A parachutist falling toward Earth opens their parachute and reaches a constant terminal velocity. At this point, what is the relationship between the magnitude of the gravitational force Fg and the magnitude of the upward air drag force Fd?
Answer: Fg=Fd
The parachutist is moving at a constant terminal velocity. According to Newton's First Law, this means the net force acting on the parachutist is zero. The forces are the downward gravitational force Fg and the upward drag force Fd. For the net force to be zero, these forces must be equal in magnitude and opposite in direction. Thus, Fg=Fd.
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:
Answer: do nothing, as no net force is required to maintain a constant velocity.
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.
A hockey puck slides on a sheet of frictionless ice at a constant speed of 10 m/s. What is the net force acting on the puck?
Answer: Zero.
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 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?
Answer: The ball remains at rest with respect to the ground.
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.
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?
Answer: Its inertia maintains constant velocity when the net external force is approximately zero.
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.
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?
Answer: Inertia maintains its state of rest because the net external force is approximately zero.
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.
An object moves with a constant velocity in a straight line. Which of the following statements about the object must be true?
Answer: The net force on the object is zero.
Newton's First Law states that an object will maintain a constant velocity if and only if the net force acting on it is zero. There can be multiple forces acting on the object, such as gravity and a normal force, but they must vectorially sum to zero for the velocity to be constant.
An elevator is moving upward at a constant speed of 2.0 m/s. A person of mass M is standing on a scale inside. What is the magnitude of the normal force exerted by the elevator floor on the person, which is what the scale reads?
Answer: Equal to Mg
The elevator is moving at a constant velocity, so its acceleration is zero. By Newton's First Law, the net force on the person must be zero. The forces acting on the person are the upward normal force N and the downward gravitational force Mg. For the net force to be zero, N−Mg=0, which means N=Mg.
A box slides down a frictionless inclined plane at constant velocity. A student observes this and concludes that Newton's first law applies because the velocity is constant. What is wrong with this reasoning?
Answer: The reasoning is incorrect because a box cannot slide down a frictionless incline at constant velocity - it must accelerate due to gravity
On a frictionless inclined plane, the only forces acting on the box are gravitational force (mg downward) and normal force (perpendicular to the surface). The component of gravitational force along the incline (mg sin θ) is unbalanced, so there must be net force down the incline. This means the box cannot move at constant velocity - it must accelerate down the incline. The scenario described is physically impossible. Newton's first law requires zero net force for constant velocity. Choice A incorrectly ignores force analysis. Choice C incorrectly limits first law to horizontal motion. Choice D incorrectly suggests normal force creates net force - normal force is perpendicular to motion.
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?
Answer: The object instantaneously stops and remains at rest.
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 car travels around a circular track at constant speed. The driver claims the car obeys Newton's first law because "it maintains constant velocity." A physics student argues this violates Newton's first law. Who is correct and why?
Answer: The student is correct because velocity is a vector quantity, and changing direction means changing velocity, requiring net force
Newton's first law states that an object remains at rest or in uniform motion (constant velocity) unless acted upon by a net external force. Velocity is a vector with both magnitude and direction. In circular motion at constant speed, the direction continuously changes, meaning velocity is not constant. This requires centripetal force and violates Newton's first law. Choice A incorrectly equates constant speed with constant velocity. Choice C misunderstands that any change in velocity (including direction) requires force. Choice D incorrectly mentions tangential acceleration - in uniform circular motion, there's only centripetal acceleration.
A satellite orbits Earth in a perfectly circular orbit at constant speed. An engineer argues that the satellite demonstrates Newton's first law because "no propulsion is needed - it just continues its natural motion." A physicist disagrees. Who is correct?
Answer: The physicist is correct because circular motion requires continuous centripetal acceleration, which necessitates net force and violates Newton's first law
This question tests your understanding of Newton's first law and circular motion—a common area where students mix up constant speed with uniform motion.
The physicist is correct. While the satellite moves at constant speed, it's continuously changing direction, which means it has acceleration. According to Newton's second law, F=ma, any acceleration requires a net force. In this case, Earth's gravitational pull provides the centripetal force Fc=rmv2 that keeps the satellite in circular motion. Since there's a net force acting on the satellite, Newton's first law (which applies only when net force is zero) doesn't govern this situation.
Choice B incorrectly focuses on kinetic energy. While the satellite's kinetic energy remains constant, Newton's first law concerns velocity (a vector), not energy. Constant speed with changing direction still violates the first law.
Choice C misapplies the concept by ignoring that "uniform motion" in Newton's first law means constant velocity (both speed and direction), not just motion without friction. The satellite's direction continuously changes.
