【People's Education Press】High School Physics - Required Course 1: Crossing from Intuition to Reason: Is Force Necessary to Maintain Motion?

This lesson marks physics' profound transition from the 'description of motion' inKinematics to the exploration of motion's causes inDynamics a deep leap. For two millennia, Aristotle's intuitive experience dominated human understanding: he believed that a force must be applied for an object to remain in motion.

Core Cognitive Logic

Causal Logic Chain: Kinematics focuses only on geometric quantities like velocity and acceleration; Dynamics establishes a bridge between force (cause) and change in motion state (result) through Newton's laws.
Idealization Thinking: Galileo deduced through inclined plane experiments that, under ideal conditions wherenet external force is zerothe object will maintain its velocity unchanged.
The Essence of Inertia: Every object possesses the inherent property of maintaining its original state of motion, andmassis the sole measure of this property (inertia).

Common Misconception (Pitfall)

Newton's first law describes a state that isan idealized state. It is the product of scientific reasoning based on limited experiments using logical thinking. In real-world Earth environments, friction cannot be fully eliminated, making it impossible to directly verify objects free from external forces through experiment. Never mistake it as a mere experimental conclusion.

QUESTION 1

According to Newton's first law, if all external forces acting on an object suddenly disappear, the object will:

come to an immediate stop

gradually slow down and eventually stop

move at constant velocity in a straight line or remain at rest

accelerate in a straight line along its original direction

QUESTION 2

Which of the following statements about 'inertia' is correct?

An object has no inertia when at rest, only when in motion

The greater an object's speed, the greater its inertia

Mass is the only measure of an object's inertia

The greater the force acting on an object, the greater its inertia

QUESTION 3

As shown in Figure 3.5-5, an object is in equilibrium under five concurrent forces. If force $F_1$ is removed while the other four forces remain unchanged, draw the magnitude and direction of the resultant force of these four forces on the diagram.

The resultant force has magnitude equal to $F_1$, and direction the same as $F_1$

The resultant force has magnitude equal to $F_1$, and direction opposite to $F_1$

The resultant force is zero

Cannot be determined

QUESTION 4

Galileo's ideal inclined plane experiment demonstrated that:

Force is required to maintain an object's motion

If there were no friction, an object would continue moving indefinitely

Force is needed to start an object's motion and also to stop it

Force is the cause of acceleration

QUESTION 5

From Newton's second law, we know that even an extremely small force can produce acceleration. Yet, when we try to lift a very heavy box, we fail. Is this contradictory to Newton's second law? How should this phenomenon be explained?

There is a contradiction, indicating Newton's second law applies only to microscopic objects

There is no contradiction. Because the net external force on the box remains zero

There is no contradiction. Because Newton's second law does not apply to stationary objects

There is a contradiction. The force is too small to produce acceleration