Contact and field forces Transitional Reasoning Download PDF

Questions

1. 1.

   An electromechanical door chime uses an energized electromagnet to pull an iron striker across an open air gap without touching it. The striker then collides directly against a metal chime bar to produce a sound, after which a compressed mechanical spring pushes the striker back to its starting position. Which statement correctly classifies the primary forces driving these three stages of motion?

   • A) The electromagnetic pull is a field force because it acts across space without physical contact, whereas the impact on the chime bar and the push from the spring are contact forces because they require direct physical touch.
   • B) The electromagnetic pull and the push from the spring are field forces because both can act through stored energy, whereas only the impact on the chime bar is a contact force because it involves a collision.
   • C) All three interactions are contact forces because the surrounding air molecules in the gap physically transmit the magnetic pull from the electromagnet to the striker.
   • D) The electromagnetic pull and the spring push are contact forces because both originate from physical components within the mechanism, whereas the impact on the chime bar is a field force because it transfers energy through sound vibrations.
2. 2.

   A science class investigates the operating principles of two direction-sensing devices: a magnetic compass and an aerodynamic wind vane. Both instruments are placed inside a transparent chamber from which all atmospheric air is completely evacuated using a vacuum pump. When an external magnet is moved near the chamber, the compass needle immediately pivots toward it, but when an internal motorized fan is turned on, the wind vane remains completely stationary. Which statement correctly evaluates why these two instruments behave differently inside the evacuated chamber?

   • A) The compass needle rotates because the external magnet exerts a contact force transmitted through the chamber walls, whereas the wind vane fails to rotate because field forces cannot act in a vacuum.
   • B) The compass needle rotates because magnetic forces are field forces that act across space without a material medium, whereas the wind vane remains stationary because aerodynamic forces are contact forces requiring physical collisions with air molecules.
   • C) Both instruments rely on contact forces, but the magnetic field creates microscopic mechanical collisions across the vacuum, while the fan cannot generate sufficient contact force without a solid connection.
   • D) Both instruments rely on field forces, but the magnetic field can penetrate glass, whereas the aerodynamic field produced by the fan is completely absorbed by the evacuated environment.
3. 3.

   During an automated manufacturing process, an engineer tests two devices designed to pull steel pins along an assembly line. Device 1 exerts a pulling force on a pin only while its probe physically touches the pin's surface, and the force drops immediately to zero the moment any separation distance exists. Device 2 exerts a pulling force on a pin across an open air gap, and as the separation distance increases from 1 mm to 10 mm, the pulling force gradually weakens but continues to act without physical contact. Which statement correctly evaluates the forces exerted by Device 1 and Device 2 based on how they transmit force across distance?

   • A) Device 1 exerts a field force because its pull is concentrated at a single touch point, whereas Device 2 exerts a contact force that uses intervening air molecules to push the pin.
   • B) Both devices exert field forces because both alter the motion of metallic objects, but Device 1 has an extremely short field range that abruptly terminates at its surface.
   • C) Device 1 exerts a contact force because it requires physical contact and cannot act across a separation distance, whereas Device 2 exerts a field force because it acts through a field across space without physical contact.
   • D) Device 1 exerts a field force because physical contact eliminates field resistance, whereas Device 2 exerts a contact force because its pulling strength decreases as distance increases.
4. 4.

   A student rubs an inflated rubber balloon with wool to give it an electric charge and places it against a flat, vertical wooden wall. When released, the balloon remains stationary, sticking to the wall without sliding downward. Which statement correctly classifies the forces maintaining the balloon in equilibrium?

   • A) Two field forces act on the balloon (downward gravity and horizontal electrostatic attraction), balanced by two contact forces from the wall (upward static friction and outward normal force).
   • B) Three contact forces act on the balloon (upward static friction, outward normal force, and electrostatic attraction), while downward gravity is the only field force present.
   • C) Two field forces act on the balloon (downward gravity and an upward electrostatic force), while the outward normal force from the wall is the only contact force present.
   • D) Four contact forces act on the balloon, because once the surfaces touch, all field interactions are replaced by mechanical contact forces.
5. 5.

