Newton's third law Transitional Reasoning Download PDF

Questions

1. 1.

   A student swings a rubber ball attached to a light string in a horizontal circle at constant speed, feeling a steady inward tension in the string. The student claims that because the string exerts an inward horizontal force on the ball, Newton's third law requires an equal outward force acting on the ball to keep the forces balanced. How should this student's claim be evaluated according to Newton's third law of motion?

   • A) The claim is incorrect because Newton's third law action-reaction pairs act on two different interacting bodies; the reaction to the string's inward pull on the ball is an equal outward pull exerted by the ball on the string.
   • B) The claim is correct because an object traveling at a constant speed must experience zero net force, meaning the inward tension must be balanced by an outward force acting on the ball.
   • C) The claim is incorrect because Newton's third law applies exclusively to bodies moving in a straight line, not to objects undergoing continuous centripetal acceleration.
   • D) The claim is correct because the ball exerts an outward inertial force on itself that directly cancels the inward tension force to maintain a constant circular radius.
2. 2.

   During an ice skating training session on a frictionless horizontal rink, Skater M (mass 70 kg) pushes forward with an average force of 140 N against the back of Skater N (mass 50 kg), who stands relaxed with arms crossed. A student observing the interaction claims that because Skater N is completely passive, Skater N does not exert a backward force on Skater M. According to Newton's third law of motion, how does the magnitude of the horizontal force exerted by Skater N on Skater M compare to 140 N, and why?

   • A) It is 0 N, because a body must actively exert muscular effort to apply a contact force to another object.
   • B) It is 100 N, because the reaction force exerted by Skater N is reduced in proportion to the ratio of their masses (50 kg to 70 kg).
   • C) It is exactly 140 N, because contact forces arise from mutual interactions between two bodies, meaning Skater N inherently exerts an equal and opposite force on Skater M regardless of who initiated the push.
   • D) It is less than 140 N, because an unbalanced difference in interaction forces between the two skaters is required for Skater N to accelerate forward.
3. 3.

   An engineering student analyzes the horizontal acceleration of a robotic rover across a flat test track. As the rover's powered wheels rotate, they grip the track surface and push backward against the ground. According to Newton's third law of motion, which statement correctly identifies the force directly responsible for the rover's forward acceleration and its corresponding interaction partner?

   • A) The upward normal force exerted by the track on the wheels accelerates the rover forward, and its interaction partner is the downward gravitational pull of Earth on the rover.
   • B) The forward driving force exerted by the motor on the wheel axle accelerates the rover, and its interaction partner is the backward resistance exerted by the rover's chassis on the motor.
   • C) The forward static frictional force exerted by the track on the wheels accelerates the rover, and its interaction partner is the equal-magnitude backward frictional force exerted by the wheels on the track.
   • D) The backward frictional force exerted by the wheels on the track accelerates the rover, and its interaction partner is the forward drag force exerted by the surrounding air on the rover body.
4. 4.

   In a physics laboratory, two identical calibrated spring scales, Scale X and Scale Y, are connected hook-to-hook horizontally. The outer ring of Scale Y is fastened rigidly to a wall, while a student pulls horizontally on the outer ring of Scale X until Scale X indicates a steady reading of 40 N. Based on Newton's third law of motion, which statement correctly predicts the reading on Scale Y and provides the correct physical explanation?

   • A) Scale Y reads 0 N because the rigid wall acts as a static anchor that absorbs the tension rather than exerting a pulling force.
   • B) Scale Y reads 40 N because the force exerted by Scale X on Scale Y is equal in magnitude to the reaction force exerted by Scale Y on Scale X.
   • C) Scale Y reads 80 N because the 40 N pulling force applied by the student combines with the 40 N reaction force from the rigid wall.
   • D) Scale Y reads 20 N because the 40 N pulling force applied by the student is divided equally across the two scales in series.
5. 5.

   A student sits inside a low-friction wheeled wagon on a level floor and attempts to accelerate the wagon forward by pushing forcefully against the wagon's rigid front dashboard with both hands. Despite applying a substantial continuous force, the wagon and student remain stationary. Which statement correctly explains why this pushing action cannot accelerate the wagon-student system?

   • A) The forward force exerted by the hands is absorbed by the wagon's rigid frame before it can generate acceleration.
   • B) The student's mass is insufficient relative to the wagon to overcome inertia and produce forward momentum.
   • C) The forward force on the dashboard and the backward reaction force on the student act internally within the system, producing zero net external force.
   • D) The reaction force from the dashboard acts externally against the floor through the wheels, neutralizing the forward push.
6. 6.

