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Units of motion for OCR A-level Physics

Units of motion

This page covers the following topics:

1. Errors in measurement

During experiments, random errors may occur which will affect precision and cause values of measurements to be different from the real values. Random errors can be reduced by repeating a process at least three times and calculating the mean, or by using computers to take measurements to reduce human error by, for example, eliminating the reaction time.

Systematic errors may also occur due to a faulty experimental method, which affects the accuracy of the measurements obtained, as it will make the values be either too great or too small by the same amount every time. Systematic errors can be reduced by calibrating apparatus and taking measurements at eye level to avoid parallax error.

Uncertainties can be calculated to find the range in which the true value of a reading (usually from an apparatus, electronic) or measurement (usually manual) is expected to lie.

uncertainty in a reading = ยฑ(half the smallest division)
uncertainty in a measurement = ยฑ(at least 1 smallest division)

When adding and subtracting data, the absolute uncertainties can be added, whereas when multiplying and dividing the data, the percentage uncertainties can be added.

Errors in measurement

1

A reading is taken from a thermometer that has the smallest division of 1ยฐC . Calculate the uncertainty of this reading.

The uncertainty for a reading is ยฑhalf the smallest division.
The smallest division of a thermometer is 1ยฐC, so uncertainty = ยฑ0.5ยฐC.

ยฑ0.5ยฐC

A reading is taken from a thermometer that has the smallest division of 1ยฐC . Calculate the uncertainty of this reading.

2

An experiment involving timing a free-falling mass using a stopwatch is conducted to calculate acceleration due to gravity. Identify two possible sources of error in this experiment.

Possible answers: incorrectly callibrated scales when measuring the mass, systematic error in ruler when measuring height through which the mass falls, human random error in measuring time.

incorrectly callibrated scales when measuring the mass, human random error in measuring time

An experiment involving timing a free-falling mass using a stopwatch is conducted to calculate acceleration due to gravity. Identify two possible sources of error in this experiment.

3

The speed of a toy trolley going down the ramp is found by measuring the length of the ramp using a ruler and using lightgates and a computer to measure the time, and using the equation for speed. Suggest why using lightgates and a computer to measure time is likely to introduce less error than using a stopwatch.

Using a stopwatch would increase the random error due to reaction time. Therefore, the lightgates and computer are more reliable since they eliminate human error.

Using computer setup would eliminate the reaction time.

The speed of a toy trolley going down the ramp is found by measuring the length of the ramp using a ruler and using lightgates and a computer to measure the time, and using the equation for speed. Suggest why using lightgates and a computer to measure time is likely to introduce less error than using a stopwatch.

4

Suggest a method which could help reduce systematic error.

Possible answers: calibrating apparatus before use, taking measurements at eye level.

calibrating apparatus before use

Suggest a method which could help reduce systematic error.

5

Define random errors.

Random errors are ones which affect precision and cause values of measurements to be different from the real values.

unpredictable errors affecting precision of measurements

Define random errors.

End of page

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