   In a dynamic loudspeaker, an alternating electrical current passes through a lightweight voice coil suspended in the gap of a permanent magnet, causing the coil to vibrate rapidly without touching the magnet. This vibrating coil is firmly bonded to a flexible cone diaphragm that pushes directly against the adjacent air to generate audible sound waves. Which statement correctly evaluates how forces drive the motion of the coil and the air in this loudspeaker system?

   • A) Both interactions rely on contact forces, because the magnetic interaction requires the surrounding air to transmit force to the coil, just as the diaphragm requires air contact to produce sound.
   • B) The magnet drives the voice coil via a field force that acts across space without physical contact, whereas the cone exerts a contact force on the air through direct physical collisions with gas molecules.
   • C) The voice coil is propelled by a contact force through electromagnetic friction at its boundary, whereas the diaphragm influences distant air molecules via an acoustic field force.
   • D) Both interactions rely on field forces, because energy is transferred to the air and to the coil without forming a permanent physical bond between the interacting bodies.
6. 6.

   An aerospace engineer monitors a satellite traveling through the outermost edge of Earth's atmosphere, where it experiences both a downward gravitational pull toward Earth and an opposing drag force from collisions with sparse gas molecules. The flight team plans to boost the satellite into a much higher orbit where atmospheric particles are completely absent. Which statement correctly evaluates the classification of these forces and explains why only one of them continues to act in the higher orbit?

   • A) Gravitational force is a contact force that requires the atmosphere as a transmitting medium, meaning both forces cease to act once the satellite enters the vacuum of the higher orbit.
   • B) Gravitational force is a field force that acts across empty space without physical contact, whereas atmospheric drag is a contact force that ceases once collisions with matter no longer occur.
   • C) Both forces are field forces because the satellite moves freely without touching solid ground, but drag ceases because field forces cannot exist in regions of zero pressure.
   • D) Atmospheric drag is a field force exerted by the surrounding gas across open space, whereas gravitational force is a contact force requiring physical interaction with Earth's atmospheric envelope.
7. 7.

   Chemical processing plants often transfer hazardous fluids using magnetic-drive pumps rather than conventional direct-drive pumps. In a conventional pump, an electric motor turns a solid drive shaft that directly touches and locks against the impeller, transmitting rotational motion through mechanical contact forces. In a magnetic-drive pump, the motor spins an external ring of magnets that rotates an isolated internal magnetic impeller across an unbroken solid barrier without any physical touch between them. Which statement correctly evaluates the nature of the forces used to drive the impeller in these two pump systems?

   • A) Both pump designs rely strictly on contact forces, because magnetic fields require direct physical touch between metallic surfaces to transfer mechanical energy.
   • B) The standard pump transmits torque via contact forces requiring direct physical touch between components, whereas the magnetic-drive pump transmits torque via non-contact field forces that act across space through the barrier.
   • C) The standard pump relies on field forces generated by friction along the drive shaft, whereas the magnetic-drive pump converts magnetic interactions into contact forces at the barrier wall.
   • D) The standard pump utilizes contact forces between solid parts, whereas the magnetic-drive pump transmits torque through contact forces passed mechanically from atom to atom through the barrier.
8. 8.

   An engineering team evaluates two frictionless transport prototypes: a hovercraft supported by a cushion of high-pressure air and a maglev vehicle suspended by magnetic repulsion between the car and its track. The team considers what would occur if both systems were operated inside an environmental chamber from which all atmospheric gas is evacuated. Which statement correctly evaluates the nature of the upward supporting force in each system and predicts its behavior in a vacuum?

