AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Acceleration: initial velocity, u, in metres per second, m/s
Initial velocity, symbol u, is the velocity of an object at the moment timing starts, measured in metres per second (m/s).
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 23
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Initial velocity, symbol u, is the velocity of an object at the moment timing starts, measured in metres per second (m/s). Velocity is a vector: it has size and direction, so u can be positive or negative depending on the chosen positive direction. For example, a car already rolling at 8 m/s when a stopwatch starts has u = 8 m/s; a ball thrown upwards at 12 m/s, with upwards taken as positive, has u = +12 m/s, while the same ball moving downwards has negative velocity. In acceleration calculations, u is the starting value and the final velocity v is the value after the time interval. The change in velocity is v − u, and dividing this by the time taken gives the acceleration. Always state the direction convention and convert units such as km/h to m/s before substituting.
What earns the marks
- States that u is the velocity at the start of the timed interval, not necessarily zero.
- Recognises that velocity is a vector, so u has both magnitude and direction and may be positive or negative.
- Uses the unit metres per second (m/s) and converts values such as km/h into m/s before calculation.
- Substitutes u correctly into a = (v − u) ÷ t, treating v as the final velocity after the interval.
- Interprets a negative value of u as motion in the direction chosen as negative, not as an impossible speed.
Common mistakes and how to put them right
Treating u as always zero.
Put it right by checking whether the object was already moving when timing began.
Confusing u with the final velocity v.
Put it right by labelling the starting value u and the value after the interval v before substituting.
Ignoring direction and giving u as a positive number when the object moves opposite to the chosen positive direction.
Put it right by stating the positive direction and including the sign.
Exam tips
- Underline the words 'initial velocity' in a question and write u beside the value before choosing an equation.
- Convert all speeds to m/s first, then substitute, so the acceleration comes out in m/s².
- State the positive direction in one short sentence whenever a velocity could be negative.
Try the lesson
The start of the Acceleration: initial velocity, u, in metres per second, m/s lesson, made from checked teaching notes for this specification point.
Have a go first
A runner starts from rest and accelerates to 6 m/s. What is the initial velocity, u, of the runner?
Listen
Initial velocity, symbol u, is the velocity of an object at the moment timing starts, measured in metres per second (m/s). Velocity is a vector: it has size and direction, so u can be positive or negative depending on the chosen positive direction. For example, a car already rolling at 8 m/s when a stopwatch starts has u = 8 m/s; a ball thrown upwards at 12 m/s, with upwards taken as positive, has u = +12 m/s, while the same ball moving downwards has negative velocity. In acceleration calculations, u is the starting value and the final velocity v is the value after the time interval. The change in velocity is v − u, and dividing this by the time taken gives the acceleration. Always state the direction convention and convert units such as km/h to m/s before substituting. A strong answer earns marks for these points. States that u is the velocity at the start of the timed interval, not necessarily zero. Recognises that velocity is a vector, so u has both magnitude and direction and may be positive or negative. Uses the unit metres per second (m/s) and converts values such as km/h into m/s before calculation. Substitutes u correctly into a = (v − u) ÷ t, treating v as the final velocity after the interval. A common slip: Treating u as always zero. Instead, checking whether the object was already moving when timing began. Exam tip: Underline the words 'initial velocity' in a question and write u beside the value before choosing an equation.
See it
Key words
Say what each word means before you turn the card.
Initial velocity
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Velocity
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Change in velocity
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Direction convention
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See every definition
- Initial velocity
- The velocity of an object at the moment timing starts, given the symbol u and measured in metres per second (m/s).
- Velocity
- A vector quantity, meaning it has both size and direction, so it can be positive or negative depending on the chosen positive direction.
- Change in velocity
- The final velocity v minus the initial velocity u, and dividing this by the time taken gives the acceleration.
- Direction convention
- The chosen positive direction, which must always be stated because a negative value of u means motion in the direction chosen as negative.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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