内力和外力

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.

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.

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.

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.

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.

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.

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.