   • A) The hovercraft relies on a field force because no solid parts touch, so both vehicles will maintain their suspended height equally well in a vacuum.
   • B) The hovercraft is supported by contact forces from gas molecules striking its base and loses its lift in a vacuum, whereas the maglev vehicle relies on a magnetic field force that persists without a material medium.
   • C) Both vehicles rely on contact forces transmitted through the surrounding air, so both systems will fail to maintain levitation once the chamber is evacuated.
   • D) The hovercraft relies on contact forces that increase in strength in a vacuum, whereas the maglev vehicle relies on an electric field force that requires ambient air to function.
9. 9.

   A student compares two desktop displays that hold identical steel spheres of equal mass stationary at the same height above a table. In Display 1, the sphere rests directly on top of a rigid vertical post. In Display 2, the sphere hovers completely motionless in mid-air beneath an overhead electromagnetic unit without touching any solid structure. Neglecting any buoyant force from the air, which statement correctly evaluates the forces maintaining equilibrium in the two displays?

   • A) Both displays are held in equilibrium by an upward contact force balancing a downward field force.
   • B) Display 1 is held in equilibrium entirely by contact forces, whereas Display 2 is held in equilibrium entirely by field forces.
   • C) Display 1 is held in equilibrium by a contact force balancing a field force, whereas Display 2 is held in equilibrium entirely by field forces.
   • D) Display 1 is held in equilibrium entirely by field forces, whereas Display 2 is held in equilibrium by an upward contact force from the air balancing a field force.
10. 10.

    A coastal research facility uses two different systems to study water movement: a mechanical wave tank in which a motorized plate repeatedly pushes directly against the water, and an astronomical model illustrating how the Moon pulls on Earth's ocean water to produce tidal bulges. Which statement correctly evaluates the forces driving water motion in these two systems?

    • A) The motorized plate exerts a contact force that requires physical touching at the boundary with the water, whereas the Moon exerts a gravitational field force that acts across empty space without physical contact.
    • B) The motorized plate exerts a contact force that requires a solid barrier to transmit, whereas the Moon exerts a field force that requires Earth's atmosphere as a physical medium to transmit the gravitational pull.
    • C) The motorized plate exerts a field force because the generated waves travel through the water without the plate touching every molecule, whereas the Moon exerts a contact force through direct friction with the atmosphere.
    • D) The motorized plate exerts a contact force that can act across an air gap at high speeds, whereas the Moon exerts a gravitational force that ceases to exist in the vacuum of space.
11. 11.

    In an analytical laboratory, two instruments deflect the motion of substances along curved paths. In a mechanical centrifuge, sample tubes are kept moving in a circle because the rigid walls of the spinning metal rotor push continuously against the outer surfaces of the tubes. In a magnetic mass analyzer, a beam of positively charged ions is deflected into a curved path across an open chamber by a magnetic field without making physical contact with any structure. Which statement correctly evaluates the forces responsible for maintaining the curved paths in these two instruments?

    • A) Both instruments rely on contact forces, because the magnetic field in the mass analyzer acts as an invisible material membrane that mechanically collides with the ions to bend their trajectory.
    • B) The centrifuge relies on a field force because the rotor creates a centrifugal field throughout the chamber, whereas the mass analyzer relies on a contact force because the magnetic field only acts when ions touch magnetic field lines.
    • C) The centrifuge relies on a contact force because the push requires direct physical touching between the solid rotor and the tubes, whereas the mass analyzer relies on a field force because the magnetic interaction acts across space without physical contact.
    • D) Both instruments rely on field forces, because any force that produces inward acceleration along a curved path must act through fundamental fields rather than mechanical surface contact.
12. 12.

    An industrial air-cleaning system uses two stages to remove airborne dust particles: Stage 1 directs the stream through a dense fibrous mesh where particles are halted upon colliding with solid fibers, while Stage 2 guides the stream through an electrostatic chamber where negatively charged dust particles are drawn across an open space toward positively charged collector plates. An engineer notes that the collector plates in Stage 2 exert a deflecting force on the charged particles across the gap without any physical connection, whereas Stage 1 exerts no force unless a particle physically touches a fiber. Which statement correctly evaluates the forces acting on the particles in these two stages based on the distinction between contact and field forces?