   During a physics laboratory investigation, two identical low-friction carts, P and Q, are placed facing each other on a level track, with Cart P carrying a strong neodymium magnet and Cart Q carrying a much weaker ceramic magnet oriented so their like poles repel. A student predicts that Cart Q will experience a larger repulsive force than Cart P because Cart P possesses a significantly stronger magnetic field. How should the student's prediction be evaluated in accordance with Newton's laws?

   • A) The prediction is correct because a magnet with a stronger magnetic field always exerts a greater force on an external object than that object can exert back on the magnet.
   • B) The prediction is incorrect because Newton's third law applies to non-contact magnetic interactions, ensuring both carts experience repulsive forces of exactly equal magnitude regardless of field strength differences.
   • C) The prediction is incorrect because Newton's third law applies exclusively to direct mechanical contact forces, so the mutual magnetic forces depend entirely on the mass ratio of the carts.
   • D) The prediction is correct because the weaker magnet must first become fully polarized by the stronger magnet before it can exert any measurable counter-force.
7. 7.

   An engineering team is testing an ion thruster designed to maneuver a scientific probe through the vacuum of deep space, far from any celestial atmosphere. A critic argues that the thruster will not be able to accelerate the probe once in space because there is no surrounding air for the exhaust stream to push against. Based on Newton's third law of motion, which evaluation correctly explains how the probe accelerates in a vacuum?

   • A) The thruster exerts a force that accelerates propellant ions backward, so the exiting ions simultaneously exert a forward reaction force of equal magnitude directly on the probe.
   • B) The escaping ions build an artificial pocket of atmospheric pressure behind the nozzle that pushes the probe forward through empty space.
   • C) The engine eliminates the need for an interaction pair because Newton's third law only governs objects interacting within a physical medium.
   • D) The vacuum of space pulls on the ejected propellant particles, creating an external suction force that draws the attached probe forward.
8. 8.

   During a field day tug-of-war contest, Team X pulls Team Y across the marker line, causing both teams to accelerate toward Team X's side. A student watching the contest claims that Team X won because the pull exerted by Team X on the rope was greater in magnitude than the pull exerted by Team Y on the rope. How should this student's claim be evaluated in light of Newton's laws of motion?

   • A) The claim is correct because an object or system can only accelerate in the direction of the larger of the two interacting tension forces acting at the ends of the rope.
   • B) The claim is incorrect because the rope exerts equal-magnitude forces on both teams; Team X wins because the ground exerts a greater static friction force on Team X than on Team Y.
   • C) The claim is incorrect because the rope tension drops to zero once acceleration begins; Team X wins solely because its members have a greater total mass than Team Y.
   • D) The claim is correct because Newton's third law applies only to stationary systems; once acceleration occurs, the pulling force of the winning team exceeds the reaction force.
9. 9.

   A student observes a stationary heavy wooden box resting on a level floor and argues that the downward gravitational force exerted by Earth on the box and the upward normal force exerted by the floor on the box constitute a Newton's third law action-reaction pair because they are equal in magnitude and opposite in direction. Which statement correctly identifies the flaw in the student's reasoning and names the true reaction force to the gravitational force acting on the box?

   • A) Action-reaction pairs only apply to moving objects, and the true reaction force is the downward contact force exerted by the box onto the floor.
   • B) The two identified forces act on the same object rather than mutually between two interacting bodies, and the true reaction force is the upward gravitational pull exerted by the box on Earth.
   • C) Action-reaction forces cannot be perpendicular to the contact surface, and the true reaction force is the horizontal static friction between the floor and the box.
   • D) Normal forces are contact forces whereas gravity is non-contact, meaning the true reaction force is the upward buoyant force exerted by the surrounding air on the box.
10. 10.

    During a collision experiment on a horizontal frictionless track, a 1.5 kg motorized cart strikes a stationary 0.5 kg target cart. Sensitive force sensors mounted on the contact surfaces record a peak impact force of 18 N acting on the target cart during the impact. According to Newton's third law and Newton's second law, what peak force does the motorized cart experience, and how does the magnitude of its acceleration compare to that of the target cart during the interaction?

    • A) A peak force of 54 N, and an acceleration of greater magnitude than the target cart
    • B) A peak force of 18 N, and an acceleration of smaller magnitude than the target cart
    • C) A peak force of 18 N, and an acceleration of equal magnitude to the target cart
    • D) A peak force of 6 N, and an acceleration of smaller magnitude than the target cart