Choice D is factually wrong—satellites in stable orbits don't need thrusters to maintain their circular paths. Gravity alone provides the necessary centripetal force.
Remember this key distinction: Newton's first law applies only to objects with zero net force, which means either at rest or moving in a straight line at constant speed. Any curved path, even at constant speed, requires continuous acceleration and therefore net force, making the first law inapplicable.
Which of the following objects is in a state of translational equilibrium?
Answer: A block being pulled at a constant velocity across a rough horizontal surface.
Translational equilibrium occurs when the net force on an object is zero, resulting in a constant velocity. A block moving at a constant velocity has zero acceleration, so the net force on it must be zero. The other options describe objects that are accelerating because their direction of velocity is changing (A, C) or they are under the constant influence of gravity (B).
A student is in a car that is accelerating forward from rest. The student observes a toy hanging from the rearview mirror swing backward and remain at a steady angle with respect to the vertical. From the perspective of a reference frame attached to the accelerating car, the toy is stationary. Why does Newton's First Law appear to be violated in this frame?
Answer: The car's frame is non-inertial, and Newton's First Law only applies in inertial frames.
The accelerating car is a non-inertial reference frame. In this frame, the toy is at rest (constant velocity of zero), but the vector sum of the real forces (tension and gravity) is not zero; it points forward. This apparent violation occurs because Newton's First Law is only valid in inertial (non-accelerating) frames of reference.
A particle is subject to two forces, F1=(3i^−4j^) N and F2. The particle is moving with a constant velocity of v=(2j^) m/s. What is the force F2?
Answer: (−3i^+4j^) N
The particle is moving with a constant velocity, which means its acceleration is zero. According to Newton's First Law, the net force on the particle must be zero. The net force is the vector sum Fnet=F1+F2. For this to be zero, F2 must be the negative of F1. So, F2=−F1=−(3i^−4j^)=(−3i^+4j^) N.
An arrow is shot from a bow. After it leaves the bowstring, and ignoring the effects of air resistance, which of the following best describes the net force acting on the arrow while it is in mid-flight?
Answer: Only the downward force of gravity.
Once the arrow leaves the bow, the bow no longer exerts a force on it. The concept that a forward force is needed to maintain motion is a common misconception that contradicts Newton's First Law. Ignoring air resistance, the only significant force acting on the arrow during its flight is the force of gravity, which acts downward.
A book sits on a table. A student argues that this demonstrates Newton's first law because "the book remains at rest." However, the book experiences gravitational force downward and normal force upward. Why does this situation actually exemplify Newton's first law?
Answer: Newton's first law applies because the gravitational and normal forces are equal in magnitude and opposite in direction, resulting in zero net force
When analyzing forces and motion, always start by identifying all forces acting on an object and determining the net force. Newton's first law states that an object at rest stays at rest (or an object in motion stays in motion) when the net force acting on it is zero.
The book experiences two forces: gravitational force pulling it downward with magnitude mg and normal force from the table pushing upward. Since the book remains at rest, these forces must be equal in magnitude and opposite in direction, creating a net force of zero. This zero net force is precisely what Newton's first law describes - when no net force acts on an object, it maintains its state of motion (in this case, remaining at rest).
Choice A incorrectly suggests that gravity and normal force are internal forces. Both are external forces acting on the book - gravity from Earth and normal force from the table. Choice B reflects the outdated Aristotelian view that objects have "natural tendencies" toward rest or motion. Newton's first law doesn't depend on natural states but on net forces. Choice C makes a common error by thinking that the presence of individual forces means Newton's first law doesn't apply. However, Newton's first law specifically addresses situations where forces are present but balanced.
Remember this key distinction: Newton's first law applies when net force equals zero (which can mean either no forces or balanced forces), while Newton's second law (Fnet=ma) applies when net force is non-zero. Static equilibrium is actually a special case of Newton's first law, not a separate concept.
Which of the following best describes an inertial frame of reference?
Answer: A frame of reference in which an object with zero net force acting upon it moves with a constant velocity.
An inertial reference frame is defined as a frame in which Newton's First Law holds true. That is, an object subjected to zero net force will move with a constant velocity (which can be zero). Accelerating frames are non-inertial.
A spider of mass m hangs from a single strand of silk, which is attached to the ceiling. The spider descends, lowering itself at a constant speed. What is the tension T in the strand of silk?
Answer: Equal to mg
The spider is moving at a constant speed, which means its velocity is constant. By Newton's First Law, the net force acting on the spider must be zero. The forces are the upward tension T from the silk and the downward gravitational force mg. For the net force to be zero, T−mg=0, so T=mg.
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?
Answer: Its inertia maintains constant velocity unless a net external force acts on it.
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.