    • A) Stage 1 uses a field force because the air current mediates the push on the particles, while Stage 2 uses a contact force because the electric field must physically compress the intervening air to push the particles.
    • B) Stage 1 relies on contact forces that require direct physical contact between the fibers and the particles, whereas Stage 2 relies on an electric field force that acts through space without requiring physical contact.
    • C) Both stages rely fundamentally on field forces because the subatomic origin of every macroscopic contact force is an exchange of field interactions without true mechanical contact.
    • D) Stage 1 relies on contact forces that can exert influence over large separations through the surrounding air, whereas Stage 2 relies on field forces that only exist when a particle makes direct contact with a charged plate.
13. 13.

    A lightweight scrap of paper rests stationary on a horizontal table while a negatively charged rod is held fixed directly above it without touching it. The upward electric pull from the rod causes the paper to lift off the table and accelerate upward through the air toward the rod. Which statement correctly analyzes the forces acting on the paper just before lift-off (while touching the table) and in midair (before touching the rod), neglecting air resistance?

    • A) Just before lift-off, the electric force is classified as a contact force because the charged rod uses the surrounding air as a physical medium to transmit its pull to the paper.
    • B) In midair, the upward acceleration requires an upward contact force from the surrounding air to overcome the downward pull of gravity.
    • C) Just before lift-off, the paper experiences both contact and field forces, whereas in midair, it is acted upon solely by field forces because gravitational and electric interactions do not require physical contact.
    • D) In midair, the gravitational force stops acting once physical contact with the table is lost, so only the contact force exerted by the electric field influences the paper.
14. 14.

    A gymnast bounces vertically on a trampoline in a gymnasium. At the lowest point of the bounce, the stretched fabric of the mat exerts an upward push on the gymnast's feet while Earth pulls downward on the gymnast. A moment later, the gymnast rises into midair and reaches the highest point of the jump, where physical contact with the mat has ended and air resistance is negligible. Which statement correctly classifies the forces acting on the gymnast at these two points in the motion?

    • A) At the lowest point, both a contact force and a field force act on the gymnast, whereas at the highest point, only a field force acts.
    • B) At the lowest point, only contact forces act on the gymnast, whereas at the highest point, both contact and field forces act.
    • C) At both points, only contact forces act on the gymnast because the surrounding air directly touches the gymnast to transmit gravity.
    • D) At both points, only field forces act on the gymnast because the upward restoring push from the mat originates from atomic interactions.
15. 15.

    A student holds a solid plastic block completely submerged and stationary in a container of water by pressing downward on its top surface with a vertical rod. In this state, three distinct forces act on the block: the downward push from the rod, the upward buoyant force from the surrounding water, and the downward gravitational force from Earth. Which statement correctly categorizes these forces into contact forces and field forces?

    • A) Only the push from the rod is a contact force, while both the buoyant force and the gravitational force are field forces because they act across the entire body.
    • B) All three forces are contact forces because the submerged block is in continuous physical contact with the surrounding fluid medium.
    • C) The push from the rod and the gravitational force are contact forces, while the buoyant force is a field force arising from depth-dependent pressure.
    • D) The push from the rod and the buoyant force are contact forces, while the gravitational force is a field force.
16. 16.

    A student investigates two methods of deflecting a positively charged lightweight sphere suspended by an insulating thread. In Trial 1, a directed stream of air from a nozzle pushes the sphere sideways to maintain an equilibrium angle. In Trial 2, the nozzle is turned off, and a negatively charged plate is placed at a distance along the same horizontal line, holding the sphere at an equilibrium angle across the open gap without touching it. Which statement correctly compares the fundamental mechanisms of the horizontal forces acting on the sphere in Trial 1 and Trial 2?

