A small toy car is pushed along a straight track. It starts from rest, accelerates uniformly for several seconds, then travels at a constant speed, and finally slows down and stops. Describe the motion of the toy car during each of these three stages, explaining the forces acting on it at each stage.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A small toy car is pushed from rest along a straight, flat track. During stage one it accelerates uniformly. During stage two it travels at constant speed. During stage three it decelerates and stops.
Model answer (5 marks)
Stage 1 – The toy car is pushed from rest and accelerates uniformly. The applied pushing force is larger than the resistive forces (friction and air resistance), giving a net force in the direction of motion and therefore a constant acceleration.
Stage 2 – The car moves at a constant speed. The driving force equals the resistive forces, so the net force is zero and the car continues with uniform motion.
Stage 3 – The car slows and stops. Either the driving force is removed or the resistive forces exceed the driving force, giving a net force opposite to the direction of motion, which causes a constant deceleration until the car comes to rest.
Stage 2 – The car moves at a constant speed. The driving force equals the resistive forces, so the net force is zero and the car continues with uniform motion.
Stage 3 – The car slows and stops. Either the driving force is removed or the resistive forces exceed the driving force, giving a net force opposite to the direction of motion, which causes a constant deceleration until the car comes to rest.
Examiner tips
- Use the word "net force" and state its direction for each stage
- Show that acceleration is positive, zero, then negative
- Mention the specific resistive forces (friction, air resistance)
Common mistakes
- Confusing the direction of the net force in stage 3
- Forgetting to state that acceleration is zero in stage 2
- Using vague terms like "push" instead of "applied force"
Mark scheme (5 marks)
- During stage one, the driving/pushing force is greater than the resistive forces (friction/air resistance), so there is a resultant force in the direction of motion causing acceleration.
- During stage two, the driving force equals the resistive forces (friction/air resistance), so the resultant force is zero and the car travels at constant/uniform speed.
- During stage three, the resistive forces (friction/air resistance) are greater than the driving force (or the driving force is removed entirely), giving a resultant force opposing motion, causing the car to decelerate and stop.
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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