Newton's First Law
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AP Physics C: Mechanics › Newton's First Law
Based on the scenario, a spacecraft coasts in a vacuum with engines off, and external forces are negligible, so net $F=0$. What prevents the spacecraft from spontaneously changing speed or direction?
A constant thrust remains stored in the spacecraft and continuously pushes it forward.
A net force is always required to maintain motion, so it must be accelerating slightly.
The absence of air creates a suction force that locks its velocity in place.
Its inertia maintains constant velocity unless a net external force acts on it.
Explanation
This question tests understanding of Newton's First Law of Motion, focusing on inertia and translational dynamics in AP Physics C: Mechanics. Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity unless acted upon by a net external force. In the scenario with the spacecraft coasting in a vacuum with engines off and negligible external forces, the net force is zero, allowing the spacecraft to maintain its velocity indefinitely. Choice B is correct because it properly identifies that the spacecraft's inertia maintains constant velocity unless a net external force acts on it, which is the fundamental principle of Newton's First Law in space. Choice C is incorrect because it states that net force is always required to maintain motion, which contradicts Newton's First Law - force is only needed to change motion, not maintain it. To help students: Use space examples to illustrate perpetual motion in the absence of forces. Discuss how this differs from everyday experience where friction typically stops motion.
Based on the scenario, a car moves at constant speed on a straight road where drag and rolling resistance are negligible, so net $F=0$. Why does the car continue its state of motion until braking provides a net force?
It continues because the road exerts a forward normal force that sustains motion.
It continues because its acceleration is zero only when mass is zero.
It continues because constant velocity requires a constant forward net force at all times.
It continues because inertia maintains constant velocity when no net external force acts.
Explanation
This question tests understanding of Newton's First Law of Motion, focusing on inertia and translational dynamics in AP Physics C: Mechanics. Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity unless acted upon by a net external force. In the scenario with the car moving at constant speed on a straight road with negligible resistance, the net force is zero, allowing the car to maintain its velocity. Choice A is correct because it properly explains that the car continues its motion due to inertia maintaining constant velocity when no net external force acts, which is the essence of Newton's First Law. Choice B is incorrect because it suggests constant velocity requires constant forward force, when in fact constant velocity occurs when net force is zero. To help students: Clarify the common misconception that motion requires force - force is only needed to change motion. Use examples of objects in space or on ice to illustrate motion without continuous force.
Based on the scenario, a helium balloon floats steadily in still air, with buoyant force balancing weight so net $F=0$. Why does the balloon remain nearly motionless until a breeze exerts a force?
Inertia maintains its state of rest because the net external force is approximately zero.
Inertia is a force that holds the balloon in place against any possible air currents.
The balloon remains still because objects at rest have no forces acting on them.
The balloon stays motionless because buoyancy always increases until motion becomes impossible.
Explanation
This question tests understanding of Newton's First Law of Motion, focusing on inertia and translational dynamics in AP Physics C: Mechanics. Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity unless acted upon by a net external force. In the scenario with the helium balloon floating steadily in still air, the buoyant force exactly balances the weight, resulting in zero net force on the balloon. Choice A is correct because it properly identifies that inertia maintains the balloon's state of rest when the net external force is approximately zero, which is a direct application of Newton's First Law. Choice B is incorrect because it mischaracterizes inertia as a force that holds objects in place, when inertia is actually the tendency to resist changes in motion state. To help students: Use balanced force examples from everyday life. Emphasize that objects can be at rest with multiple forces acting, as long as they sum to zero net force.
Based on the scenario, a hockey puck slides straight on smooth ice with negligible friction and no net $F$. Why does the puck maintain constant velocity until a player’s stick applies an external force?
A constant forward force from the ice must continuously act to maintain motion.
The puck accelerates because any moving object must experience a net external force.
Its inertia actively produces forward motion, so no forces are ever needed.
Its inertia maintains constant velocity when the net external force is approximately zero.
Explanation
This question tests understanding of Newton's First Law of Motion, focusing on inertia and translational dynamics in AP Physics C: Mechanics. Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity unless acted upon by a net external force. In the scenario with the hockey puck on smooth ice with negligible friction, the puck experiences essentially zero net external force, allowing its inertia to maintain its state of motion. Choice A is correct because it accurately describes how inertia maintains the puck's constant velocity when the net external force is approximately zero, which is the fundamental principle of Newton's First Law. Choice B is incorrect because inertia is not a force that produces motion; it's simply the tendency of objects to resist changes in their motion state. To help students: Use demonstrations with air hockey tables or dry ice pucks to show motion with minimal friction. Emphasize that inertia is not a force but a property of matter that resists changes in motion.