1.3.3Dynamics
Measuring Acceleration
5 min read 5 parts Lesson 10 of 22
Section 1 of 5The Principle
Acceleration cannot be read from a meter; it is calculated from several measured quantities.
What to measure
- To find acceleration we need the initial velocity, the final velocity and the time taken to change between them.
Velocities
- Each velocity is found from the length of an interrupt card divided by the time it blocks a light beam.
Time between
- The time between the two readings is measured by the timer or computer, from the first beam break to the second.
Equation
- These values are then substituted into a = (v − u) ÷ t to find the acceleration of the trolley.
Section 2 of 5Two Set-ups
Two Set-ups
Two light gates
- Place two gates a known distance apart on a slope;
- one timer records the card time at each gate and the time between them.
Double interrupt card
- A card with two blocking sections passes one gate, so one gate gives both velocities and the time between them.
Computer software
- Data-logging software displays the velocities and time between automatically and can even calculate the acceleration.
Slope
- The trolley is released from rest at the top of a ramp, so it accelerates steadily as it moves down.
| Measurement | Symbol | How it is found |
|---|---|---|
| Initial velocity | u | card length ÷ time at first gate |
| Final velocity | v | card length ÷ time at second gate |
| Time between gates | t | from the timer |
Section 3 of 5Improving Reliability
1
Release the trolley from the same point every time so that the starting conditions are identical.
2
Repeat the experiment several times and calculate the mean acceleration.
3
Use a card of accurately measured length because its length directly affects both velocities.
4
Check that the gates are aligned so the card cuts the beam cleanly each time.
5
Compare results from different slope angles to investigate how a steeper slope gives a greater acceleration.
Section 4 of 5Worked Example
Question
- A double card with each section 0.050 m long passes a light gate.
- The first section blocks the beam for 0.025 s, the second for 0.020 s, and 0.50 s passes between the two readings.
- Calculate the acceleration.
- u = 0.050 ÷ 0.025 = 2.0 m/s.
- v = 0.050 ÷ 0.020 = 2.5 m/s.
- a = (v − u) ÷ t = (2.5 − 2.0) ÷ 0.50 = 0.50 ÷ 0.50 = 1.0 m/s².What the symbols mean: a = acceleration (m s−2) · v = final velocity · u = initial velocity (m s−1) · Δv = change in velocity · t = time taken (s)
Answer Acceleration = 1.0 m/s².
Section 5 of 5Exam Checklist & Tips
1.3.3 – Key Points for the Exam
- Double interrupt card A card with two sections that cut the beam separately.
- Light gate A sensor that records when a beam of light is blocked.
- Mean The average of several repeated measurements.
- Data logger A device or software that records measurements automatically.
Common Mistakes
- Using the total time the trolley was moving rather than the time between the two velocity readings.
- Mixing up u and v, which gives a negative acceleration for a trolley that is speeding up.
- Using the length of the whole card instead of the length of each separate section.
- Releasing the trolley from different heights each time and averaging the results.