Skills in the study of physics practice questions.

Which of these is an SI base unit?

• the newton (N)
• kilogram
• joule
• watt

Answer: kilogram

The seven SI base units are kg, m, s, A, K, mol and cd. The newton, joule and watt are derived units.

Which of the following is a vector quantity?

• speed
• mass
• velocity
• temperature

Answer: velocity

Vectors have magnitude and direction. Velocity, force, acceleration and momentum are vectors; speed, mass and temperature are scalars.

What is meant by the resolution of a measuring instrument?

• The smallest change in the quantity that the instrument can detect
• The largest value the instrument is able to display on its scale
• The difference between the true value and the value that is measured
• The full range of values over which the instrument has been calibrated

Answer: The smallest change in the quantity that the instrument can detect

Resolution is the finest change an instrument can register (e.g. 1 mm on a metre rule). A common error is to confuse it with accuracy, which is about how close a reading is to the true value.

What is the main advantage of using a data logger with a motion sensor rather than a stopwatch to record a fast collision?

• It samples many readings per second, capturing events too fast to time by hand
• It automatically removes every systematic error present in the sensor calibration
• It guarantees that the recorded data contain no random error of any kind at all
• It makes the raw measurements more accurate than the true accepted physical value

Answer: It samples many readings per second, capturing events too fast to time by hand

The strength of a data logger is a high sampling rate and no human reaction-time delay. It does not remove systematic errors built into the sensor's calibration.

A spreadsheet fits a linear trendline to a set of data points and reports R² = 0.998. What does this value indicate?

• The data are described very well by a straight-line relationship
• The gradient of the fitted line of best fit is equal to 0.998
• The percentage uncertainty in each plotted data point is about 0.2%
• The independent variable is definitely the physical cause of the change

Answer: The data are described very well by a straight-line relationship

R² close to 1 means the points lie very close to the fitted straight line. It is not the gradient, an uncertainty, or evidence of causation.

A student investigates how the extension of a spring depends on the load hung from it. Which quantity is the independent variable?

• The load hung from the spring
• The extension of the spring
• The spring constant of the spring
• The temperature of the room

Answer: The load hung from the spring

The independent variable is the one deliberately changed by the experimenter (the load). The extension is the dependent variable, and other factors are kept controlled.