Newton's First Law
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AP Physics C: Mechanics › Newton's First Law
A particle is in translational equilibrium. Which of the following statements must be true about the particle?
The particle's speed must be constant, but its direction of motion may be changing.
The sum of the magnitudes of all forces acting on the particle must be zero.
The particle must be at rest relative to the observer.
The particle's velocity vector must be constant.
Explanation
Translational equilibrium means the net force on the particle is zero. According to Newton's First Law, this implies that the particle's acceleration is zero, which means its velocity must be constant. A constant velocity vector means both the speed and the direction of motion are unchanging. Being at rest is a special case where the constant velocity is zero.
An object is moving to the right with a constant velocity. If a single, constant force is then applied to the object, which of the following is NOT a possible resulting motion?
The object instantaneously stops and remains at rest.
The object continues moving right but its speed increases.
The object's path curves, and it begins to move upwards and to the right.
The object continues moving right but its speed decreases.
Explanation
A net force causes acceleration, which means the velocity must change. However, velocity is the integral of acceleration, so it must change continuously over time. An object cannot instantaneously stop unless an infinite force is applied over an infinitesimal time. The object would first have to decelerate to zero velocity.
A hockey puck slides on a sheet of frictionless ice at a constant speed of $$10 \text{ m/s}$$. What is the net force acting on the puck?
Zero.
A horizontal force that is directly proportional to the puck's speed.
A constant horizontal force in the direction of the puck's velocity to maintain the speed.
A force with a magnitude equal to the puck's weight, acting perpendicular to the ice.
Explanation
The puck is moving with a constant velocity (constant speed and direction). According to Newton's First Law, if an object's velocity is constant, the net force acting on it must be zero. The vertical forces (gravity and normal force) cancel, and there is no horizontal force in the absence of friction or propulsion.
A heavy crate rests motionless on a horizontal floor. According to Newton's First Law, what can be concluded about the forces acting on the crate?
A static friction force is the primary force preventing the crate from starting to move on its own.
The gravitational force and the normal force are the only forces, and they happen to be equal and opposite.
The vector sum of all forces acting on the crate is zero, maintaining its state of rest.
The gravitational force is the only force acting on the crate, but it is not strong enough to cause motion.
Explanation
Since the crate is at rest, its velocity is constant (zero). Newton's First Law states that for an object to have a constant velocity, the net force (the vector sum of all forces) acting on it must be zero. Other forces besides gravity and the normal force could be present, but the net effect of all forces must be zero.
A heavy ball is at rest in the middle of a frictionless horizontal wagon. The wagon is suddenly pulled forward with a constant acceleration. What is the motion of the ball as observed from a stationary frame of reference on the ground?
The ball remains at rest with respect to the ground.
The ball accelerates backward with respect to the ground.
The ball accelerates forward with the wagon due to inertia.
The ball moves backward with a constant velocity relative to the ground.
Explanation
From the perspective of an observer on the ground (an inertial frame), the ball is initially at rest. Since the wagon bed is frictionless, there is no horizontal force exerted on the ball when the wagon is pulled. According to Newton's First Law, with zero net horizontal force, the ball's state of horizontal motion will not change. It will remain at rest relative to the ground.
A spaceship is drifting in deep space, far from any significant gravitational influences, with its engines turned off. It is moving at a constant velocity. To maintain this constant velocity, the spaceship must:
fire its engines in the direction opposite to its velocity to prevent it from speeding up on its own.
continuously fire its engines with a small, constant force to counteract cosmic drag.
periodically fire its engines to provide boosts that compensate for a natural tendency to slow down.
do nothing, as no net force is required to maintain a constant velocity.
Explanation
According to Newton's First Law, an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force. In deep space with engines off, the net force is zero. Therefore, the spaceship will continue to move at a constant velocity without any action.
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?
Its inertia maintains constant velocity unless a net external force acts on it.
A net force is always required to maintain motion, so it must be accelerating slightly.
A constant thrust remains stored in the spacecraft and continuously pushes it forward.
The absence of air creates a suction force that locks its velocity in place.
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 inertia maintains constant velocity when no net external force acts.
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 the road exerts a forward normal force that sustains motion.
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 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.
Inertia maintains its state of rest because 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 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?
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.
The puck accelerates because any moving object must experience a net external force.
A constant forward force from the ice must continuously act to maintain motion.
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.