5.5
Internal and External Forces
Newton's first law states that a net external force causes a change in motion. External forces act on an object or system, originating outside of the…
What makes a car accelerate from rest?
Is it the force of the car's engine on the drive shaft or the force of its axles on the wheels? Or, is it the frictional force of the road on the car's wheels?
The drive shaft translates the internal force of the engine to the axles. The axles then rotate the wheels. The rotating wheels experience an external frictional force by the road and the car moves forward.
If there is no frictional force, say when a car is stuck on an icy road, the wheels will rotate but the car would remain stuck in the same place.
Consequently, it is the friction exerted by the road that propels the car forward.
Such forces that originate outside the system of interest are external forces. Only net external forces can change an object's state of motion.
Whereas, the internal forces originate inside a system and are exchanged within the parts of the system. They do not cause any acceleration.
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.