Motion QWC practice question
Question
A student uses a trapdoor and electromagnet arrangement to measure the acceleration of free fall. When the electromagnet releases a steel ball, an electronic timer starts. The ball falls through the trapdoor, which breaks an electrical circuit and stops the timer. The student measures the distance fallen as 0.50 m and records a fall time of 0.32 s.
Explain why this experimental method can give a reliable measurement of gravitational acceleration, and discuss what sources of error might affect the accuracy of the result.
Student guidance
Write a clear linked explanation using precise physics vocabulary.
Expected physics points
- Key physics ideas to look for:
- Free fall under gravity with zero initial velocity
- Kinematic equation s = ½gt² or rearrangement a = 2s/t²
- The role of the electromagnet in ensuring the ball starts from rest
- The role of the trapdoor in defining the distance and stopping the timer
- Air resistance acts upward, opposing motion, reducing the net downward acceleration measured
Mark scheme points
- Key physics ideas to look for:
- Free fall under gravity with zero initial velocity
- Kinematic equation s = ½gt² or rearrangement a = 2s/t²
- The role of the electromagnet in ensuring the ball starts from rest
- The role of the trapdoor in defining the distance and stopping the timer
- Air resistance acts upward, opposing motion, reducing the net downward acceleration measured
Indicative content
- Key physics ideas to look for:
- Free fall under gravity with zero initial velocity
- Kinematic equation s = ½gt² or rearrangement a = 2s/t²
- The role of the electromagnet in ensuring the ball starts from rest
- The role of the trapdoor in defining the distance and stopping the timer
- Air resistance acts upward, opposing motion, reducing the net downward acceleration measured
- Timing uncertainty (reaction time, electronic timer precision) introduces random error into t
- Distance measurement uncertainty propagates into the calculated acceleration
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