    • A) Trial 1 is driven by a field force generated by fluid pressure gradients, whereas Trial 2 is driven by a contact force mediated by charge transfer across the gap.
    • B) Both trials are driven by contact forces because the sphere in Trial 2 requires intervening air molecules to transmit the electrostatic pull toward the plate.
    • C) Trial 1 is caused by a contact force resulting from physical collisions with moving air molecules, whereas Trial 2 is caused by a field force that acts across space without requiring a material medium.
    • D) Both trials are driven by field forces because the air nozzle and the charged plate both exert their influences across an open gap without touching the sphere directly.
17. 17.

    An uncharged glider coasts along a horizontal track and gradually comes to rest due to friction between the solid surfaces. In a subsequent run, a magnet is mounted on the front of the glider and a like-poled magnet is fixed at the end of the track, causing the moving glider to slow down, halt, and reverse direction before touching the end barrier. Which statement correctly compares the nature of the primary forces responsible for stopping the glider in these two trials?

    • A) Both forces are contact forces because the magnetic repulsion requires surrounding air molecules to transfer the force to the glider.
    • B) Both forces are field forces because gravitational attraction must hold the glider against the track for any deceleration to occur.
    • C) Friction is a contact force requiring direct physical contact between surfaces, whereas magnetic repulsion is a field force acting across space without physical contact.
    • D) Friction is a field force because it opposes motion through a resistive field, whereas magnetic repulsion is a contact force mediated by physical field lines.
18. 18.

    A student places a block with an embedded permanent magnet onto an electronic balance, which registers the upward contact force supporting the block. When the student holds a second magnet stationary directly above the block with repelling poles facing each other, the balance reading increases even though the two magnets never touch. Which statement correctly explains this observation based on the concepts of contact and field forces?

    • A) The balance reading increases because the magnetic field acts exclusively on the internal mechanism of the balance while bypassing the block entirely.
    • B) The non-contact magnetic repulsion adds a downward field force to the block, requiring a greater upward contact force from the balance surface to maintain equilibrium.
    • C) The balance reading increases because the upper magnet converts the gravitational field force into an upward contact force between the magnets.
    • D) The non-contact magnetic repulsion produces an upward field force on the balance surface, reducing the contact force needed to support the block.
19. 19.

    A science teacher releases a flat sheet of paper and a dense steel marble simultaneously from the same height inside a tall vertical tube filled with air, observing that the marble hits the base first. When the air is completely pumped out of the tube to create a vacuum, both objects fall side by side and reach the base at the exact same instant. Which conclusion correctly accounts for this observation using the principles of contact and field forces?

    • A) Air resistance is a contact force that vanishes in a vacuum because it requires physical collisions with fluid particles, whereas gravity is a field force that acts through space without requiring matter.
    • B) Air resistance is a field force that ceases to exist without atmospheric pressure, whereas gravity is a contact force that requires ambient air molecules to transfer Earth's downward pull to the objects.
    • C) Both air resistance and gravity are contact forces, but the paper only experiences gravity once air molecules are present to establish physical contact between the paper and the ground.
    • D) Both air resistance and gravity are field forces, but removing the air neutralizes the electrostatic attraction that previously accelerated the dense marble more than the paper.
20. 20.

    In a physics laboratory, a small steel sphere suspended from a light thread hangs at rest inside an evacuated glass chamber where air has been completely removed. When a strong bar magnet is held stationary outside the chamber, the sphere deflects toward the magnet and remains balanced at an angle without touching the glass wall. Which option correctly classifies every force acting on the stationary steel sphere as either a contact force or a field force?

    • A) Contact forces: Tension force and magnetic force; Field force: Gravitational force
    • B) Contact force: Gravitational force only; Field forces: Tension force and magnetic force
    • C) Contact force: Tension force only; Field forces: Gravitational force and magnetic force
    • D) Contact forces: None; Field forces: Tension force, gravitational force, and magnetic force