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Measuring Speed (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Leander Oates

Updated on

Measuring speed

Equipment for measuring distance and time

  • Speed is the distance travelled per unit time

  • Speed isn't usually measured directly, but it can be calculated from measurements of distance and time

  • The right equipment to use depends on how long or short the distance or time intervals are

Measuring distance

  • Short distances can be measured with a ruler, metre stick or tape measure

  • Long distances can be measured with a trundle wheel

A trundle wheel measuring 100 metres between two street lamps on a road, displaying distance with a red arrow and label.
Trundle wheels are useful for measuring long distances, such as the length of a road or a race track

Measuring time

  • Long time intervals can be measured with a stopwatch

  • Short time intervals can be measured using light gates connected to a timer or a computer

A trolley passing through a light gate, blocking a light beam, with a connected timer measuring the duration of the light blockage.
Light gates are used for measuring short time intervals, such as the time for a card attached to a trolley to pass through it

Examiner Tips and Tricks

A light gate on its own cannot take measurements; it is the timer it connects to that records the time interval.

Measuring average speed

  • Average speed is the total distance travelled over the total time taken

    • It is measured over long distances and long time intervals

  • Apparatus for measuring average speed:

    • Trolley with card attached

    • Ramp

    • Metre stick or measuring tape

    • Two light gates connected to a computer or electronic timer

Experimental set-up for measuring average speed

A trolley on a ramp with a card passes through two light gates connected to a timer, measuring 1.05s over a metre stick distance.
The average speed of the trolley between two points can be determined by measuring the time taken for the trolley to pass between two light gates

  • Method:

    • Set up the ramp with two light gates attached to a computer or timer

    • Measure the distance between the light gates with a metre stick

    • Release the trolley from the top of the ramp

    • Timer starts when the trolley passes the first light gate and stops when it passes the second light gate

    • Calculate the average speed of the trolley using the equation:

average space speed space equals space fraction numerator distance space between space light space gates over denominator time space for space trolley space to space pass space between space light space gates end fraction

Measuring instantaneous speed

  • Instantaneous speed is the speed of an object at a given instant in time

    • It is measured over very short distances and very short time intervals

  • Apparatus for measuring instantaneous speed:

    • Trolley with card attached

    • Ramp

    • Ruler

    • Light gate connected to a computer or electronic timer

Experimental set-up for measuring instantaneous speed

A trolley on a ramp passing through a light gate attached to a timer, measuring 0.01s. Labels: card length, ruler, light gate, trolley.
The instantaneous speed of the trolley at a point can be determined by measuring the time taken for a known length of card to pass through a light gate

  • Method:

    • Set up the ramp with one light gate attached to a computer or timer (or use the same apparatus as above, but connect each light gate to a separate timer)

    • Measure the length of the card using a ruler

    • Release the trolley from the top of the ramp

    • As the trolley passes the light gate, the timer records the time the card blocks the light beam

    • Calculate the instantaneous speed of the trolley using the equation:

instantaneous space speed space equals space fraction numerator length space of space card over denominator time space for space card space to space cut space beam end fraction

Examiner Tips and Tricks

Whenever you’re asked to describe an experiment, if you need to use an equation to calculate something, write this down first. The relationship between the quantities in the equation may give you some hints about the variables you might need to measure.

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics Content Creator

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Leander Oates

    Reviewer: Leander Oates

    Expertise: Physics Content Creator

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.