A student pushes a heavy crate along a factory floor at a constant velocity. Explain why a constant velocity means the resultant force on the crate must be zero, and state what this tells you about the relationship between the pushing force and the friction force acting on the crate.

Model answer (4 marks)

Examiner tips

• Show the link between constant velocity and zero acceleration first.
• State Newton’s first law to justify zero resultant force.
• Explain that the two horizontal forces must cancel – equal magnitude, opposite direction.
• Use the exact wording ‘equal in magnitude’ and ‘opposite directions’.

Common mistakes

• Confusing ‘no acceleration’ with ‘no force’ – remember the resultant force must be zero.
• Forgetting to mention that the forces act in opposite directions.
• Using vague terms like ‘balanced’ without specifying magnitude and direction.