel a car forward instead of the engine's internal force?
The engine's internal force is transmitted through the drive shaft and axles to rotate the wheels, but only the external frictional force from the road actually propels the car forward. Without this friction, such as on an icy road, the wheels spin but the car remains stationary. Only net external forces can change an object's state of motion, making friction the critical force for acceleration.
Q2: What is the difference between internal and external forces?
Internal forces originate inside a system and are exchanged between parts of the same object, producing no acceleration. External forces act on an object from outside the system, originating from other objects. When analyzing motion, only external forces matter because they cause changes in an object's state of motion, while internal forces cancel out within the system.
Q3: How do external forces affect an object's acceleration?
A larger net external force produces a larger acceleration on an object. Acceleration is directly proportional to and in the same direction as the net external force acting on the system. Different external forces exerted on the same mass produce different accelerations, demonstrating the relationship between force and motion.
Q4: When two students push a stalled car, which forces are internal and which are external?
The force exerted by the two students on the car is an external force because it originates outside the system. The force the driver exerts to hold the steering wheel is an internal force because it acts between elements within the system of interest. Identifying these force types helps determine which forces actually cause the car's acceleration.
Q5: What types of forces are classified as external forces?
External forces generally include applied force, normal force, tension force, friction force, and air resistance force. These forces originate outside the system and act on an object due to other objects or environmental factors. Understanding these force types is essential for analyzing motion and applying Newton's laws to real-world scenarios.
Q6: Why do internal forces not cause acceleration in a system?
Internal forces are exchanged between parts of the same object and always occur in pairs that cancel each other out within the system. Since they balance internally, they produce no net effect on the system's overall motion. Only unbalanced external forces can change an object's state of motion or produce acceleration.
Q7: How can drawing free body diagrams help identify internal versus external forces?
Free body diagrams isolate a single object and show only the external forces acting on it, making it easier to distinguish external forces from internal forces. By visualizing which forces originate outside the system of interest, students can better apply Newton's laws and analyze motion accurately. This visual approach clarifies which forces contribute to acceleration.