According to Newton's first law,

Question

BlackTom AI · Accepted Answer

An object with zero net force has zero acceleration, so its velocity (both magnitude and direction) remains constant. Relevant concepts: Newton’s first law (law of inertia), vector nature of velocity, and Newton’s second law F_net = ma.

Step-by-step reasoning and equations:
1. Newton’s second law gives F_net = m a. If no force acts (F_net = 0), then m a = 0.
2. For nonzero mass m, a = 0. Acceleration a is the time derivative of velocity v: a = dv/dt.
3. dv/dt = 0 implies v is constant in time. Integrating: v(t) = v0 (a constant vector). Integrating again for position: x(t) = v0 t + x0.
4. Because velocity v is a vector, “constant v” means both constant speed (|v|) and constant direction. A special case v0 = 0 describes an object at rest remaining at rest.

Why the correct answer is correct:
- Newton’s first law states that in the absence of a net external force, an object continues in uniform motion along a straight line. This is exactly the statement that both speed and direction remain constant when no force acts.

Why the other options are incorrect:
- “An applied force always causes a change in the speed of an object.” Incorrect: a force perpendicular to velocity (pure centripetal force) changes direction but not speed (e.g., uniform circular motion).
- “The rate of change of speed of an object is larger, the larger the force acting on the object.” Misleading: the acceleration magnitude is proportional to net force (a = F_net/m), but the rate of change of speed equals the tangential component of acceleration (a_t = F_t/m). A large force perpendicular to velocity does not change speed.
- “An applied force always causes a change in the direction of travel of an object.” Incorrect: a force parallel to velocity changes speed but not direction; no force causes neither to change.

Conclusion: The correct answer is: "if no force is acting on an object, then the object's speed and direction of travel will both be constant." This matches the provided answer.

According to Newton's first law,单项选择题

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