1.2.7Development of Practical Skills in Physics

Use of Software and Tools

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1.2.7 Use of Software and Tools — OCR A Level Physics A notes: Use spreadsheets, data loggers, sensors and graph-plotting software effectively, and recognise their strengths and limitations.Modern physics relies on computers and electronic sensors.

Sampling rate
Resolution
Calibration
light gate
Computer simulations
Video analysis

Section 1 of 9Why use software and electronic tools

Modern physics relies on computers and electronic sensors.

Why use software and electronic tools

They record data faster than a person can, make many repeat readings, calculate and plot automatically, and reveal trends in large data sets.

A student who can use them well can investigate fast processes (a bouncing ball, a collision) that a stopwatch could never time.

But the tools do not think: the physicist must decide what to measure, check the output makes sense and judge its uncertainty.

Section 2 of 9Data loggers, sensors and light gates

A data logger records a signal from a sensor (temperature, light, sound, force, motion, p.d.) at regular intervals. Key features are:

Sampling rate

Sampling rate readings per second. It must be high enough to follow the change. A pulse lasting 10 ms needs sampling at least every 1 ms. Too slow a rate misses peaks.

Resolution

Resolution the smallest change the sensor and its analogue-to-digital converter can detect.

Range

Range the largest and smallest values it can measure without overloading.

Calibration

Calibration the sensor reading must be checked against a known standard.

A light gate times how long a beam is interrupted.

For a card of length d passing in time t the average speed is v = d/t.

Two gates give the time between them and hence an acceleration.

Resolution can be 0.001 s or better, removing human reaction time.

Other sensors include motion sensors (ultrasound), force sensors, and the stopwatch in video analysis.

Section 3 of 9Spreadsheets and graph software

A spreadsheet is ideal for repetitive calculation.

Enter raw data in columns, then use formulae (for example, a column for T = t/20 or T²) and fill down.

Trendline

Built-in functions such as average, standard deviation, logarithm and the trendline or linear regression give gradient and intercept quickly.

Benefits are speed, fewer arithmetic slips, and quick re-plotting if the choice of axes changes.

Limits to bear in mind:

Software will plot a best-fit line through anything; check the points actually look linear.

Trendline outputs are shown to many digits; round them sensibly.

Error bars and worst-fit lines must be added deliberately; the default line does not give an uncertainty.

Check units and axis labels, which are often missing by default.

Section 4 of 9Simulations and video analysis

Computer simulations let you change a variable that would be impractical in the lab (for example, the gravitational field strength), and test a model against data. A simulation is only as good as its model: it may ignore air resistance or friction, so agreement with a simulation is not the same as agreement with reality.

Video analysis uses a recording at a known frame rate.

If a video runs at 30 frames per second, the time between frames is 1/30 s = 0.033 s.

Position is read from a scale in the picture (a metre rule in the same plane as the motion).

This allows velocity and acceleration to be extracted from a projectile or a collision, and an object’s motion to be checked against theory.

Errors arise from parallax if the camera is not square-on and from the finite frame rate.

Section 5 of 9Choosing the right tool and recording what you did

Select the tool by the scale of the measurement: a stopwatch is adequate for a 10 s swing, but a data logger is needed for a collision lasting 2 ms.

Choosing the right tool and recording what you did

Always record the model of instrument, the sampling rate and the range chosen so that the work can be repeated.

Keep raw data files; do not overwrite them with processed values.

Software helps with the skills of Plotting and Interpreting Graphs (1.2.3), but understanding the physics behind a gradient is still your job.

Section 6 of 9Good practice with electronic data

Before logging, test the sensor with a known input to confirm it responds as expected.

Good practice with electronic data

During analysis, look at the raw plot first and decide whether the signal is noisy; if so, take a short average rather than a single point.

Save data in more than one place, and record units and the time of each run in the file name.

Section 7 of 9Worked examples

A card of length 0.050 m interrupts a light gate for 0.0125 s. Calculate the speed of the trolley carrying it. If the gate has timing resolution 0.0005 s, give the percentage uncertainty in the speed from timing.
A video recorded at 25 frames per second shows a ball falling. Its position changes from 0.10 m to 0.34 m below the release point over 6 frames. Find the average speed over those frames.
A temperature sensor records a rapid cooling that takes 2.0 s to complete. What is the minimum sampling rate needed to obtain at least 20 readings during the cooling?

Section 8 of 9Key terms

Term	Meaning
Data logger	An electronic device that records readings from sensors at set time intervals.
Sampling rate	The number of readings taken per second.
Sensor	A device that converts a physical quantity into an electrical signal.
Trendline	A best-fit line generated by software through plotted data.
Light gate	A sensor that times how long a light beam is blocked.
Frame rate	Number of video frames recorded each second.

Section 9 of 9Exam technique

Accepting a trendline or software output without checking that the data really are linear.

Quoting the gradient to all the digits that the spreadsheet displays.

Choosing a sampling rate that is too low to catch a rapid change.

Assuming a simulation shows real behaviour when it ignores friction or air resistance.

Forgetting to calibrate or zero a sensor before logging.