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PHYSICS, M2 EQ-Bank 1

Jordan is trying to push a heavy filing cabinet across the office floor, but it’s not budging. The cabinet has been used by physics students, and someone has stuck a post-it note on it with a “reminder” that reads:

“According to Newton’s third law, the cabinet pushes back with the same force you apply — so you’ll never move it!”

Jordan sighs and tries harder. Another note on the cabinet reads:

“Don’t bother! The law of conservation of momentum says that if the cabinet is at rest, its momentum must stay at zero forever.”

Identify and explain the two misunderstandings about Newton’s third law and the law of conservation of momentum. Use correct physics principles to explain how Jordan can, in fact, move the filing cabinet.    (5 marks)

Show Answers Only

Newton’s third law misconception:

  • Newton’s third law states that every action has an equal and opposite reaction, but these force pairs act on different objects.
  • Jordan pushes the cabinet, and the cabinet pushes back on him with equal force. These forces do not cancel because they act on separate objects.
  • The motion of the cabinet depends on the net force acting on it, according to Newton’s second law:  \(F_{\text{net}} = ma\).

 Conservation of momentum misconception:

  • Momentum is conserved only in a closed system with no external net forces.
  • Since Jordan exerts an external force on the cabinet and the floor provides friction, the cabinet is not a closed system.
  • Therefore, momentum conservation does not prevent Jordan from moving the cabinet.

 How Jordan can move the cabinet:

  • By bracing his feet, Jordan increases backward friction against the floor, allowing him to exert a greater forward force.
  • If this force exceeds static friction, the cabinet accelerates and its momentum changes:  \(\Delta p = F \Delta t\).
Show Worked Solution

Newton’s third law misconception:

  • Newton’s third law states that every action has an equal and opposite reaction, but these force pairs act on different objects.
  • Jordan pushes the cabinet, and the cabinet pushes back on him with equal force. These forces do not cancel because they act on separate objects.
  • The motion of the cabinet depends on the net force acting on it, according to Newton’s second law:  \(F_{\text{net}} = ma\).

 Conservation of momentum misconception:

  • Momentum is conserved only in a closed system with no external net forces.
  • Since Jordan exerts an external force on the cabinet and the floor provides friction, the cabinet is not a closed system.
  • Therefore, momentum conservation does not prevent Jordan from moving the cabinet.

 How Jordan can move the cabinet:

  • By bracing his feet, Jordan increases backward friction against the floor, allowing him to exert a greater forward force.
  • If this force exceeds static friction, the cabinet accelerates and its momentum changes:  \(\Delta p = F \Delta t\).