A wooden crate is pushed across a rough concrete warehouse floor at a constant velocity by a worker applying a horizontal force. Explain why the worker must continue to apply a force to keep the crate moving at constant velocity, and describe what would happen to the motion of the crate if the worker suddenly doubles the pushing force.

Edexcel A-Level Physics (9PH0) — 9.1 Kinetic theory and ideal gases · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A wooden crate of weight 400 N is pushed across a rough concrete warehouse floor. The worker applies a constant horizontal force and the crate moves at a steady speed.

Model answer (5 marks)

1. The rough floor creates a friction force that acts opposite to the direction of motion.
2. At constant speed the net horizontal force is zero, so the worker’s applied force must balance the friction force.
3. If the worker stopped applying a force, the friction force would be unopposed and the crate would decelerate until it stopped.
4. If the worker suddenly doubled the applied force, the applied force would exceed the friction force.
5. The unbalanced force would produce a net force in the direction of motion, causing the crate to accelerate forward.

Examiner tips

  • Use the word "balance" to show forces are equal at constant speed. Show the direction of friction and the applied force. Explain the consequence of an unbalanced force (deceleration or acceleration).

Common mistakes

  • Confusing friction as a force that only acts when the crate is moving; it acts whenever surfaces are in contact. Assuming the crate will keep moving without force. Not recognising that doubling the force creates a net force and acceleration.

Mark scheme (5 marks)

  1. Friction acts on the crate in the opposite direction to motion (because the surfaces are rough / in contact)
  2. At constant velocity the forces are balanced / resultant force is zero, so the applied force must equal friction
  3. If the worker applied no force, friction would decelerate / slow the crate (because there would be a resultant force in the direction of friction)
  4. When the applied force is doubled, the pushing force is greater than friction, so there is a resultant force in the direction of motion
  5. The resultant force causes the crate to accelerate (in the direction of the applied force)

Key terms in this question

constant velocity

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