Vertical Motion and Calculating g

Overview

Vertical motion is still SUVAT, but the direction choice becomes more important. This lesson also connects the calculation model to an experiment for measuring the acceleration of free fall.

What You Need to Know

• Near the Earth’s surface, free-fall acceleration is usually taken as about 9.81 m s^-2 downward.
• If upward is positive, the acceleration due to gravity is negative.
• At the highest point of a vertical throw, the velocity is zero for an instant, but the acceleration is still downward.
• A falling-object experiment needs a measured distance and time, with attention to reaction time, release method, and repeated measurements.

How to Work Through It

1. Start with a simple drop problem and write the sign convention before calculating.
2. Work through a vertical throw where the object moves upward, stops briefly, then falls.
3. Connect the equations to a practical method for finding g.
4. Evaluate which measurements would most affect the quality of the result.

Check Your Understanding

• Why is acceleration not zero at the top of a throw?
• What sign should g have if upward is chosen as positive?
• Which measurements are needed to determine g from a falling object?
• How would air resistance affect the result compared with the ideal model?

Common Mistakes

• Changing the positive direction halfway through a question.
• Treating the highest point as a place where acceleration disappears.
• Using g = 9.81 without considering whether it should be positive or negative.
• Describing a practical method without explaining how the data leads to g.

Next Steps

• Redo one vertical motion question with the opposite sign convention to check your understanding.
• Keep the free-fall practical in mind when you return to Paper 3 skills.