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Chapter 2: Problem 6

What do we mean by "external" force? In light of Newton's third law of motion, why can't an internal force alone produce an acceleration of a system?

Short Answer

Expert verified
Answer: Internal forces alone cannot produce acceleration in a system because they occur in pairs with equal magnitude but opposite directions, as stated by Newton's third law of motion. These pairs of forces cancel each other out, resulting in a net force of zero. According to Newton's second law of motion, if the net force is zero, there will be no acceleration. Therefore, an external force is necessary to produce acceleration in a system.

Step by step solution

01

Define External Force

An external force is a force that comes from outside a system and acts upon an object within the system. It can cause a change in the object's velocity or produce acceleration. Examples of external forces include gravity, friction, and applied force.
02

Define Internal Force

An internal force is a force that originates from within the system, acting between objects or particles inside the system. Internal forces occur in pairs, for example, the force of an object pushing or pulling on another object within the system.
03

Explain Newton's Third Law of Motion

Newton's third law of motion states that for every action, there is an equal and opposite reaction. In other words, when Object A exerts a force on Object B, Object B also exerts an equal and opposite force on Object A. Mathematically, this can be represented as: F_(A→B) = -F_(B→A).
04

Relate Internal Forces to Newton's Third Law of Motion

Internal forces are subject to Newton's third law because they occur in pairs, as we mentioned earlier. When two objects within a system exert forces on each other, these forces have the same magnitude but are in opposite directions.
05

Explain Why Internal Forces Alone Can't Produce Acceleration

Since internal forces occur in pairs that have equal magnitude but opposite directions, their contributions to the net force on the system will cancel each other out. As a result, the net force acting on the system because of internal forces will be zero. According to Newton's second law of motion (F = ma), if the net force is zero, there will be no acceleration (a = 0). Therefore, internal forces alone cannot produce acceleration of a system. In conclusion, an external force is necessary to produce acceleration in a system because internal forces, which are subject to Newton's third law of motion, cancel each other out and cannot cause acceleration by themselves.

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Most popular questions from this chapter

The terminal speed of a ping-pong ball is about \(20 \mathrm{mph}\). From the top of a tall building on a windless day, a ping-pong ball is thrown downward with an initial speed of \(50 \mathrm{mph}\). Describe what happens to the ball's speed as it moves downward from the moment it is thrown to the moment when it hits the ground.

As any car travels with constant velocity on a straight, flat section of highway, the road still exerts both a vertical (upward or downward) force and a horizontal (forward or backward) force on the car. Identify the specific direction of each of these forces.

A single-engine airplane usually has its propeller in the front. Boats and ships usually have their propeller(s) in the rear. From the perspective of Newton's second law of motion, is this significant?

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